This document discusses the process of determining and correcting refractive errors through objective and subjective refraction methods. It describes retinoscopy and trial frame techniques used in subjective refraction to find the optimal lens prescription. Specifically, it details how Jackson's cross cylinder and fogging techniques are used to refine the cylindrical correction and verify the accuracy of the prescription before the final step of binocular balancing for the patient.

1. Dr. Atul Kumar Anand
Senior Resident
AIIMS Patna

2. Procedure of determining & correcting
refractive error is refraction
Two methods:-
Objective : retinoscopy, refractometry,
keratometry
Subjective

3. Also KA skiascopy or shadow test
To find out error of refraction by method of
neutralization
Objective method

4. Dark room : 6 m long
Trial box & frame
Vision box
Retinoscope:- two types
 mirror retinoscope ( single plane mirror or
pristley-smith mirror –plane & concave )
Self illuminated retinoscope:- spot or streak
Concave mirror useful in hazy media & high
degree of ametropia.

7. 2 types:-
Dry retinoscopy ( without cyclopegics)
Wet retinoscopy
Done at 1 m distance usually

10. With plane mirror at 1 m, depending on
movement of red reflex, results are
No movement of red reflex indicate myopia
of 1D
With movement of red reflex indicate
myopia < 1D or emmetropia or hyper
Against movement of red reflex indicate
myopia > 1D

12. Movement of red reflex neutralized by
addition of increasing convex ( + )
spherical lens ( when red reflex move with
movement of retinoscope ) or concave (-)
spherical lens (when refex move against )

14. Deduction for distance 1D for 1 m or 1.5 D
for 2/3 m
Deduction for cycloplegic when used ( 1D for
atropine, 0.5D for homatropine or 0.75 D for
cyclopetolate )
Refractive error = retinoscopic finding –
distance - cycloplegic

17. For prescription of most suitable lens
Done after objective refraction ( retino or
AR )
When cycloplegic used, done after 3-4
days ( for homide or pentol), 3 wk for
topine :- PMT

18. Done by trial & error method (after retino
or AR) with trial box & frame, separately for
each eye
Most suitable lens prescribed
Strongest convex lens for hypermetropia
Weakest concave lens for myopia

19. Most suitable lens after subj. verification is
refined before final prescription
1st refine cylinder then sphere usually
Refining the cylinder: - by jackson’s cross
cylinder or astigmatic fan
Jackson’s cross cylinder: - combination of 2
cylinders of equal strength but oppst. Sign
placed with their axes at rt. Angle to each
other
Used to verify both strength & axis of cyld.

20.  Commonly used cross
cylinders are of +-
0.25D &
+-0.5D

21. To check power of cyld. , cross cyld. Of +-
0.25D is placed with its axis parallel to axis of
cylinder in trial frame 1st with same sign then
with oppst. Sign.
In 1st position, cylindrical correction is
enhanced by 0.25D & in 2nd diminished by
same amount.
When visual improvement does not occur in
either position, power of cyld. in trial frame
correct

22.  Cross cyld (+-0.5D)placed before eye with its axis at
45 degree to axis of cross cyld in trial frame (1st with
-0.5D cyld & then +0.5D cyld or vice versa)
 If pts. notices no difference between 2 positions,
axis of correcting cyld in trial frame is correct .
However if visual improvement is attained in one of
position, a ‘plus’ correcting cyld should be rotated in
direction of plus cyld component of cross cyld & vice
versa.
 Test repeated several times until neutral point
reached

23. Pin hole test:- improvement in VA while
looking through a pin hole indicates that
optical correction in trial frame is incorrect
Fogging technique

24. Final step
Allows both eyes to have retinal image
simultaneously in focus
Aniseikonia of 5 to 7 % (anisometropia of
2.5D to 3.5D) is tolerable theoretically

25. Done after 40 yr age
Better to under correct the presbyopia