Low Vision

Question 1

Angular Magnification

Answer 1

Ratio of the original retinal image size to the retinal image size viewed through an optical system
(object size and location does not change)

Question 2

Equation for Power of a HHM

Answer 2

Store HHM label is for a standard 25cm WD so M=F/4

F = 4M

Question 3

Types of Mag used in a HHM

Answer 3

RDM and Angular mag

Question 4

Magnification of a HHM using original distance from target

Answer 4

M = (original distance from target)(F)

Question 5

Feq when using a HHM or SM with an add.

How does moving the HHM change Feq

Answer 5

Feq = F1 + F2 - dF1F2

moving the HHM closer gives more + Feq

Question 6

FOV through a HHM

-Where is it largest and what kind of image

Answer 6

FOV = (lens diameter) / (power) (lens distance from eye)

Largest FOV at lens primary focal pt -> creates enlarged virtual image at infinity

Question 7

Types of mag in a SM

Answer 7

Lateral mag and RDM

Total mag is LM x RDM

Question 8

Object and Image attributes for a SM

Answer 8

The object is located within the primary focal point so the image is upright, magnified and virtual and the pt will need to wear an add to compensate for the diverging light

Question 9

Equation for total magnification of a SM/add combo

Answer 9

M = Fe/4

Max Mag = 1 + Fsm/4

Question 10

Equation for Mag in a telescope

Answer 10

M = -Foc/Fobj

can also use Dent/Dex (diameter of entrance/exit pupils)

Question 11

Interpret telescope notation 5 x 45

Answer 11

Mag is 5x

Objective lens diameter (entrance pupil) is 45mm

Question 12

Factors for FOV in a telescope

Answer 12

Exit pupil diameter (can be increased by increasing entrance pupil/obj. lens diameter)
Diameter of entrance pupil of pts eye
Distance from telescope exit pupil to pts entrance pupil

Question 13

Aspects of a Keplerian Telescope

Answer 13

Positive obj. and ocular
Forms an inverted image so a 3rd inverting lens is needed
Longer and Heavier but available in higher Mag
Exit pupil is outside scope so good FOV but it is small so the image is DIM

Question 14

Aspects of a Galilean telescope

Answer 14

Positive obj. and negative ocular
Forms an upright image
Smaller and lighter but only available up to 4x
Exit pupil is inside scope so poor FOV but it is large so the image is BRIGHT and alignment is less critical

Question 15

Ways to make a telemicroscope

Answer 15

Add a reading cap

Adjust tube length (technically not a telemicroscope)

Question 16

magnification of a telemicroscope reading cap

Answer 16

Mc = F x (original distance from object to EYE)

-use standard distance of 0.25m if distance is not provided

Question 17

Total magnification of a telemicroscope

Answer 17

Total = Mc x Mt

Question 18

Ways to expand FOV

Answer 18

Minus lens (-5 to -10)
Reverse Telescopes (usually used for spotting)

Question 19

Describe ways to relocate FOV

Answer 19

Mirrors: for pts with temporal loss, induce a scotoma, create confusion/nausea due to the image in the mirror being reversed
Prisms: place base towards defect, reduces need for scanning

Question 20

Spectacle Mounted Telescope Types

Answer 20

Center Fit: mounted in the line of sight, requires minimal training, good for continuous viewing but don’t walk around in them
Bioptic Fit: mounted in the upper part of the glasses, requires significant training, used for spotting

Question 21

How many Americans legally blind/ legally impaired

Answer 21

1. 1 mil. blind

4. 3 mil. impaired

Question 22

Linear size of 1M

Answer 22

1.45 mm

Question 23

How to reduce convergence demand on high powered glasses needed for near vision

Answer 23

BI prism (usually F+2)

• Also can use a HHM with a diameter as large as the pts PD
• pt with really high power can occlude an eye

Question 24

Define legal blindness

Answer 24

Pt cannot read any letters on the 20/100 line in the better seeing eye with best correction
VF diameter is less than 20 degrees in the better seeing eye

Question 25

Devices to measure VF for legal blindness

Answer 25

Static or Automated threshold/Humphrey

Static or Kinetic Goldmann

Question 26

1 M equivalent at 40 cm

Answer 26

20/50

Question 27

Symptoms for scotomas to the left vs right of fixation

Answer 27

Left: lost of place when reading
Right: slow reading speed

Question 28

Eccentric Viewing

Answer 28

Viewing with an off fovea healthy retinal location known as the Preferred Retinal Locus
-Cannot eccentric view in legal blindness measurement

Question 29

Kestenbaums Rule

Answer 29

Take the inverse of the pts DISTANCE BCVA to determine add

• Assumes a 1M goal
• distance doesn’t always correspond to near

Question 30

Lighthouse Method

Answer 30

Predicted add = (current NVA/goal NVA) x 1/working distance

-more accurate than kestenbaum

Question 31

Order to show a pt devices

Answer 31

Near then distance

Question 32

Tests for Contrast Sensitivity

Answer 32

Pelli Robson: large letters of uniform size, each row gets lighter
Vistech: sine wave gratings of various contrast and frequency
Balie-Lovie: 3 different contrast level charts

Question 33

Methods to decrease glare and enhance contrast

Answer 33

ND filters: reduce glare but no effect on contrast

Blue blockers/amber tints: enhance contrast and reduce glare by reducing short wavelength transmission

Question 34

Best way to assess central visual field

Answer 34

Scanning Laser Ophthalmoscope

-Goldman perimetry can be used but with less effectiveness