Course: clinical examination
Subjective
Refraction
Kiran Alvi
Learning objectives:
Subjective Verification
Balancing of Accommodation
Tests of Subjective refraction
• To refine the prescription obtain from
objective refraction, with the help of lens and
patient involvement is necessary.
Principle of subjective refraction
1. Accommodation-relaxed state
2. Maximum PLUS, minimum minus
3. Always trial frame before prescribing
4. Take into account vertex distance
especially for high prescription individuals
duochrome or bichromatic test
• Principle : it is based on chromatic
aberration
• Indication:
to access the type of refractive error
To find out either patient is under corrected or
over corrected
Duechrome
Procedure
• Occlude one eye, put prescription(obtain from
objective refraction) in trial frame
• Ask the patient: “which letters/dots are
clearer red or green, or are they the
same?"
• If patients appreciate both colors equally,
this suggests the best vision sphere has been
obtained and the circle of least confusion is on
the retina, pt is emmetrope.
For myopes
• If myopic patient appreciate red colour then
he is still myopic. (he is undercorrected)
• You have to increase power of lens
• If myopic patient appreciate green colur then
he became hyperopic. (he is overcorrected)
• Decrease power of lens.
For hyperopes
• If hyperopic patient appreciate green colour
then he is still hyperopic. (he is
undercorrected)
• You have to increase power of lens
• If hyperopic patient appreciate red colur then
he became myopic. (he is overcorrected)
• Decrease power of lenses.
Best sphere test
• Principle: fogging and relax
• Indication:
to access the type of refractive error
To find out either patient is under corrected or
over corrected
Procedure
• Use 2 sphere having power of +0.25D and
-0.25D
• Occlude one eye and instruct pt to look at
visual acuity chart, insert both lenses +0.25D
and -0.25D one by one in trial frame.
• Ask pt on which lens chart become blur, if
with both lenses chart become blur then pt
become emmetrope with correction (give
same nmber which inserted in trial frame).
For myopes
• If myopic patient appreciate -0.25D(chart clear
with -0.25d) then he is still myopic. (he is
undercorrected)
• You have to increase power of lens
• If myopic patient appreciate +0.25D then he
became hyperopic. (he is overcorrected)
• Decrease power of lens.
• Note: increase or decrease power until with
both lenses chart blur.
Hyperopia
• If hyperopic patient appreciate +0.25D then
he is still hyperopic. (he is undercorrected)
• You have to increase power of lens
• If hyperopic patient appreciate -0.25D then he
became myopic. (he is overcorrected)
• Decrease power of lenses.
• Note: increase or decrease power until with
both lenses chart blur.
+1 Blur test
• Principle: base on fogging method
• Indication:
to access the type of refractive error
To find out either patient is under corrected or
over corrected
Procedure
• Each snellen line has value of +0.25D
• With +1D lower 4lines of chart will blur.
• If patients said that all four lines are blur then
we prescribe same number of glasses
obtained from objective refraction.
For hyperopes
• If less then 4 lines are blur then pt is
undercorrected
• If more then 4lines are blur then pt is
overcorrected
For myopes
• If less then 4 lines are blur then pt is
overcorrected
• If more then 4lines are blur then pt is
undercorrected.
Refining of cylinder
• It is always better to first refine the cylinder
• and then sphere
• Cylinder can be refined by using Jackson's
• cross cylinder
• The cross cylinder is a combination of two
cylinders of equal strength but with opposite
signs placed with their axis at right angle to
each other and mounted in a handle
• The commonly used cross cylinders are of
• ＋－ 0.25 and ＋－ 0.50
Uses
• To find out either astigmatism is present or
not
• To refine axis of cylinder
• To find power of cylinder
• Any question?
Learning outcomes:
Solved Problems related to subjective refraction
Discussed tests of binocular balancing
Learnt equalization of Accommodation
References:
Clinical Practice of Optics by A.K Khurana
Clinical Optics by AR Elkington
Internet
• Thanks
