Low Vision Formulas

Question 1

Moderate Low Vision

Answer 1

20/70 - 20/160

Question 2

Severe Low Vision

Answer 2

20/200 - 20/400

Question 3

M units

Answer 3

Metric distance at which lower case letter would subtend 5 min of arc.
20/50 = 1.00M = 8N = 1.45mm ht = 0.40 logMAR

Question 4

Feq

Answer 4

Equivalent power, total dioptric strength needed by patient to achieve their near visual goal

Question 5

Kestenbaum rule

Answer 5

BCVA: denom/numerator for 1M (reciprocal of vision), and then do Kest/goal if their goal is something other than 1M.
Ex. Kest/0.5M would be twice as much power needed than for 1M goal

Question 6

Effective Mag

Answer 6

D/4

Ex. +20/4 = 5X

Question 7

Conventional Mag

Answer 7

(D/4)+1

Ex. +20/4 +1 = 6X

Question 8

F1

Answer 8

Lens held away from spec plane

Question 9

F2

Answer 9

Lens at spec plane (the ADD)

Question 10

c

Answer 10

Separation of the lenses

Question 11

JND

Answer 11

Denom when numerator is 20 of BCVA (better eye), show the lenses +/- half of the JND

Question 12

How to find Feq for a near goal with near VA given

Answer 12

(Working distance D) near VA / goal VA
Ex. Patient with 0.4/3.2M VA wants to see 0.6M —> (3.2/0.6)(+2.50) = +13.33D Feq

Question 13

Microscopes (spec-mounted convex lenses)

Answer 13

Feq = F2
- largest FOV, but poor working distance, RDM, focal length is of the prescribed microscope, but remember to include pt’s eye’s inherent power in the calculation (myopes require less plus, hyperopes require more plus), remember convergence demand issue

Question 14

Convergence demand for microscope lenses

Answer 14

Need to add BI if between +4 and +12 lenses, BI = D + 2 for each lens

Question 15

Hand-held magnifiers

Answer 15

Feq = F1, RDM and angular mag.
For spot reading mostly, held at focal length from reading material. Use long formula if using it with an ADD. Feq = F1 + F2 - cF1F2 where c is the distance bw HHM and ADD

Question 16

Break-even point

Answer 16

1 focal length of magnifier from spectacle plane

Question 17

FOV and HHM

Answer 17

FOV decreases as HHM is moved away from the eye. FOV = d*f/h where d is the lens diameter, f is focal length, and h is the lens to eye distance

Question 18

Stand magnifiers

Answer 18

Feq = ER x F2, RDM and angular mag. Requires an add. Object to lens distance is fixed, with an upright virtual magnified image.

Enlargement ratio ER = U/V
V=U+P
Wd=1/F2= [z+v]
z is distance between specs/add and the F1 magnifier

Question 19

Electronic magnification

Answer 19

Feq = TM x F2
TM = image height/object height
F2 is the add (1/wd)
Good for large Feqs and poor VAs, but expensive

Question 20

Telemicroscopes

Answer 20

Feq=TS x F2
RDM and angular mag, F2 is the reading cap (on objective lens)

Question 21

Mag

Answer 21

Does not provide clarification, but produces a large enough retinal image for identification

Question 22

Relative distance magnification

Answer 22

Original object to eye distance / new object to eye distance
Most common form of mag in low vision devices, like bringing something closer to the eye

Question 23

Relative size magnification

Answer 23

new object size / original object size , or final object height / original object
Large print books, increasing font size

Question 24

Angular magnification

Answer 24

Ratio of angular size of the image to the angular size of the object at whatever distance the object is located, without changing object size or viewing distance. Ex. Telescopes, HHM, SM

Question 25

Transverse magnification aka lateral or linear mag

Answer 25

Final height of image / original height of image
Ex. Overhead projector

Question 26

Telescope mag formulas

Answer 26

M= -Doc/Dobj
M= ent pupil diameter / exit pupil diameter

Question 27

Distance of lenses in a telescope formula

Answer 27

d= fobj+foc

Question 28

Tube length and power of telescope

Answer 28

Increasing tube length, decreases power

Question 29

Keplerian

Answer 29

(+) ocular, (+) objective, longer tube length, larger FOV, better image quality. Exit pupil floats outside system. REAL and INVERTED image.
More expensive, higher powers. Uncorrected myope will increase the mag

Question 30

Galilean

Answer 30

(-)ocular, (+)objective, shorter tube length, <4X powers, exit pupil fills ocular lens diameter,lower cost, easier use. VIRTUAL and UPRIGHT image. Uncorrected hyperope will increase mag

Question 31

Reverse telescopes

Answer 31

Expands FOV by minifying