for Optometry students

1. Calculation of Magnification
in Low Vision
Mohammad Arman Bin Aziz
Instructor Optometrist
Institute of Community Ophthalmology

2. Can visual acuity be equated with
functional vision?
Visual Acuity = Functional Vision

3. Magnification
• Ratio of image size to object size (for a lens
system)
• M = I/O = h’/h = l’/l = ’/ 
• Many different types each with different
meaning

4. Different Terms
 Angular magnification
 Apparent magnification
 Spectacle magnification
 Relative distant magnification
 Nominal magnification
 Perceived magnification
 Actual magnification
 True magnification
 Iso-accommodative magnification
 Manufacture rating
 Magnifying power

5. Low vision Magnification
• Relative size
• Relative distant
• Angular
• Apparent
• Relative
• Iso-accommodative
• Equivalent Viewing Power (EVP)
– Newer concept in Low vision

6. Low Vision Devices
1. Telescopic lenses
2. Magnifying lenses
3. Electronic magnifiers
4. Field enhancers

7. Relative Size Magnification
'

= = tan '
tan 
h2
h1
=
l
h1 
h2
l
'

8. Relative Size Magnification
Large Print Cards Large Print Calculator

9. Relative Size Magnification
Zoomtext for enlargement

10. Relative Distance Magnification
h
l1
E
h
l2
E

'
'

= = tan '
tan 
l1
l2
=

11. Relative Distance Magnification
• Achieved by decreasing the distance between
the object & eye
• Need either accommodation/ Plus lenses to
maintain clear focus
– due to the large accommodative demand created
by short viewing distance

12. Relative Distance Magnification

13. Angular Magnification
'

=
h
l
E
l
F
h
E
xf
'

14. Angular Magnification
Telescope
Magnifiers

15. Name Method Examples
Relative Size
Magnification
Increasing the
actual size of the
object being
viewed
Larger print
material
Relative Distance
Magnification
Reducing the
distance between
the object and the
eye
Move object closer
to the eye
Angular
Magnification
Increasing angular
subtense of the
image being
viewed
Telescope,
magnifier
Three Types of Magnification in low vision

16. Projection Magnification
Closed-circuit Television (CCTV)

17. Projection Magnification
CCTV connected to a computer

18. Projection Magnification
Reading with Traveller

19. Projection Magnification
MaxPort Magnifier with eyeglasses as display unit
FlipperPort Magnifier
for distant viewing

20. Adaptive Technology – hardware and software
 Screen enlargers and screen magnifiers
 Screen reviewers and screen readers
 On-screen keyboards
 Keyboard enhancement
 Voice input aids or speech recognition
 Alternative input devices

21. Apparent Magnification (Bailey & Bennett)
 Perceived Magnification
 Ratio between the angle subtended by the image at
entrance pupil of eye to the angle subtended by the
object without magnifier
 System
 Object at anterior focal plane of lens
 Object not at focal plane of lens
 For accurate apparent magnification
 Viewing distance / Equivalent power of magnifier
should specified

22. Apparent Magnification (Bailey & Bennett)
•

23. Relative Magnification
 Effective or conventional magnification
 Retinal image size produced by the magnifier to
the retinal image size produced by the object,
 when viewed at a standard distance
 (LDDV = 25 cm)with out magnifier
 RM = angle subtended at eye by image produced by lens
angle subtended at unaided eye by object at LDDV
= ' / 25

24. Relative Magnification
Mrel = - F(-d)
= F/D
– If d = LLDV = 25 cm
= F/ 4 = Trade or
manufacture
rating
Magnification
Equivalent power of
magnifier (F)= M x4

25. Relative Magnification
 For : Relative = Actual magnification
 Will be true for following conditions
 Patient should be emmetropic or corrected for any ametropia
 For Myopia = Actual Magnification increase
 For Hyperopia = Smaller magnification than specified
 Object at anterior focal plane of the magnifier, so that image formed
at infinity
 If located at less distance = Actual magnification increase
 If located further away = Actual magnification decrease
 Reference object distance
 For ref. Distance 25 cm, M = F/4
 But in real life situation
 Reading distance , 33 cm . M = F/3
 Reading distance, 40 cm = F/2.5

26. Iso-accommodative Magnification
(manufacture rating magnification)
 The ratio of the angle subtended at the
entrance pupil of the eye by the magnified
image to the angle subtended by object
 When viewed from same distance
 Type of Relative magnification
 Object is located outside the anterior focal plane of
magnifying lense

27. Iso-accommodative Magnification
(manufacture rating magnification
• Miso-acc = 1 + F/ D

28. Iso-accommodative Magnification
(manufacture rating magnification
 For Ref. Distance 25 cm
 Miso-acc = 1 + F/4
 Three assumption inherent in above formula
 Magnifier to eye distance is negligible
 The reference viewing distance is 25 cm
 The image produced by the magnifier is also at 25 cm
 Thus accommodative state / add for near = 4Ds
 For object & Image

29. Terms of Magnification in Low vision
Term Viewing distance
Apparent
Magnification
No specific viewing
distance
Relative
Magnification
A standard distance chosen
for comparison (usually 25
cm)
Iso-
accommodative
Magnification
Same distance of the object
and image from the eye

30. Equivalent Viewing Power(EVP)
 Discard ill-defined magnifications:
 according to Bailey. Better to specify every optical LVD in term of EVP
 Magnifying effect of eye represented by EVP
 EVP = Equivalent focal length of the lens system
 EVP = X D of a lens system
 Provides the same resolution as if the naked eye were
viewing the object at ‘x’ m away with out magnifer
 Where, X = 1/x

31. Equivalent Viewing Power(EVP)

32. Equivalent Viewing Power(EVP)
 EVP represents
 intrinsic property of an optical system that corresponds to
the resolution afforded by the system
 If EVP of a system that gives certain resolution to the patient
is know,
 the resolution capability by any other system can be known by
simple proportion
 Eg:
 If NVA with + 2.50 D is 6/18 at 40 cm, an EVP of + 7.50 Ds
magnifier will increase VA to 6/6 at 13 cm.

33. Equivalent Viewing Power(EVP)
 By knowing EVP of a optical system is known
 A logical & efficient conversion of one magnifying
optical system to another
 Eg: If a +10.00 D add is required to read 2M print at
10 cm
 For a CCTV, with 5 x magnification viewed at 50 cm
& +2.00D add will enable patient to read 2M print

34. Prediction of Magnification for Low Vision
 Accurate corrected near visual acuity
assessment
 Always in Metric system (1M = N8)
 For unknown sized reading material
 Conversion to metric system =
letter size in mm/1.45 mm

35. Rule of 1000
 Average number of letters + space counted in
1 inch
 Divide that number to 1000 for metric system
 Resultant is reduced snellen denominator
 Eg: 40 spaces & letters in 1 inch of text patient
wants to read
 1000/40 = 25 : Reduced acuity size of the print= 20/25

36. Methods to determine Magnification
1. Lebenson’s Method of reciprocal vision
2. Kestenbaum’s Method
3. Ratio between Best near VA to target VA
4. Reading power needed to read 1 M print
5. Lovie’s Method
6. Ian Bailey method:
Equivalent Viewing distance (EVP)

37. Lebenson’s Method of reciprocal vision
 Find Best corrected distance acuity & a near target
acuity
 M = ratio of denominator of distance Snellen
fraction to denominator of near snellen fraction of
target acuity
 Eg:
 BDA = 20/400 , TNA = 20/50 (1M)
 M = 400/50 = 8 x, Power (D) = M x 4 = + 32 Ds

38. Kestenbaum’s Method
 Required Dioptric power of add =
 Reciprocal of best corrected distance acuity
 Eg:
 If BCDA = 20/400, Power of add = 400/20 = +
20Ds
 Both above method can give wrong M
value:
 Distance acuity is poor predictor of near
acuity

39. Ratio between Best near VA to target VA
 Patient BNA at testing distance (TD) is recorded in M
notation
 Target near acuity (TNA) is determine, let “X” be new
reading distance
 Then, BNA/ TNA = TD/ X, X = TNA/BNA x TD
 Now, power of Add = 1/X in m

40. Ratio between Best near VA to
target VA: Example
 If, BNA = 4M at 0.4 m, TNA = 1M , X = ?
 X = TNA/ BNA x TD
= ¼ x 40 = 10 cm
Add required (D) = 100/X = 100/10 = +10 Ds
 Patient need to hold the reading material at
10 cm with + 10 Ds magnifier

41. Reading power needed to read 1 M print
 Measure BNA at 40 cm (16 inches)
 Theoretical add power to read 1M print
= Multiply BNA, M value by 2,50 Ds
 Eg:
 If 4 M read at 40 cm
 Add power = 2.50 x 4 = + 10.00 Ds

42. Lovie’s Method
 To find the patient goal and expected reading
rate
 Reading rate (words per minute) for normal
 To spot: 80 wpm – Enables identification of single
word
 To be fluent : 160 wpm – Enables reading accurately
 Maximum: 320 wpm – Enables reading accurately at
a high speed

43. Lovie’s Method
 Needs threshold than desired print size
 Spot: one Line smaller
 Fluent: Three lines smaller
 Maximum: Five line
 Eg: BNA is 1.6 M , required TNVA is 1 M
 To be fluent threshold = 3 lines minimum = 0.5 M
 Keep in mind:
 “Magnification is more important than the field
for best reading vision with visual impairment”

44. Ian Bailey
Equivalent Viewing distance (EVP)
 Defn:
 It is the distance at which the object itself would
subtend an angle that is equal to the angle that
is being subtended by the image.
 We need to adopt EVD forgetting every
magnification

45. References
• Essentials in Low vision practice by Richard S.
Brilliant
• Principles of low vision by Christine Dickinson
• Low Vision Manual by Jothathon Jackson