1

Aberrations
 Aberrations are phenomena that degrade
the quality of the image formed by an optical
system
 2

Aberrations
 Aberrations are phenomena that degrade
the quality of the image formed by an optical
system
 Degradation results when light rays from a
given object-point fail to form a single sharp
image
 3

Aberrations
 Aberrations are phenomena that degrade
the quality of the image formed by an optical
system
 Degradation results when light rays from a
given object-point fail to form a single image-
point
 4

Aberrations
 Aberrations are phenomena that degrade
the quality of the image formed by an optical
system
 Degradation results when light rays from a
given object-point fail to form a single image-
point
 It’s important to recognize that aberrations
are the rule, not the exception
 Aberration-free vision essentially never occurs
 5

Aberrations
 Some aberrations are attributable to
corrective lenses
 6

Aberrations
 Some aberrations are attributable to
corrective lenses
 Others are intrinsic to the eye itself
 7

Aberrations
 Some aberrations are attributable to
corrective lenses
 Others are intrinsic to the eye itself
 Three familiar forms:
 Spherical error (myopia/hyperopia)
 Cylinder (astigmatism)
 Chromatic aberration
 8

Aberrations
 Some aberrations are attributable to
corrective lenses
 Others are intrinsic to the eye itself
 Three familiar forms:
 Spherical error (myopia/hyperopia)
 Cylinder (astigmatism)
 Chromatic aberration
 9

Aberrations
 Back in the day, only three aberrations
were addressed by clinicians:
1) Spherical error (ie, myopia/hyperopia)
2) Regular astigmatism
 Regular meaning ‘that which can be corrected with
cylindrical lenses’
3) Irregular astigmatism
 Irregular meaning ‘that which can’t be corrected with
cylindrical lenses’
 10

Aberrations
 Back in the day, only three aberrations
were addressed by clinicians:
1) Spherical error (ie, myopia/hyperopia)
2) Regular astigmatism
 Regular meaning ‘that which can be corrected with
cylindrical lenses’
3) Irregular astigmatism
 Irregular meaning ‘that which can’t be corrected with
cylindrical lenses’
 11

Aberrations
 Back in the day, only three aberrations
were addressed by clinicians:
1) Spherical error (ie, myopia/hyperopia)
2) Regular astigmatism
 Regular meaning ‘that which can be corrected with
cylindrical lenses’
3) Irregular astigmatism
 Irregular meaning ‘that which can’t be corrected with
cylindrical lenses’
 12

Aberrations
 Back in the day, only three aberrations
were addressed by clinicians:
1) Spherical error (ie, myopia/hyperopia)
2) Regular astigmatism
 Regular meaning ‘that which can be corrected with
cylindrical lenses’
3) Irregular astigmatism
 Irregular meaning ‘that which can’t be corrected with
cylindrical lenses’
 13

Aberrations
 Back in the day, only three aberrations
were addressed by clinicians:
1) Spherical error (ie, myopia/hyperopia)
2) Regular astigmatism
 Regular meaning ‘that which can be corrected with
cylindrical lenses’
3) Irregular astigmatism
 Irregular meaning ‘that which can’t be corrected with
cylindrical lenses’
 14

Aberrations
 Back in the day, only three aberrations
were addressed by clinicians:
1) Spherical error (ie, myopia/hyperopia)
2) Regular astigmatism
 Regular meaning ‘that which can be corrected with
cylindrical lenses’
3) Irregular astigmatism
 Irregular meaning ‘that which can’t be corrected with
cylindrical lenses’
 15

Aberrations
 Back in the day, only three aberrations
were addressed by clinicians:
1) Spherical error (ie, myopia/hyperopia)
2) Regular astigmatism
 Regular meaning ‘that which can be corrected with
cylindrical lenses’
3) Irregular astigmatism
 Irregular meaning ‘that which can’t be corrected with
cylindrical lenses’

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected (by glasses) even if they had been measureable
 16

Aberrations
Old Lingo

Sphere
Myopia
Hyperopia

‘Regular
Cylinder
Astigmatism’

This is how we thought of

aberrations back in the day
Any component
of refractive error
‘Irregular that could not be
Astigmatism’ remediated with
spherical and/or
cylindrical lenses
 17

Aberrations
 Wavefront analysis did away with the first
two words

problem

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 18

Aberrations: Wavefront Analysis

 Wavefront analysis did away with the first
problem

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 19

Aberrations: Wavefront Analysis

 Wavefront analysis did away with the first
problem
 Allows clinicians to identify/quantify many of the
refractive problems previously consigned to the
irregular-astigmatism wastebasket

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 20

Aberrations: Wavefront Analysis

 Wavefront analysis did away with the first
problem
 Allows clinicians to identify/quantify many of the
refractive problems previously consigned to the
irregular-astigmatism wastebasket
 Several different technologies for measuring the
wavefront have been developed, but one dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 21

Aberrations: Wavefront Analysis

 Wavefront analysis did away with the first
problem
 Allows clinicians to identify/quantify many of the
refractive problems previously consigned to the
irregular-astigmatism wastebasket
 Several different technologies for measuring the
wavefront have been developed, but one dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 22

Aberrations: Wavefront Analysis

Wavefront analysis did away with the first
How does the Hartmann-Shack wavefront sensor (HSWS) work?

Essentially, by reversing the function of the eye. Instead of treating the eye as a light-gathering device, it treats
the eye as a light-emitting device. It then analyzes the wavefront of light emitted by the eye with respect to
problem
how ‘pure’ (ie, how uniform and free of warpage) it is.

How does Allows clinicians to identify/quantify many of the

 the HSWS turn the eye into a light-emitting device?
By firing a low-power laser at the fovea, which reflects off the fovea. This reflected light then passes through
refractive problems previously consigned to the
the focusing structures of the eye (ie, the lens and cornea, and leaves the eye.

irregular-astigmatism
OK, so the HSWS wastebasket
turns the eye into a flashlight of sorts. How does this allow for identification and
quantification of aberrations?
The HSWS Several different
contains an array of sensors thattechnologies for
measure the ‘emitted’ light. If measuring the
the refracting structures of the eye
were perfect (ie, aberration-free), the wavefront of the emitted light would be perfectly flat; any deviation from
wavefront
flatness represents have
aberration, which been
in turn reflects developed, butfocusing
imperfections in the eye’s onestructures.
dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 23

Aberrations: Wavefront Analysis

Wavefront analysis did away with the first
How does the Hartmann-Shack wavefront sensor (HSWS) work?

Essentially, by reversing the function of the eye. Instead of treating the eye as a light-gathering device, it treats
the eye as a light-emitting device. It then analyzes the wavefront of light emitted by the eye with respect to
problem
how ‘pure’ (ie, how uniform and free of warpage) it is.

How does Allows clinicians to identify/quantify many of the

 the HSWS turn the eye into a light-emitting device?
By firing a low-power laser at the fovea, which reflects off the fovea. This reflected light then passes through
refractive problems previously consigned to the
the focusing structures of the eye (ie, the lens and cornea, and leaves the eye.

irregular-astigmatism
OK, so the HSWS wastebasket
turns the eye into a flashlight of sorts. How does this allow for identification and
quantification of aberrations?
The HSWS Several different
contains an array of sensors thattechnologies for
measure the ‘emitted’ light. If measuring the
the refracting structures of the eye
were perfect (ie, aberration-free), the wavefront of the emitted light would be perfectly flat; any deviation from
wavefront
flatness represents have
aberration, which been
in turn reflects developed, butfocusing
imperfections in the eye’s onestructures.
dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 24

Aberrations: Wavefront Analysis

Wavefront analysis did away with the first
How does the Hartmann-Shack wavefront sensor (HSWS) work?

Essentially, by reversing the function of the eye. Instead of treating the eye as a light-gathering device, it treats
the eye as a light-emitting device. It then analyzes the wavefront of light emitted by the eye with respect to
problem
how ‘pure’ (ie, how uniform and free of warpage) it is.

How does Allows clinicians to identify/quantify many of the

 the HSWS turn the eye into a light-emitting device?
By firing a low-power laser at the fovea, which reflects off the fovea. This reflected light then passes through
refractive problems previously consigned to the
the focusing structures of the eye (ie, the lens and cornea, and leaves the eye.

irregular-astigmatism
OK, so the HSWS wastebasket
turns the eye into a flashlight of sorts. How does this allow for identification and
quantification of aberrations?
The HSWS Several different
contains an array of sensors thattechnologies for
measure the ‘emitted’ light. If measuring the
the refracting structures of the eye
were perfect (ie, aberration-free), the wavefront of the emitted light would be perfectly flat; any deviation from
wavefront
flatness represents have
aberration, which been
in turn reflects developed, butfocusing
imperfections in the eye’s onestructures.
dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 25

Aberrations: Wavefront Analysis

Wavefront analysis did away with the first
How does the Hartmann-Shack wavefront sensor (HSWS) work?

Essentially, by reversing the function of the eye. Instead of treating the eye as a light-gathering device, it treats
the eye as a light-emitting device. It then analyzes the wavefront of light emitted by the eye with respect to
problem
how ‘pure’ (ie, how uniform and free of warpage) it is.

How does Allows clinicians to identify/quantify many of the

 the HSWS turn the eye into a light-emitting device?
By firing a low-power laser into the eye that reflects off the fovea. The reflected light then passes through the
refractive problems previously consigned to the
focusing structures of the eye (ie, the lens and cornea), and leaves the eye.

irregular-astigmatism
OK, so the HSWS wastebasket
turns the eye into a flashlight of sorts. How does this allow for identification and
quantification of aberrations?
The HSWS Several different
contains an array of sensors thattechnologies for
measure the ‘emitted’ light. If measuring the
the refracting structures of the eye
were perfect (ie, aberration-free), the wavefront of the emitted light would be perfectly flat; any deviation from
wavefront
flatness represents have
aberration, which been
in turn reflects developed, butfocusing
imperfections in the eye’s onestructures.
dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 26

Aberrations: Wavefront Analysis

Wavefront analysis did away with the first
How does the Hartmann-Shack wavefront sensor (HSWS) work?

Essentially, by reversing the function of the eye. Instead of treating the eye as a light-gathering device, it treats
the eye as a light-emitting device. It then analyzes the wavefront of light emitted by the eye with respect to
problem
how ‘pure’ (ie, how uniform and free of warpage) it is.

How does Allows clinicians to identify/quantify many of the

 the HSWS turn the eye into a light-emitting device?
By firing a low-power laser into the eye that reflects off the fovea. The reflected light then passes through the
refractive problems previously consigned to the
focusing structures of the eye (ie, the lens and cornea), and leaves the eye.

irregular-astigmatism
OK, so the HSWS wastebasket
turns the eye into a flashlight of sorts. How does this allow for identification and
quantification of aberrations?
The HSWS Several different
contains an array of sensors thattechnologies for
measure the ‘emitted’ light. If measuring the
the refracting structures of the eye
were perfect (ie, aberration-free), the wavefront of the emitted light would be perfectly flat; any deviation from
wavefront
flatness represents have
aberration, which been
in turn reflects developed, butfocusing
imperfections in the eye’s onestructures.
dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 27

Aberrations: Wavefront Analysis

Wavefront analysis did away with the first
How does the Hartmann-Shack wavefront sensor (HSWS) work?

Essentially, by reversing the function of the eye. Instead of treating the eye as a light-gathering device, it treats
the eye as a light-emitting device. It then analyzes the wavefront of light emitted by the eye with respect to
problem
how ‘pure’ (ie, how uniform and free of warpage) it is.

How does Allows clinicians to identify/quantify many of the

 the HSWS turn the eye into a light-emitting device?
By firing a low-power laser into the eye that reflects off the fovea. The reflected light then passes through the
refractive problems previously consigned to the
focusing structures of the eye (ie, the lens and cornea), and leaves the eye.

irregular-astigmatism
OK, so the HSWS wastebasket
turns the eye into a flashlight of sorts. How does this allow for identification and
quantification of aberrations?
The HSWS Several different
contains an array of sensors thattechnologies for
measure the ‘emitted’ light. If measuring the
the refracting structures of the eye
were perfect (ie, aberration-free), the wavefront of the emitted light would be perfectly flat--any deviation from
wavefront
flatness represents have
aberration, which been
in turn reflects developed, butfocusing
imperfections in the eye’s onestructures.
dominates
current clinical practice:
The Hartmann-Shack wavefront sensor

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 28

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia
Hyperopia

‘Regular
Cylinder
Astigmatism’

Any component
of refractive error
‘Irregular that could not be
Astigmatism’ remediated with
spherical and/or
cylindrical lenses
 29

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive
+ vs (-) defocus

Hyperopia = Negative
+ vs (-) defocus

‘Regular
Cylinder
Astigmatism’

Any component
of refractive error
‘Irregular that could not be
Astigmatism’ remediated with
spherical and/or
cylindrical lenses
 30

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus To remember which is which,

Myopia = Positive defocus note that each is the same
Hyperopia = Negative defocus as the error lens
responsible for each status

‘Regular
Cylinder
Astigmatism’

Any component
of refractive error
‘Irregular that could not be
Astigmatism’ remediated with
spherical and/or
cylindrical lenses
 31

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

‘Regular
Cylinder = Cylinder
Astigmatism’

Any component
of refractive error
‘Irregular that could not be
Astigmatism’ remediated with
spherical and/or
cylindrical lenses
 32

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

‘Regular
Cylinder = Cylinder
Astigmatism’

Any component
of refractive error
‘Irregular that could not be
Astigmatism’ remediated with
spherical and/or
cylindrical lenses
 33

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus ‘Lower-order
Aberrations’
‘Regular
Cylinder = Cylinder
Astigmatism’

Any component
of refractive error
‘Irregular that could not be
Astigmatism’ remediated with
spherical and/or
cylindrical lenses
 34

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus ‘Lower-order
Aberrations’
‘Regular
Cylinder = Cylinder
Astigmatism’

Spherical
two words
aberration
Any component
of refractive error Coma
‘Irregular that could not be
=
Astigmatism’ remediated with Trefoil
spherical and/or
cylindrical lenses (Others, less
clinically relevant)
 35

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus ‘Lower-order
Aberrations’
‘Regular
Cylinder = Cylinder
Astigmatism’

Spherical
aberration
Any component
of refractive error Coma
‘Irregular that could not be
=
Astigmatism’ remediated with Trefoil
spherical and/or
cylindrical lenses (Others, less
clinically relevant)
 36

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus ‘Lower-order
Aberrations’
‘Regular
Cylinder = Cylinder
Astigmatism’

Spherical
aberration
Any component
of refractive error Coma
‘Irregular that could not be ‘Higher-order
=
Astigmatism’ remediated with Trefoil Aberrations’
spherical and/or
cylindrical lenses (Others, less
clinically relevant)
 37

Aberrations
Old Lingo New Lingo
(from wavefront analysis)

Sphere = Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus ‘Lower-order
Aberrations’
‘Regular
We will address
Astigmatism’
Cylinder these= Cylinder

in greater detail later

in this slide-set Spherical
aberration
Any component
of refractive error Coma
‘Irregular that could not be ‘Higher-order
=
Astigmatism’ remediated with Trefoil Aberrations’
spherical and/or
cylindrical lenses (Others, less
clinically relevant)
 38

Aberrations
Not paraxial (close to optical axis, but not parallel to it)

ni = 1.0 nt = 1.34

Optical
Paraxial rays
axis

Not paraxial (nearly parallel to optical axis, but not close to it)

When dealing with refraction at a curved surface, we work only with the paraxial rays:
Those that are both close to the optical axis and nearly parallel to it.

(The above was presented first in the slide-set

Basic Optics, Chapter 17. If you have no idea
what it’s about, consider reviewing that chapter.)
 39

Aberrations
Not paraxial (close to optical axis, but not parallel to it)

ni = 1.0 nt = 1.34

Optical
Paraxial rays
axis

Not paraxial (nearly parallel to optical axis, but not close to it)

When dealing with refraction at a curved surface, we work only with the paraxial rays:
Those that are both close to the optical axis and nearly parallel to it.

Until now, we have focused exclusively on the optics of paraxial rays. But
to understand higher-order aberrations, we have to consider the optics of
nonparaxial rays.
 40

Aberrations
Not paraxial (close to optical axis, but not parallel to it)

ni = 1.0 nt = 1.34

Optical
Paraxial rays
axis

Not paraxial (nearly parallel to optical axis, but not close to it)

When dealing with refraction at a curved surface, we work only with the paraxial rays:
Those that are both close to the optical axis and nearly parallel to it.

Until now, we have focused exclusively on the optics of paraxial rays. But
to understand higher-order aberrations, we have to consider the optics of
nonparaxial rays.

The clinically most important higher-order aberration

stemming from nonparaxial rays is sphericaltwo words
aberration,
so we’ll discuss it first.
 41

Aberrations
Not paraxial (close to optical axis, but not parallel to it)

ni = 1.0 nt = 1.34

Optical
Paraxial rays
axis

Not paraxial (nearly parallel to optical axis, but not close to it)

When dealing with refraction at a curved surface, we work only with the paraxial rays:
Those that are both close to the optical axis and nearly parallel to it.

Until now, we have focused exclusively on the optics of paraxial rays. But
to understand higher-order aberrations, we have to consider the optics of
nonparaxial rays.

The clinically most important higher-order aberration

stemming from nonparaxial rays is spherical aberration,
so we’ll discuss it first.
 42

Aberrations: Spherical
 A spherical lens is one for which the
refracting surface(s) have a single
radius of curvature
three words
 43

Aberrations: Spherical
 A spherical lens is one for which the
refracting surface(s) have a single
radius of curvature
 44

Aberrations: Spherical
 A spherical lens is one for which the
refracting surface(s) have a single
radius of curvature

Sphere

Spherical lens

Note that a spherical lens need not be a sphere!

For a lens to be ‘spherical,’ its refracting surface(s)
must have a single radius-of-curvature—as if the
lens was sliced off of a sphere.
 45

Aberrations: Spherical
Spherical lens

 A spherical lens is one for which the

refracting surface(s) have a single
radius of curvature

Sphere

Note that a spherical lens need not have a single

refracting surface.
 46

Aberrations: Spherical
 A spherical lens is one for which the
refracting surface(s) have a single
radius of curvature Solid block of glass

Minus
lens
cut Sphere-shaped
out void in the glass
of
the
glass

Spherical (minus) lens

Note that a spherical lens need not be a

plus lens, either.
 47

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single
radius of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?

Sphere

Spherocylindrical lens?

Rhetorical question—advance when ready

 48

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power.
Sphere

Spherocylindrical lens?
 49

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere. Sphere

Spherocylindrical lens?
 50

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.

Can you think of an everyday (hint: and delicious) object from

which a slice could be taken that would qualify as a S-C lens?
 51

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.

Can you think of an everyday (hint: and delicious) object from

which a slice could be taken that would qualify as a S-C lens?
Yes—a donut.
 52

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.

Can you think of an everyday (hint: and delicious) object from

which a slice could be taken that would qualify as a S-C lens?
Yes—a donut. Every point on the surface of a donut has two
radii—one determined by its distance from the center of the
donut’s hole, the other by its distance from the center of the part
you bite into.
 53

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.

Can you think of an everyday (hint: and delicious) object from

which a slice could be taken that would qualify as a S-C lens?
Yes—a donut. Every point on the surface of a donut has two
radii—one determined by its distance from the center of the
donut’s hole, the other by its distance from the center of the part
you bite into. So, just as a spherical lens is created by taking a
slice off a sphere, a spherocylindrical lens is created by taking
a slice off a donut.
 54

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.
There is a more formal/precise name for the shape from
which a spherocylindrical lens is sliced—what is it?
Can you A
think of an everyday (hint: and delicious) object from
torus
which a slice could be taken that would qualify as an S-C lens?
Yes—a donut. Every
Similarly, point on thename
this more-formal surface
givesofrise
a donut has two
to an alternate
radii—one determined
name by its distance
for a spherocylindrical from the
lens—what center of the
is it?
A toric
donut’s hole; thelens
other by its distance from the center of the part
you bite into. So, just as a spherical lens is created by taking a
slice off a sphere, a spherocylindrical lens is created by taking
a slice off a donut.
 55

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.
There is a more formal/precise name for the shape from
which a spherocylindrical lens is sliced—what is it?
Can you A
think of an everyday (hint: and delicious) object from
torus
which a slice could be taken that would qualify as an S-C lens?
Yes—a donut. Every
Similarly, point on thename
this more-formal surface
givesofrise
a donut has two
to an alternate
radii—one determined
name by its distance
for a spherocylindrical from the
lens—what center of the
is it?
A toric
donut’s hole; thelens
other by its distance from the center of the part
you bite into. So, just as a spherical lens is created by taking a
slice off a sphere, a spherocylindrical lens is created by taking
a slice off a donut.
 56

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.
There is a more formal/precise name for the shape from
which a spherocylindrical lens is sliced—what is it?
Can you A
think of an everyday (hint: and delicious) object from
torus
which a slice could be taken that would qualify as an S-C lens?
Yes—a donut. Every
Similarly, point on thename
this more-formal surface
givesofrise
a donut has two
to an alternate
radii—one determined
name by its distance
for a spherocylindrical from the
lens—what center of the
is it?
A toric
donut’s hole; thelens
other by its distance from the center of the part
you bite into. So, just as a spherical lens is created by taking a
slice off a sphere, a spherocylindrical lens is created by taking
a slice off a donut.
 57

Aberrations: Spherical
spherocylindrical
 A spherical lens is one for which the
^
refracting surface(s) have a single two
radius
i of curvature
What about the refracting surface of a spherocylindrical (S-C) lens?
Recall that, by definition, a S-C lens has two different powers
oriented at right angles to one another. This means every point
on its surface has two radii—one for each power. Thus, such a
lens could not be created by slicing off a section from a sphere.
There is a more formal/precise name for the shape from
which a spherocylindrical lens is sliced—what is it?
Can you A
think of an everyday (hint: and delicious) object from
torus
which a slice could be taken that would qualify as an S-C lens?
Yes—a donut. Every
Similarly, point on thename
this more-formal surface
givesofrise
a donut has two
to an alternate
radii—one determined
name by its distance
for a spherocylindrical from the
lens—what center of the
is it?
A toric
donut’s hole; thelens
other by its distance from the center of the part
you bite into. So, just as a spherical lens is created by taking a
slice off a sphere, a spherocylindrical lens is created by taking
a slice off a donut.
 58

Aberrations: Spherical

Let’s drill down on how spherical aberration comes to pass:

 59

Aberrations: Spherical
Spherical lens

Object
point

Consider an object-lens system as above.

Let’s drill down on how spherical aberration comes to pass:

 60

Aberrations: Spherical
Spherical lens

Lens axis Paraxial rays

Object Image
point point

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

Let’s drill down on how spherical aberration comes to pass:

 61

Aberrations: Spherical
Spherical lens

Nonparaxial rays

Lens axis Paraxial rays

Object
point

Nonparaxial rays

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

However, when we look at the behavior of the non-paraxial rays, we find they do
not focus at the same location as the paraxial rays; rather, because they are more
sharply refracted, they focus anterior to the paraxial focal point.

Let’s drill down on how spherical aberration comes to pass:

 62

Aberrations: Spherical
Spherical lens

Nonparaxial rays

Lens axis Paraxial rays

Object
point

Nonparaxial rays

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

However, when we look at the behavior of the non-paraxial rays, we find they do
not focus at the same location as the paraxial rays; rather, because they are more
sharply refracted, they focus anterior to the paraxial focal point.
Why are nonparaxial rays refracted more than paraxial rays on a spherical lens?
Recall that Snell’s Law states that the angle of refraction is a function of the angle of
incidence. For paraxial rays, the angle of incidence is determined solely by the radius-
of-curvature of the lens. However, the angle-of-incidence for non-paraxial rays is a
function of both the radius of curvature and the fact that the surface of the lens
becomes more and more oblique as you move away from the lens axis; ie, the lens
periphery ‘turns away’ from the incoming light, thereby increasing the angle of
incidence in a way unrelated to the radius of curvature.
 63

Aberrations: Spherical
Spherical lens

Nonparaxial rays

Lens axis Paraxial rays

Object
point

Nonparaxial rays

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

However, when we look at the behavior of the non-paraxial rays, we find they do
not focus at the same location as the paraxial rays; rather, because they are more
sharply refracted, they focus anterior to the paraxial focal point.
Why are nonparaxial rays refracted more than paraxial rays on a spherical lens?
Snell’s Law states that the angle of refraction is a function of the angle of incidence.
For paraxial rays, the angle of incidence is determined solely by the radius-of-
curvature of the lens. However, the angle-of-incidence for non-paraxial rays is a
function of both the radius of curvature and the fact that the surface of the lens
becomes more and more oblique (relative to the path of the light) as you move away
from the lens axis; ie, the lens periphery ‘turns away’ from the incoming light, thereby
increasing the angle of incidence in a way unrelated to the radius of curvature.
 64

Aberrations: Spherical
Spherical lens

Nonparaxial rays

Lens axis Paraxial rays

Object
point

Nonparaxial rays

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

However, when we look at the behavior of the non-paraxial rays, we find they do
not focus at the same location as the paraxial rays; rather, because they are more
sharply refracted, they focus anterior to the paraxial focal point.
Why are nonparaxial rays refracted more than paraxial rays on a spherical lens?
Snell’s Law states that the angle of refraction is a function of the angle of incidence.
For paraxial rays, the angle of incidence is determined solely by the radius-of-
curvature of the lens. However, the angle-of-incidence for non-paraxial rays is a
function of both the radius of curvature and the fact that the surface of the lens
becomes more and more oblique (relative to the path of the light) as you move away
from the lens axis; ie, the lens periphery ‘turns away’ from the incoming light, thereby
increasing the angle of incidence in a way unrelated to the radius of curvature.
 65

Aberrations: Spherical
Spherical lens

Nonparaxial rays

Lens axis Paraxial rays

Object
point

Image
Nonparaxial rays (blur circle)

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

However, when we look at the behavior of the non-paraxial rays, we find they do
not focus at the same location as the paraxial rays; rather, because they are more
sharply refracted, they focus anterior to the paraxial focal point. By the time these rays
reach the focal plane for the paraxial rays, they are diverging. Thus, they contribute
not to a focal point, but rather to a somewhat defocused area called a blur circle.
 66

Aberrations: Spherical
Spherical lens

Nonparaxial rays

Lens axis Paraxial rays

Object
point

Image
Nonparaxial rays (blur circle)

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

However, when we look at the behavior of the non-paraxial rays, we find they do
not focus at the same location as the paraxial rays; rather, because they are more
sharply refracted, they focus anterior to the paraxial focal point. By the time these rays
reach the focal plane for the paraxial rays, they are diverging. Thus, they contribute
not to a focal point, but rather to a somewhat defocused area called a blur circle.

When progressively peripheral rays are refracted more and more sharply,
the lens is said to possess positive spherical aberration.
 67

Aberrations: Spherical
Non - Spherical lens
Nonparaxial rays

Lens axis Paraxial rays

Object
point

Image
(blur circle)
Nonparaxial rays

If we deal only with the paraxial rays, we find their focus closely approximates a
perfect point, as predicted by first-order optics.

However, when we look at the behavior of the non-paraxial rays, we find they do less
not focus at the same location as the paraxial rays; rather, because they are more
sharply refracted, they focus anterior to the paraxial focal point.
posterior

On the other hand, when progressively peripheral rays are refracted less
and less sharply, the lens is said to possess negative spherical aberration.
 68

Aberrations: Spherical

(All these rays

are from the
Lens same Lens
axis axis
point
on the object
at infinity.)
 69

Aberrations: Spherical

(All these rays

are from the
Lens same Lens
axis axis
point
on the object
at infinity.) Because of spherical
aberration, an optical
interval is produced
in which rays from
different ‘zones’ of the
lens are focused, but
all others aren’t.
 70

Aberrations: Spherical

(All these rays

are from the
Lens same Lens
axis axis
point
on the object
at infinity.)

A viewing screen placed

anywhere within this
interval would yield a
crisp image (due to rays
passing through one
zone of the lens), with
surrounding ‘halos’
representing rays from
the other zones (and
therefore out-of-focus
at this point in the interval)
 71

Aberrations: Spherical

(All these rays

are from the
Lens same Lens
axis axis
point
on the object
at infinity.)

And because it is an optical instrument…

 72

Aberrations: Spherical

(All these rays

are from the
Optical same Optical
axis axis
point
on the object
at infinity.)

And because it is an optical instrument…the eye is subject to the same phenomenon.

 73

Aberrations: Spherical

(Remember, Sharply-focused
all these rays retinal image point
are from the

same point
on the object
at infinity.)

When the pupil is small, light reaching the retina consists largely of paraxial rays; ie, rays passing through the
central portion of the cornea.

Object Image
point point
 74

Aberrations: Spherical

(Remember, Poorly-focused
all these rays retinal blur circle
are from the

same point
on the object
at infinity.)
Note the myopic shift--
ie, peripheral rays are
focused in the vitreous

When the pupil is small, light reaching the retina consists largely of paraxial rays; ie, rays passing through the
central portion of the cornea. However, when the pupil is large, rays passing through the peripheral
cornea come into play, and spherical aberration causes these rays to be focused more anteriorly, resulting
in a myopic component to the final image.

Object Image
point blur circle
 75

Aberrations: Spherical

(Remember, Poorly-focused
all these rays retinal blur circle
are from the

same point
on the object
at infinity.)
Note the myopic shift--
ie, peripheral rays are
focused in the vitreous

When the pupil is small, light reaching the retina consists largely of paraxial rays; ie, rays passing through the
central portion of the cornea. However, when the pupil is large, rays passing through the peripheral
cornea come into play, and spherical aberration causes these rays to be focused more anteriorly, resulting
in a myopic component to the final image. Spherical aberration is a factor in the phenomenon called night
myopia, in which pts complain of blurred vision brought on by dusk- and night-time illumination levels.

Object Image
point blur circle
 76

Aberrations: Spherical

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm (Why µm? Don’t ask.)

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 77

Aberrations: Spherical

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 78

Aberrations: Spherical
Why is the unit of spherical aberration microns--a unit of distance? What distance is being referred to?
It refers to the distance between the location where central rays for a focal point and where the
peripheral rays form a focal point

But as can be seen in the figure, the location of the focal point for rays passing through the corneal
periphery is a function of ‘how peripheral’ those rays are. Given this, how can one measure spherical
aberration?
By convention, rays passing through the cornea 6 mm from the optical axis are used

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 79

Aberrations: Spherical
Why is the unit of spherical aberration microns--a unit of distance? What distance is being referred to?
It refers to the distance between the location where central rays form a focal point and where the
peripheral rays form a focal point

But as can be seen in the figure, the location of the focal point for rays passing through the corneal
periphery is a function of ‘how peripheral’ those rays are. Given this, how can one measure spherical
aberration?
By convention, rays passing through the cornea 6 mm from the optical axis are used

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 80

Aberrations: Spherical
Why is the unit of spherical aberration microns--a unit of distance? What distance is being referred to?
It refers to the distance between the location where central rays form a focal point and where the
peripheral rays form a focal point

But as can be seen in the figure, the location of the focal point for rays passing through the corneal
periphery is a function of ‘how peripheral’ those rays are. Given this, how can one measure spherical
aberration?
By convention, rays passing through the cornea 6 mm from the optical axis are used

(All these rays

are from the
Optical same Optical
axis point axis
on the object ? ? ?
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 81

Aberrations: Spherical
Why is the unit of spherical aberration microns--a unit of distance? What distance is being referred to?
It refers to the distance between the location where central rays form a focal point and where the
peripheral rays form a focal point

But as can be seen in the figure, the location of the focal point for rays passing through the corneal
periphery is a function of ‘how peripheral’ those rays are. Given this, how can one measure spherical
aberration?
By convention, rays passing through the cornea 6 mm from the optical axis are used

(All these rays

are from the
Optical same 3 mm Optical
axis point axis
3 mm
on the object
at infinity.) 0.27 µm

(Drawing not to scale, obviously)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 82

Aberrations: Spherical

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 83

Aberrations: Spherical

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 84

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52

Why didn’t we evolve corneas with a Q factor of -0.52?

Well, know one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would require a radically different angle between the cornea and the sclera--an angle that could not be
achieved given the biomechanics and size of the normal human globe. Thus, a Q factor of -0.52 would
require a very radical ‘re-design’ of the globe--and thus of the orbits, and the cranium, and etc.

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 85

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52

Why didn’t we evolve corneas with a Q factor of -0.52?

Well, know one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would require a radically different angle between the cornea and the sclera--an angle that could not be
achieved given the biomechanics and size of the normal human globe. Thus, a Q factor of -0.52 would
require a very radical ‘re-design’ of the globe--and thus of the orbits, and the cranium, and etc.

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 86

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52

Why didn’t we evolve corneas with a Q factor of -0.52?

Well, know one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would require a radically different angle between the cornea and the sclera--an angle that could not be
achieved given the biomechanics and size of the normal human globe. Thus, a Q factor of -0.52 would
require a very radical ‘re-design’ of the globe--and thus of the orbits, and the cranium, and etc.

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 87

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52

Why didn’t we evolve corneas with a Q factor of -0.52?

Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would require a radically different angle between the cornea and the sclera--an angle that could not be
achieved given the biomechanics and size of the normal human globe. Thus, a Q factor of -0.52 would
require a very radical ‘re-design’ of the globe--and thus of the orbits, and the cranium, and etc.

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 88

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52

Why didn’t we evolve corneas with a Q factor of -0.52?

Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would require a radically different angle between the cornea and the sclera--an angle that could not be
achieved given the biomechanics and size of the normal human globe. Thus, a Q factor of -0.52 would
require a very radical ‘re-design’ of the globe--and thus of the orbits, and the cranium, and etc.

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 89

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52
eyes
Why didn’t we evolve corneas with a Q factor of -0.52? We did!
Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would requirewhile
Interestingly, a radically
we didn’t different
evolveangle
corneasbetween
with a the cornea
Q factor and the
of -0.52, we sclera--an
did evolve eyesanglewith
thatit.could not be
The human
achieved given the
lens of a young adultbiomechanics
has an average and
Q size
valueofofthe normal
about -0.25.human globe.
Thus, the Thus,
entire a Q factor
refracting of of
system -0.52 would
the average
young adult
require a veryhuman eye
radical has a totalofQthe
‘re-design’ factor very closethus
globe--and to -0.52,
of theand thusand
orbits, has the
littlecranium,
to no spherical
and etc.aberration!

(All these rays

are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 90

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52
eyes
Why didn’t we evolve corneas with a Q factor of -0.52? We did!
Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would requirewhile
Interestingly, a radically different
we didn’t evolveangle
corneasbetween
with a the cornea
Q factor and the
of -0.52, we sclera--an
did evolve eyesanglewith
thatit.could not be
The human
achieved given the
lens of a young adultbiomechanics
has an average and
Q size
valueofofthe normal
about -0.25.human globe.
Thus, the Thus,
entire a Q factor
refracting of of
system -0.52 would
the average
young adult
require a veryhuman
radicaleye has a totalofQthe
‘re-design’ factor very closethus
globe--and to -0.52,
of theand thusand
orbits, has the
littlecranium,
to no spherical
and etc.aberration!
‘Young adult’ seems to be emphasized, implying that the Q factor is not -0.25
in older adults. What happens to the Q factor of the lens as we age?
It becomes progressively less negative, ultimately reaching a value of 0
at about age 40
(All these rays
are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 91

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52
eyes
Why didn’t we evolve corneas with a Q factor of -0.52? We did!
Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would requirewhile
Interestingly, a radically different
we didn’t evolveangle
corneasbetween
with a the cornea
Q factor and the
of -0.52, we sclera--an
did evolve eyesanglewith
thatit.could not be
The human
achieved given the
lens of a young adultbiomechanics
has an average and
Q size
valueofofthe normal
about -0.25.human globe.
Thus, the Thus,
entire a Q factor
refracting of of
system -0.52 would
the average
young adult
require a veryhuman
radicaleye has a totalofQthe
‘re-design’ factor very closethus
globe--and to -0.52,
of theand thusand
orbits, has the
littlecranium,
to no spherical
and etc.aberration!
‘Young adult’ seems to be emphasized, implying that the Q factor is not -0.25
in older adults. What happens to the Q factor of the lens as we age?
It becomes progressively less v less negative, ultimately reaching a value of 0
more #
at about age 40 #
(All these rays
are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 92

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52
eyes
Why didn’t we evolve corneas with a Q factor of -0.52? We did!
Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would requirewhile
Interestingly, a radically different
we didn’t evolveangle
corneasbetween
with a the cornea
Q factor and the
of -0.52, we sclera--an
did evolve eyesanglewith
thatit.could not be
The human
achieved given the
lens of a young adultbiomechanics
has an average and
Q size
valueofofthe normal
about -0.25.human globe.
Thus, the Thus,
entire a Q factor
refracting of of
system -0.52 would
the average
young adult
require a veryhuman
radicaleye has a totalofQthe
‘re-design’ factor very closethus
globe--and to -0.52,
of theand thusand
orbits, has the
littlecranium,
to no spherical
and etc.aberration!
‘Young adult’ seems to be emphasized, implying that the Q factor is not -0.25
in older adults. What happens to the Q factor of the lens as we age?
It becomes progressively less negative, ultimately reaching a value of 0
at about age 40
(All these rays
are from the
Optical same Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 93

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52
eyes
Why didn’t we evolve corneas with a Q factor of -0.52? We did!
Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would requirewhile
Interestingly, a radically different
we didn’t evolveangle
corneasbetween
with a the cornea
Q factor and the
of -0.52, we sclera--an
did evolve eyesanglewith
thatit.could not be
The human
achieved given the
lens of a young adultbiomechanics
has an average and
Q size
valueofofthe normal
about -0.25.human globe.
Thus, the Thus,
entire a Q factor
refracting of of
system -0.52 would
the average
young adult
require a veryhuman
radicaleye has a totalofQthe
‘re-design’ factor very closethus
globe--and to -0.52,
of theand thusand
orbits, has the
littlecranium,
to no spherical
and etc.aberration!
‘Young adult’ seems to be emphasized, implying that the Q factor is not -0.25
in older adults. What happens to the Q factor of the lens as we age?
It becomes progressively less negative, ultimately reaching a value of 0
at about age 40
(All these rays
are from the What characteristic of older eyes—quite frustrating for anyone attempting to
Optical same retinoscope them—serves to offset the spherical-aberration-inducing loss of Optical
axis point negative Q factor in the lens? axis
on the object The pupil tends to be miotic, which blocks the more peripheral rays
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 94

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52
eyes
Why didn’t we evolve corneas with a Q factor of -0.52? We did!
Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would requirewhile
Interestingly, a radically different
we didn’t evolveangle
corneasbetween
with a the cornea
Q factor and the
of -0.52, we sclera--an
did evolve eyesanglewith
thatit.could not be
The human
achieved given the
lens of a young adultbiomechanics
has an average and
Q size
valueofofthe normal
about -0.25.human globe.
Thus, the Thus,
entire a Q factor
refracting of of
system -0.52 would
the average
young adult
require a veryhuman
radicaleye has a totalofQthe
‘re-design’ factor very closethus
globe--and to -0.52,
of theand thusand
orbits, has the
littlecranium,
to no spherical
and etc.aberration!
‘Young adult’ seems to be emphasized, implying that the Q factor is not -0.25
in older adults. What happens to the Q factor of the lens as we age?
It becomes progressively less negative, ultimately reaching a value of 0
at about age 40
(All these rays
are from the What characteristic of older eyes—quite frustrating for anyone attempting to
Optical same retinoscope them—serves to offset the spherical-aberration-inducing loss of Optical
axis point negative Q factor in the lens? axis
on the object The pupil tends to be miotic, which blocks the more peripheral rays
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 95

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52

Why didn’t we evolve corneas with a Q factor of -0.52?

Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would require a radically different angle between the cornea and the sclera--an angle that could not be
achieved given the biomechanics and size of the normal human globe. Thus, a Q factor of -0.52 would
require a very radical ‘re-design’ of the globe--and thus of the orbits, and the cranium, and etc.

(All So,
these average cornea has a spherical aberration of +0.27 µm and a Q factor of -0.26.
therays
areSurely it’s not a coincidence that these numbers almost perfectly cancel one another out?
from the
Optical same that’s exactly what it is--a coincidence
I’m afraid Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 96

Spherical Aberration
Recall that the cornea’s Q factor is -0.26. What would it be if the cornea had no spherical aberration?
About -0.52

Why didn’t we evolve corneas with a Q factor of -0.52?

Well, no one can say for sure of course. But what can be said with certainty is that a Q factor of -0.52
would require a radically different angle between the cornea and the sclera--an angle that could not be
achieved given the biomechanics and size of the normal human globe. Thus, a Q factor of -0.52 would
require a very radical ‘re-design’ of the globe--and thus of the orbits, and the cranium, and etc.

(All So,
these average cornea has a spherical aberration of +0.27 µm and a Q factor of -0.26.
therays
areSurely it’s not a coincidence that these numbers almost perfectly cancel one another out?
from the
Optical same that’s exactly what it is--a coincidence
I’m afraid Optical
axis point axis
on the object
at infinity.)

How much spherical aberration does the average human cornea possess?
About +0.27 µm

So this means the cornea possesses positive spherical aberration. But the cornea’s Q factor is negative.
What gives?
The Q factor measures the relative asphericity of the cornea. A negative Q factor simply means the
corneal periphery has less power than the central cornea; it does not mean the cornea as a whole doesn’t
have spherical aberration!
 97

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 98

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
 99

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
 100

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
In other words: Any wavefront, no matter how complex its shape,
can be ‘broken down’ into a set of Zernike shapes.
 101

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
 The set of shapes starts off very simple/basic,
becoming progressively more complex as the
series proceeds
 102

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
 The set of shapes starts off very simple/basic,
becoming progressively more complex as the
series proceeds
 The progression is described by the order of a given
shape
 103

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
 The set of shapes starts off very simple/basic,
becoming progressively more complex as the
series proceeds
 The progression is described by the order of a given
shape
 104

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
 The set of shapes starts off very simple/basic,
becoming progressively more complex as the
series proceeds
 The progression is described by the order of a given
shape
 Order start at zero,
# and goes up from there
 105

Aberrations: Zernike Polynomials

 A mathematical system for describing and

systematizing optical aberrations
 A series of shapes; when combined, they can
account for the overall contour of a wavefront
 The set of shapes starts off very simple/basic,
becoming progressively more complex as the
series proceeds
 The progression is described by the order of a given
shape
 Order start at zero, and goes up from there
 106

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape

2nd Defocus
Positive defocus
Negative defocus

2nd Cylinder

Spherical
Intentionally out of order!
While coma and trefoil
4th aberration
are of lower-order than
spherical aberration, SA
is clinically more 3rd Coma
significant.

3rd Trefoil
(Others, less
clinically relevant)
 107

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ?
1st ?
Wait--you said ZPs start at
2nd Defocus
zero and go up from there. Positive defocus
What are the 0th and 1st-
Negative defocus
order aberrations?

2nd Cylinder

Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
 108

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
Wait--you said ZPs start at
2nd Defocus
zero and go up from there. Positive defocus
What are the 0th and 1st-
Negative defocus
order aberrations?
‘Piston’ and ‘prism’
(aka tip and tilt)
2nd Cylinder

Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
 109

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
Wait--you said ZPs start at
2nd Defocus
zero and go up from there. Positive defocus
What are the 0th and 1st-
Negative defocus
order aberrations?
‘Piston’ and ‘prism’
(aka tip and tilt)
2nd Cylinder
Why haven’t we talked about piston and prism?
Because while they are technically aberrations
in the ZP system, they do not degrade the
quality of the visual image, and are thus
Spherical
clinically irrelevant
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
 110

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
Wait--you said ZPs start at
2nd Defocus
zero and go up from there. Positive defocus
What are the 0th and 1st-
Negative defocus
order aberrations?
‘Piston’ and ‘prism’
(aka tip and tilt)
2nd Cylinder
Why haven’t we talked about piston and prism?
Because while they are technically aberrations
in the ZP system, they do not degrade the
quality of the visual image and are thus
Spherical
clinically irrelevant
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
 111

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

2nd Cylinder
(positive)
‘Bowl’

Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
 112

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

2nd Cylinder

Spherical ‘Saddle’
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
 113

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

2nd Cylinder

Spherical
4th aberration
3rd Coma
3rd Trefoil ‘Bundt cake pan’

(Others, less
clinically relevant)
 114

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

2nd Cylinder

Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
‘Recliner’
 115

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

In layman’s terms, what is ndthe problem with the incoming lightCylinder

that leads to the higher-order aberration of coma?
2
Coma occurs when the source of the rays is located off the optical axis. Because of its location, light from
this source reaches one side of the pupil before the other. The result is that rays entering the ‘near’ side and the
‘far’ side of the pupil are focused not at as a single point, but rather as a point with a ‘smear’ attached (not unlike
a comet’s tail, which is why the words share a root).
Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
‘Recliner’
 116

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

In layman’s terms, what is ndthe problem with the incoming lightCylinder

that leads to the higher-order aberration of coma?
2
Coma occurs when the source of the rays is located off the optical axis. Because of its location, light from
this source reaches one side of the pupil before the other. The result is that rays entering the ‘near’ side and the
‘far’ side of the pupil are focused not at as a single point, but rather as a point with a ‘smear’ attached (not unlike
a comet’s tail, which is why the words share a root).
Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
‘Recliner’
 117

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

In layman’s terms, what is ndthe problem with the incoming lightCylinder

that leads to the higher-order aberration of coma?
2
Coma occurs when the source of the rays is located off the optical axis. Because of its location, light from
this source reaches one side of the pupil before the other. The result is that rays entering the ‘near’ side and the
‘far’ side of the pupil are focused not at as a single point, but rather as a point with a ‘smear’ attached (not unlike
a comet’s tail, which is why the words share a root).
Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
‘Recliner’
 118

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

2nd Cylinder

Spherical
4th aberration
3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
‘Three peaks’
 119

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

2nd Cylinder
In layman’s terms, what is the problem with the incoming light that leads to trefoil?
Happily, the BCSC books do not spend much time on trefoil, so you don’t need to know
much more about it than:
1) it is Spherical
a clinically significant (albeit modestly so) higher-order aberration; and
4 th
2) its shape, ie, be able to recognize its aberration
wavefront analysis profile (more on this later).

3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
‘Three peaks’
 120

Aberrations: Zernike Polynomials

Zernike Polynomial Order New Lingo Shape
0th ‘Piston’
1st ‘Prism’
2nd Defocus
Myopia = Positive defocus
Hyperopia = Negative defocus

2nd Cylinder
In layman’s terms, what is the problem with the incoming light that leads to trefoil?
Happily, the BCSC books do not spend much time on trefoil, so you don’t need to know
much more about it than:
1) it is Spherical
a clinically significant (albeit modestly so) higher-order aberration; and
4 th
2) its shape, ie, be able to recognize its aberration
wavefront analysis profile (more on this later).

3rd Coma
3rd Trefoil
(Others, less
clinically relevant)
‘Three peaks’
 121

Aberrations: Zernike Polynomials

0th

1st

2nd

3rd

4th

Spherical
aberration
In addition to the 3-D
3-D representation representation of each shape… 3-D representation
You need to be able to
recognize its 2-D image as well!
 122

Aberrations: Zernike Polynomials

0th

1st

2nd

3rd

4th

Spherical
aberration
In addition to the 3-D
2-D representation representation of each shape… 3-D representation
You need to be able to
recognize its 2-D image as well!
 123

Aberrations: Zernike Polynomials

0th

1st

2nd

3rd

4th

Spherical
aberration
And in addition to the 2- and 3-D
2-D representation representation of each shape… 3-D representation
 124

Aberrations: Zernike Polynomials

0th (As mentioned previously,
note that piston, tip and tilt
do not degrade the quality
of the image)

1st

2nd

3rd

(Note that coma looks like a comet)

4th

Spherical
aberration
And in addition to the 2- and 3-D
Optical effect
2-D representation representation of each shape…
You need to be able to recognize its of each
optical impact on an image-point
 125

Aberrations
 Wavefront-guided keratorefractive surgery
two-words

did away with the second problem

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 126

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 127

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
aberrations identified via wavefront analysis

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 128

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
aberrations identified via wavefront analysis
 That said, precisely which higher-order aberrations
should be corrected (and to what degree) is an
unsettled issue at this time

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 129

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.
That said, precisely which higher-order aberrations
In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
address
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.

unsettled issue at this time

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 130

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.
That said, precisely which higher-order aberrations
In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
address
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.

unsettled issue at this time

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 131

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.
That said, precisely which higher-order aberrations
In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
address
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.

unsettled issue at this time

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 132

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.
That said, precisely which higher-order aberrations
In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
address
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.
How does a wavefront-optimized ablative procedure differ from a so-called conventional ablative procedure?
unsettled
In a conventional issue
procedure, the ablation at this time
is determined solely by a standard phoropter-based refraction
obtained by the surgeon during pre-op. That is, the phoropter-based refraction is used to program the
correction of sphere and cyl. In a wavefront-optimized ablation, the wavefront analysis is used to program the
correxction of sphere and cyl.

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 133

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.
That said, precisely which higher-order aberrations
In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
address
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.
How does a wavefront-optimized ablative procedure differ from a so-called conventional ablative procedure?
unsettled
In a conventional issue
procedure, the ablation at this time
is determined solely by a standard phoropter-based refraction
obtained by the surgeon during pre-op. That is, the phoropter-based refraction is used to program the
correction of sphere and cyl. In a wavefront-optimized ablation, the wavefront analysis is used to program the
correction of sphere and cyl.

Essentially, irregular astigmatism was a wastebasket term for aberrations that:

1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 134

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.
That said, precisely which higher-order aberrations
In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
address
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.
How does a wavefront-optimized ablative procedure differ from a so-called conventional ablative procedure?
unsettled
In a conventional issue
procedure, the ablation at this time
is determined solely by a standard phoropter-based refraction
obtained by the surgeon during pre-op. That is, the phoropter-based refraction is used to program the
correction of sphere and cyl. In a wavefront-optimized ablation, the wavefront analysis is used to program the
correction of sphere and cyl.

In addition to wavefront-guided, wavefront-optimized and conventional approaches to ablation, there is

one more. What is it?
Topography-guided. For details on this and the other three approaches, see the slide set on
Photoablative Refractive Surgery.
Essentially, irregular astigmatism was a wastebasket term for aberrations that:
1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 135

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.
That said, precisely which higher-order aberrations
In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
address
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.
How does a wavefront-optimized ablative procedure differ from a so-called conventional ablative procedure?
unsettled
In a conventional issue
procedure, the ablation at this time
is determined solely by a standard phoropter-based refraction
obtained by the surgeon during pre-op. That is, the phoropter-based refraction is used to program the
correction of sphere and cyl. In a wavefront-optimized ablation, the wavefront analysis is used to program the
correction of sphere and cyl.

In addition to wavefront-guided, wavefront-optimized and conventional approaches to ablation, there is

one more. What is it?
Topography-guided. For details on this and the other three approaches, see the slide set on
Photoablative Refractive Surgery.
Essentially, irregular astigmatism was a wastebasket term for aberrations that:
1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable
 136

Aberrations
 Wavefront-guided keratorefractive surgery
did away with the second problem
 Allows surgeons to correct/minimize the higher-order
How does a wavefront-guided ablative procedure differ from a wavefront-optimized ablative procedure?
In a wavefront-guided procedure, the information obtained from wavefront analysis is used to correct
aberrations identified via wavefront analysis
certain higher-order aberrations along with the more-important lower-order (ie, sphere and cyl)
aberrations.

address That said, precisely which higher-order aberrations

In contrast, a wavefront-optimized procedure corrects only sphere and cylinder; no attempt is made to
 higher-order aberrations. Instead, the wavefront information is used to ‘fine tune’ the ablation in
So, there are four basic techniques for
should be corrected (and to what degree) is an
such a way as to minimize the creation or exacerbation of higher-order aberrations.
performing keratoablative refractive surgery
How does a wavefront-optimized ablative procedure differ from a so-called conventional ablative procedure?
unsettled
In a conventional issue
procedure, the ablation at this time
is determined solely by a standard phoropter-based refraction
obtained by the surgeon during pre-op. That is, the phoropter-based refraction is used to program the
correction of sphere and cyl. In a wavefront-optimized ablation, the wavefront analysis is used to program the
correction of sphere and cyl.

In addition to wavefront-guided, wavefront-optimized and conventional approaches to ablation, there is

one more. What is it?
Topography-guided. For details on this and the other three approaches, see the slide set on
Photoablative Refractive Surgery.
Essentially, irregular astigmatism was a wastebasket term for aberrations that:
1) could not be measured in the clinic; and
2) could not be corrected even if they had been measureable