OPTICS OF

AMETROPIA






By Vithiaa
• Ametropia is a state where refractive error is
present, or when distant points are not focused
properly to the retina.
• Means that the parallel rays of light coming from
infinity are focused either in front or behind the
retina which receives a blurred image.
It includes-
 Hypermetropia

 Myopia

 Astigmatism

 Aphakia
EMMETROPIA
Emmetropia is a state of refraction where an object at infinity is
focused on retina with the eye lens in neutral & relaxed state.
 Long sightedness
Images are focussed behind retina
ie. Posterior focal point is behind retina

Mechanisms-

1. AXIAL: 1mm shortening of AP diameter of eye causes 3D of hypermetropia.

2. CURVATURAL:curvature of cornea or lens or both is f latter than normal. 1mm
decrease in radius of curvature causes 6D of hypermetropia.

3. INDEX: due to change in refractive index of lens in old age and in diabetics.

4. POSITIONAL: posteriorly placed lens.

5. ABSENCE OF CRYSTALLINE LENS.

CLINICAL TYPES :

1) SIMPLE HYPERMETROPIA: due to biological variation in size and shape of eyeball.

Could be axial or curvatural.

2) PATHOLOGICAL :

A) CONGENITAL: Associated with microphthalmos, micro cornea, congenital posterior

subluxation of lens or congenital aphakia.

B) ACQUIRED:
i. SENILE: could be curvatural or index (due to cortical sclerosis)
ii. POSITIONAL : subluxation of lens
iii. APHAKIA
iv. CONSECUTIVE:surgicallyovercorrectedmyopia orpseudophakia withunder
correction
v. ORBITALMASS:tumors or edema may push the retina forward
C) FUNCTIONAL : due to paralysis of accommodation as in 3rd nerve palsy
COMPONENTS OF HYPERMETROPIA:
FACULTATIVE
MANIFEST

TOTAL ABSOLUTE

LATENT

Total - after complete cycloplegia with atropine.

Latent - about 1D of hypermetropia that is masked by
inherent ciliary tone & involuntary accommodation
Manifest - strongest convex lens correction
Facultative - corrected by patient’s accommodative effort.
Absolute - hypermetropia in excess of amplitude of accomodation
 Short sightedness.
Parallel rays coming from inf inity focus in front of
retina with accommodation at rest.

MECHANISMS:
MYOPIA

1. AXIAL: increased axial length of the eye ball.

2. Commonest form. Eg Staphyloma
3. CURVATURAL: increased curvature of cornea or lens
4. or both.
5. INDEX/REFRACTIVE: increased refractive index of lens with
nuclear sclerosis, keratoconus (cornea)
6. EXCESSIVE ACCOMMODATION: in spasm of
accommodation
1 . Far point of the myopic eye is at a f inite point in front of the eye.
2 . Nodal point of the eye is further away from the retina.
Hence the image of the object formed is larger than that of the
emmetropic eye or spectacle corrected eye. This compensates for
visual acuity to some extent.
3 . They do not need to accommodate. Hence it is not well developed
and they may suffer from convergence insuf ficiency, exophoria or early
presbyopia.
 Astigmatism is a refractive error in which the refraction varies in different meridians.
ASTIGMATISM
When refractive power of cornea & lens are not the same in all meridians, instead of single
focal point there are two focal points separated by focal interval which is called Sturm’s
conoid.
ASTIGMATISM ETIOLOGY
 CORNEAL : due to abnormalities in the curvature of cornea.
Congenital or acquired.

 LENTICULAR
- CURVATURAL : due to abnormal curvature of lens.
Eg Lenticonus (anterior or posterior)
- POSITIONAL : due to oblique placement or tilting of lens.
Eg Lens subluxation
- INDEX : due to difference of refractive index in different
meridians

 RETINAL : due to oblique placement of macula

ASTIGMATISM • Regular astigmatism – principal meridians are perpendicular.

With-the-rule astigmatism – the vertical meridian is steepest

Against-the-rule astigmatism – the horizontal meridian is steepest.

• Oblique astigmatism – the steepest curve lies in between

120 and 150 degrees and 30 and 60 degrees.

• Irregular astigmatism – principal meridians are not perpendicular. Cannot be

corrected by spectacles
 Classification

Simple astigmatism
 Simple myopic astigmatism – f irst focal line isin front of the retina, while the second is
ASTIGMATISM
on the retina.
 Simple hyperopic astigmatism – f irst focal line is on retina, while the second islocated
behind the retina.

Compound astigmatism

 Compound myopic astigmatism – both focal lines are located in front of the retina.

 Compound hyperopic astigmatism – both focallinesare located behind the retina.

Mixed astigmatism

Focal lines on both sides of the retina (straddling the retina)

 Condition in which there is different refraction of two eyes
ANISOMETROPIA
Disparity of 1D in hypermetropic individual can cause amblyopia of
the more hypermetropic eye.

In myopic individual disparity of 2D cause amblyopia. Can be

corrected with glasses
 Method used to determine if reduced visual acuity is due to
refractive error or due to ocular/neurological pathology.
PIN HOLE TEST

No improvement in VA if there is ocular pathology.

In macular disease the pin hole acuity may be worse than unaided
acuity

In high degree of ametropia (range outside +4 to -4),

retinal image is too diffuse hence difficult to achieve the
improvement.
STENOPAEIC SLIT
Used to determine refraction & principal
axes in astigmatism.

Acts as elongated pin-hole – allow light

to enter in that axis.
 Far point of the eye is the position of an object
such that its image falls on
the retina of the relaxed eye in
absence of accommodation.
FAR POINT

For emmetropia it is atinf inity.

For myopia it lies at a f inite distance in front
of the eye.
In hypermetropia it is virtual (as only
converging light can focus on the retina)
 The purpose of optical correction is to deviate the rays of light so
OPTICAL CORRECTION OF
that they appear to come from the far point.
AMETROPIA
 In uncorrected hypermetropia the image of an object falls behind the
retina
EFFECTIVE POWEROF LENSES

The purpose of convex lens is to bring theimage forward.

If the correcting lens is itself moved forward the image will
move further forward.
Eg – the effectivity of the lens is increased

Thus a weaker lens is required to project the image on the retina.

 Similarly in uncorrected myopia the image falls in front of the
retina.
EFFECTIVE POWEROF LENSES

The purpose of the concave lens is to bring the image behind.

If the correcting lens is moved further away from the eye, image
moves forward again. Eg – the effectivity of the lens is reduced.

Thus a stronger lens is required to project the image onto

retina
 The convex lens in hypermetropia hasto be
made weaker and the concave lens in myopia
EFFECTIVE POWEROF LENSES

hasto be made stronger when the lens ismoved

further away from the eye

Hence aphakics or high hyperopes pull their

glasses down their nose to read.

While myopes do not like their glassesslipping

down.
 FORMULA TO CALCULATE THE NEW FOCAL LENGTH OF LENS AT THE NEW DISTANCE-
EFFECTIVE POWEROF LENSES

F2= 1/ f1- d or F2= F1/ 1- dF1

Whereby,
F1= power of the original lens in diopters
F2= power of lens in diopters at new position
f1= focal length in meters of original lens
d= distance moved in meters. It is taken positive if moved toward the eye
and negative if moved away from the eye.
 For any prescription greater than 5D especially in aphakia
refractionist must state how far in front of the eye the trial
frame was placed, so that can adjust the power of contact
lens is used or if the glasses are to be worn at a different
distance. ics the
BACK VERTEXDISTANCE

The distance between the back of the lens and the cornea.
Measurement can be made with a ruler held parallel to the
arm of the trial frame or slipped through a steanopic slit till it
touches the closed lid. 2mm should be added to correct for
the thickness of the lid.
 Example 1:
A patient has been prescribed glasses with +16.00D sphere at a
BVD of 14mm. He selects a frame that f its him at a BVD of 16mm.
What is the power of the new lens?
BACK VERTEXDISTANCE
Answer : +15.50D

F2= F1/ 1- dF1

= 16/1-(-0.002x16)
= +15.50
 Example 2:
A aphakic patient requires a +10.00D lens at BVD 15mm. He now
wants a contact lens. What should be the power of the contact lens?
BACK VERTEXDISTANCE
Answer : +11.75D

F2= F1/ 1- dF1

= 10/1-(+0.015x10)
= +11.75
 Example 3:
A patient was given a prescription of -16.00D at a BVD of 14mm.
BACK VERTEXDISTANCE

He selects a spectacle frame of BVD 16mm. What will be the

power of the new lens?
Answer: -16.50D

F2= F1/ 1- dF1

= -16/1-(-0.002x -16)
= -16.50
 - The optical correction of ametropia is associated with a change in the
retinal image size

-The ratio between the corrected and uncorrected image size is known as
SPECTACLE MAGNIFICATION

spectacle magnif ication

Spectacle magnif ication = corrected image size

uncorrected image size

Relative spectacle magnif ication = corrected image size (RSM)

emmetropic image size
 In axial ametropia, if the correcting lens is placed at the anterior focal point of the
eye then the image size is same as in emmetropia. RSM is unity.

In axial myopia, if the correcting lens is worn nearer to eye than the anterior focal
SPECTACLE MAGNIFICATION

point, the image is increased. RSM has greater unity.

Eg : contact lens in axial myopia has magnifying effect.

In contrast to axial ametropia, image size in refractive ametropia differs even if lens is
placed at the anterior focal point.

In refractive hypermetropia the image size is increased. RSM>1

While in refractive myopia the image size is reduced. RSM<1

 In refractive ametropia, if the correcting lens is worn nearer to eye than anterior
focal point, the image size approaches the emmetropic size. RSM is unity.
 RSM = 1.36 for apakia (refractive hypermetropia) with lens at anterior focal
point Eg 23.2mm from principle plane.

RSM= 1.33 for aphakia with lens placed at 12-15mm

SPECTACLE MAGNIFICATION

RSM= 1.1 for contact lenses spectacles

contact lens
normal
 1. SPECTACLE MAGNIFICATION:
OPTICAL PROBLEMS IN APHAKIA

The spectacle magnif ication produced by aphakic

glasses is 1.33. Thus the image is one third times
larger than emmetropes.

The patient thus tends to misjudge distances.

Objects appear closer to the eye than they are. Leads
to enhanced performance in visual acuity tests.
 2. DISTORTION OF IMAGES DUE TO
ABBERRATIONS
OPTICAL PROBLEMS IN APHAKIA

Straight lines appear curved except through a small central

portion of the lens.
At the periphery of the lens the lines appear to be more
curved- pincushion effect.
Thus the environment appears ascurves as the patient
moves his eyes across different parts of the lens. Patients
adapt to this by moving their head rather than eyes.
 3) PRISMATIC EFFECTOF LENS:
The prismatic effect increases towards the
OPTICAL PROBLEMS IN APHAKIA

periphery of the lens.

It produces a troublesome ring scotoma at the
edge of the lens. Hence they can trip over unseen
objects.
The direction of the ring scotoma changes and
objects disappear into the scotoma and appear to
reappear out of it- jack in the box phenomenon.
 4) DUE TO WEIGHT OF THE GLASSES:
OPTICAL PROBLEMS IN APHAKIA

Aphakic glasses are very heavy and tend to slip down the nose.
Plastic glasses are lighter but lessscratch resistant.
Lenticular form of lenses reduce weight but also reduce f ield of vision.
 5) UNILATERALAPHAKIAWITH NORMAL EYE
OPTICAL PROBLEMS IN APHAKIA

The image in aphakic eye is one third larger hence causesaniseikonia.

Patient is unable to fuse the images of unequal size hence suffers from
diplopia.
The use of contact lenses and intra ocular implants reduce this effect.
Aniseikonic glasses though available are very heavy and costly.
QUESTION
The following are true about myopia :
a. Second principal focus lies behind retina

b. The presence of posterior staphyloma suggest axial myopia

c. Axial myopia may be caused by the cornea having too strong

refractive power
d. Nucleosclerosis is a cause of index myopia

e. High myopia may be treated with clear lens extraction

The following are true about myopia :
a. Second principal focus lies behind retina F

b. The presence of posterior staphyloma suggest axial myopia T

c. Axial myopia may be caused by the cornea having too strong

refractive power F
d. Nucleosclerosis is a cause of index myopia T

e. High myopia may be treated with clear lens extraction T

Reference :
Clinical Optics, 3rd Edition, Elkington

THANK YOU