This document provides guidance on properly measuring and prescribing glasses prescriptions. It emphasizes the importance of accurately measuring the pupilary distance (IPD) to avoid issues like decentration, tilt and induced astigmatism. It also discusses evaluating and correcting for accommodation, prescribing readers and bifocals, managing anisometropia and presbyopia, and using glasses to treat strabismus and amblyopia. Key points are measuring IPD before trials, testing reading function, and advising patients on minimizing decentration through small frames.

1. ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬
Nel nome di Dio, il
Compassionevole, il
Misericordioso

2. Glasses prescription
· Mohamed Elkadim MD
· Tanta University
· mzekadem@gmail.com

3. Why its important ?
· Its easy job with long term patient comfort.
· Glasses became expensive $$$
· Decision for refractive surgery/
keratoconus depend on the glasses
prescription.

4. Commonly not done well , good
glasses became achievement

6. IPD
· Should be measured before trial
· * Autoref
· * Ruler
· * Frame √√

7. Facial asymmetry/ squint

9. Centralize the cross on the pupil
center of both eyes

10. Wrong IPD = Decentration + tilt
· Decantation = Prism
· Tilt = Cylinder

11. Prism
Prism = Lens power x decentration (cm)
Ex :
Lens -10 ,, decentration 1 mm
Prism = 10 x 0.1 = 1 prism diopter
i.e. image at 1 meter distance will be
displaced 1 cm
The more power the lens the more risky is IPD change.

12. prism

13. Tilt
· Induce astigmatism at axis of tilt (180)
· This astigmatism + original
astigmatism of the lens = new
astigmatism with new axis
· Patient will prefer axis 180

14. Tilt induced astigmatism

15. Astigmatism at 180 least affected by
IPD / axis 90 max effect

16. When we change the IPD ?
· To add low power prism e.g. CI
· * Old age (reading glasses)
· * young age (latent XT)
· + lens = - IPD
· - Lens = + IPD Provided that >> patient
is not high error or astigmatism

17. Decentered minus lens out lead to base
in prism = assist convergence

18. IPD in reading glasses
· IPD in reading is 2-3 mm less than far.
· In myope no need to change IPD
· Test reading in single glasses for young

19. Testing the reading for far glasses
is important / stress test
· Check accomodation is OK
· Convergence is OK
· IPD is OK
· Astigmatism axis is OK
· Near vision is more commonly used
these days than far

20. · What is better for 20 years old myope
refraction -2 OU during reading ?
· # Glasses
· # contact lens
· # Nothing

21. Answer : Glasses
· As base in prism will assist
convergence.
· Accommodation power required for
myope wearing glasses is less than
contact lens : confirm by ray tracing

22. In summary
· IPD should be accurate in high myope, high
hyperope and any astigmatism.
· Test reading function even young age
· Advice your patient (high myopia, high
hyperopia and any astigmatism) to get small
frame to minimize decentration error

23. NEAR ADD

24. Indications
· Loss of accommodation power
· To stop accommodation power

25. Common mistakes
1. Prescribing near glasses for a case
of only hyperopia (not presbyopia)
2. Prescribing glasses not suitable for
patient near work
3. Hyperopic Jump

26. Hyperopia no presbyopia
40 years male came to clinic asking for
reading glasses ,, his Autoref and trial
were +1.5 OU ,, he never used glasses ,
and had no complaint about far vision ??

27. Patient Job and expectation
· Test the near glasses also for far.
‫الحجة‬ ‫عليه‬ ‫اقم‬
· Ask patient about specific job.

28. Why phoroptor is bad ?

29. Hyperopic Jump (also mistake for far
glasses)
· Patient 40 years old , far vision glasses
is -2 / -1.5 x 30 , trial refraction
(manifest) is -1 / -1.5 x 30 what to
prescribe for far and near ??
Respect patient accommodation ,, add plus over patient previous
glasses should range from +0.5 (in young age) to maximum +1.5
(in very poor accommodation)

30. Steps for finding near ADD
· Far trial
· Find accommodation amplitude (for
each eye separately)
· Prescribe and adjust (leaving 1/3
accommodation amplitude relaxed)

31. Accomodation amplitude
· Pull or Push method
· Each eye separately
· Rarely both eyes have different ADD

33. · If patient can read well at 20 cm
· He has accommodation amplitude +5
· He mostly don’t need near ADD
· As he can read at 30 cm with 2 D relaxed

34. Can Add exceed +3 ?
· In low vision
· Can also differ in both eyes (e.g.
unilateral pathology or pseudophakia )

35. · Bifocal / Multifocal :
Don’t advise
Perfect patient : no astigmatism no
anisometrope no cataract , no squint, not
high add …
Perfect place.
· Office Glasses : Multifocal for
intermediate and near

36. Intermediate vision
· Important in patient with no
accommodation
· Add +0.5 to far glasses ( or +0.75 to
non dominant eye )

37. Sphere

38. Accommodation spasm
· Children
· Wrong previous glasses
· Excess near work ( usually -2.5)
· Drugs
· Astigmatism

39. Sources of error in sphere other
than accommodation
1. Room distance : add - 0.25 for room
distance in hyperope.
2. BVD : if error > 5 , if frame is large
under correction usually present/
phoroptor can lead to overcorrection.
3. Night myopia : if significant : give
driving glasses with add -1

40. َ‫ي‬َ‫ل‬ ‫وا‬ُ‫ن‬َ‫م‬‫آ‬ َ‫ين‬ِ‫ذ‬َّ‫ال‬ ‫ا‬َ‫ه‬ُّ‫ي‬َ‫أ‬ ‫ا‬َ‫ي‬
ُ َّ
‫ّللا‬ ُ‫م‬ُ‫ك‬َّ‫ن‬ َ‫و‬ُ‫ل‬ْ‫ب‬
ٍ‫ء‬ْ‫ي‬َ‫ش‬ِ‫ب‬
ِ‫د‬ْ‫ي‬َ‫أ‬ ُ‫ه‬ُ‫ل‬‫َا‬‫ن‬َ‫ت‬ ِ‫د‬ْ‫ي‬َّ‫ص‬‫ال‬ َ‫ن‬ِ‫م‬
َ‫ي‬ِ‫ل‬ ْ‫م‬ُ‫ك‬ُ‫ح‬‫ا‬َ‫م‬ ِ
‫ر‬ َ‫و‬ ْ‫م‬ُ‫ك‬‫ي‬
َ‫م‬َ‫ل‬ْ‫ع‬
ْ‫ي‬َ‫غ‬ْ‫ال‬ِ‫ب‬ ُ‫ه‬ُ‫ف‬‫َا‬‫خ‬َ‫ي‬ ْ‫ن‬َ‫م‬ ُ َّ
‫ّللا‬
َ‫ب‬ ‫ى‬َ‫د‬َ‫ت‬ْ‫ع‬‫ا‬ ِ‫ن‬َ‫م‬َ‫ف‬ ِ‫ب‬
َ‫د‬ْ‫ع‬
ٌ‫م‬‫ي‬ِ‫ل‬َ‫أ‬ ٌ‫اب‬َ‫ذ‬َ‫ع‬ ُ‫ه‬َ‫ل‬َ‫ف‬ َ‫ك‬ِ‫ل‬َ‫ذ‬

41. ASTIGMATISM AND MYOPIA
SHOULD BE FULLY CORRECTED

42. In hyperope
· Under correction is allowed
· According to patient usual
accommodation state.

43. Slight under correction in high myope is allowed
(In children)
· Prescribe full cyclo refraction
· Under correction could improve
peripheral vision / magnification.

44. In children
· The aim is (not 6/6 sharp) but to :
· * Prevent amblyopia
· * Treat symptoms

45. CYLINDER

46. Source of cylinder
· Astigmatism = Anterior cornea
· +
· others
· (posterior cornea / lens / retina )

47. Error in autoref
· Accomodation
· Pupil size
· Decentered patient
· Dry eye
· Faint nebula
· Cataract
· PCO or any operated eye.
May Add cylinder
that is not present

48. Eye lid position
· Starring in the target of Autoref
Autoref removes
cylinder that is present

49. Keratoconus
· Overt Kc → Autoref totally wrong
· Form fruste Kc → Autoref increases
cylinder and shift the axis to WTR

50. In keratoconus , start point should be
cornea not autoref

51. Source of error in subjective trial

52. Problem of Frame

53. How to subjectively find the
true cylinder ?
· Axis : subjective rotation of knob
· Power : adjust power of cylinder
without changing SE
· Cross cylinder

54. Axis

55. Pwr
· Trial : -1 / -3 x 90
· Try : -1.25 / -2.5 x 90
· Also : -0.75/ -3.5 x 90

56. If cylinder is high (2 or more)
· Re try pwr and axis in binocular
fogging // remember meridional
anisometropia
· confirm axis for near vision.

57.  Use ( - ) cylinder as a routine.
 Astigmatism is an indication of cyclo refraction as it
induces ciliary spasm.
 Astigmatism > 5 is usually not tolerated due to unequal
magnification in different meridians
 In near glasses , patient usually prefer less cylinder (Try
it binocular).

58. Cross cylinder
· The role is to find
residual / missed
cylinder e.g. mild
ptosis
· Done with binocular
fogging.

59. KERATOCONUS

60. Problems
Autoref is wrong.
Sphere is usually overminus due to
accommodation spasm.
Cylinder should be taken from
pentacam or keratoscope.
 Refine axis and power several times

61. Start trial by sphere or cylinder ?

64. What axis ? What sphere?
Axis
Cylinder
Sphere
130
- 4.00
+ 2.00

67. Substitute of pentacam to find
axis

69. BINOCULAR TRIAL

70. Accommodation reflex is bilateral
· The output is equal to both eyes.
(differ only in pathology)
· The input is blurred retinal image
from both eyes.

71. So
· The state of accommodation differ
between monocular and binocular trial
· Some times torsion also differ (Axis)
· Change of accommodation can change
the power of astigmatism also

72. Some myopic patients could accommodate
to see the occluder → tested eye will have
more myopia.
Binocular balancing give better binocular
vision and stereopsis and less headache.

73. How to test with both eyes open ?
· Fogging of one eye √√√√
· Dissociate the 2 eyes
Red – green filter √√
Polarizing filter
Prism dissociation (vertical)
Prism Duochrome test
Alternate cover 

74. Fogging
· Fog one eye with +1.5 and find the highest tolerable + in the
other eye ( +0.25 then +0.5 then … ) and confirm its cylinder

75. Importance of fogging
· Reach point where
· Add +0.25 = Worse
· Add -0.25 = not better
· Diagnose accommodation spasm
· Refine cylinder also during fogging
(power and axis)

76. If accommodation spasm
· Cycloplegia.
· Or
· Repeated fogging.
·
· (children , high astigmatism, excess
near work, wrong previous glasses ,
drugs)

77. Red green (worth 4 dot test)

78. ANISOMETROPIA

79. Problems
· Different refraction = different image =
failure to fuse images =
1- Amblyopia
2- Alternate vision
3- Anisokonia (with glasses)
4- Anisophoria (with glasses)

80. Amplyopia
· Glasses + full day cover (except 1 h,
follow up vision weekly)
· Improved : CL/ PRK / ICL / Glasses
with alternate cover.
OD : - 1 → 6/6
OS : +5 → 5/60

81. Alternate vision
Glasses :
· Diplopia : CL/ PRK / ICL / Glasses
with correct OS for near.
· No Diplopia : Worth 4 dot test
OD : 0 → 6/6
OS : - 6 → 6/9

82. Alternate vision
Worth 4 dot test:
· 4 dot (fusion) : Glasses with special
precautions.
· 2/3 dots (suppression) : contact lens trial
OD : 0 → 6/6
OS : - 6 → 6/9

83. Alternate vision
Contact lens trial :
· 4 dot (fusion) : CL/ PRK / ICL / Glasses
with correct OS for near.
· 2 dots (suppression) / 5 dots (diplopia) :
Glasses and correct OS for near
OD : 0 → 6/6
OS : - 6 → 6/9

85. 50 years old male (work in watch repair)
· Nothing to be done
OD : -1 → 6/6
OS : - 13 → 6/9

86. Anisometropia glasses
· Can leave 0.5 – 1 D uncorrected myopia in
the most myopic eye.
· Decrease the back vertex distance
· Ask for low quality plastic lens (1.4) in the
low power side and high quality glass lens
(1.7) in the high power side.

87. Anisophoria
· Unequal prism effect in anisometropic
glasses when patient look up / down /
sides
· Solution : till the patient to move head
not eye = use small frame or till the
optician to decenter the lens 2 mm down.
· May need prism particularly for reading
glasses.

88. Meridional anisometropia
· Lead to Meridional Amblyopia
· Decrease the cylinder in the high
cylinder side.
OD : 0 /-1 x 90 → 6/6
OS : - 2/ -3 x 90 → 6/9

89. Don’t forget in cataract
· If one eye is pseudophakic
· Biometry should be adjusted so that no
anisometropia particularly at axis 90

90. · Expected post operative refraction:
· -1 / +2 x 90
· Pwr at axis 90 OD = +1 ; OS = - 1.5
· This lead to anisophoria in looking down
OD : cataract
K1 (90) = 46
K2 (180) = 44
Pwr = +20
OS : + 1.5/ -3 x 90 → 6/9

91. Solution
· Target refraction should be -1
· So post op refraction OD
· -2 / +2 x 90
· So power at 90 = zero

92. GLASSES AND SQUINT

93. Patient not happy during reading
· Monocular : error in astigmatism axis.
· Binocular problem :
· accommodation imbalance
· prism vertical effect,
· prism horizontal effect: CI/ wrong
near PD.

94. Other Squint-induced by glasses
· If the glasses stimulate accommodation :
· * Full correction of myopia (in high
AC/A ratio) → bifocal
· * partial correction of high hyperopia→
full correction.

95. PRESCRIPTION OF PRISM

98. · Maximum allowed 10 prism diopter in each lens
· Find correct refraction
· Patch one eye for 30 min to find the full phoria
· Trial of prism and give the patient the frame for 30
min in the clinic
· Better to divide the prism between 2 eyes.

99. Post traumatic

100. Decompensated latent XT

101. Retinoscopy

102. Why important
1. In children
2. Uncooperative adult with inaccurate
autoref.
3. Extreme error
4. Over refraction

103. RETINOSCOPY WITHOUT
NEUTRALIZATION

104. Retinoscope has 3 positions
Zero
- 2
+ 4

105. Eye has 3 possible far points

106. · When retinoscope is focused
on the far point you will see
sharp thin streak of
retinoscope on the retina

107. start with parallel rays
Emmetrope

108. Not emmetrope

109. Go near the patient and move
streak up
· + 4

110. Go back
· More and more
hyperopia > +4

111. Myope

112. Myope

113. sequence

114. This retinoscopy depend on
the ingoing rays
· Confirmation could be done with
outgoing rays ( Neutralization / null
point retinoscopy )

115. Stay at the far point in myope or the
far point with + lens in hyperope
· To see bright red reflex + any
movement lead to sharp rapid
moving reflex.