Low Vision Formulas Flashcards
Moderate Low Vision
20/70 - 20/160
Severe Low Vision
20/200 - 20/400
M units
Metric distance at which lower case letter would subtend 5 min of arc.
20/50 = 1.00M = 8N = 1.45mm ht = 0.40 logMAR
Feq
Equivalent power, total dioptric strength needed by patient to achieve their near visual goal
Kestenbaum rule
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
Effective Mag
D/4
Ex. +20/4 = 5X
Conventional Mag
(D/4)+1
Ex. +20/4 +1 = 6X
F1
Lens held away from spec plane
F2
Lens at spec plane (the ADD)
c
Separation of the lenses
JND
Denom when numerator is 20 of BCVA (better eye), show the lenses +/- half of the JND
How to find Feq for a near goal with near VA given
(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
Microscopes (spec-mounted convex lenses)
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
Convergence demand for microscope lenses
Need to add BI if between +4 and +12 lenses, BI = D + 2 for each lens
Hand-held magnifiers
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
Break-even point
1 focal length of magnifier from spectacle plane
FOV and HHM
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
Stand magnifiers
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
Electronic magnification
Feq = TM x F2
TM = image height/object height
F2 is the add (1/wd)
Good for large Feqs and poor VAs, but expensive
Telemicroscopes
Feq=TS x F2
RDM and angular mag, F2 is the reading cap (on objective lens)
Mag
Does not provide clarification, but produces a large enough retinal image for identification
Relative distance magnification
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
Relative size magnification
new object size / original object size , or final object height / original object
Large print books, increasing font size
Angular magnification
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
Transverse magnification aka lateral or linear mag
Final height of image / original height of image
Ex. Overhead projector
Telescope mag formulas
M= -Doc/Dobj
M= ent pupil diameter / exit pupil diameter
Distance of lenses in a telescope formula
d= fobj+foc
Tube length and power of telescope
Increasing tube length, decreases power
Keplerian
(+) 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
Galilean
(-)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
Reverse telescopes
Expands FOV by minifying
