Low Vision

Question 1

4 main types of magnification

Answer 1

1. RDM (D= O/N)
2. RSM (S =N/O)
3. Angular mag compares retinal image sizes
4. Lateral mag compares original object size to new image size

Question 2

Collimating magnifier

Answer 2

When exiting rays are parallel, like a HHM.

Accommodation/adds are not required.

Question 3

Total magnification provided by a HHM =

Answer 3

Focal distance between object and eye x F

Question 4

HHM is brought closer to the eye

What mag changes? What mag stays the same?

Answer 4

Angular mag does not change

Linear FOV will increase

Question 5

How to calculate Linear FOV with HHM

Answer 5

Linear FOV = lens diameter / (F x distance between HMM and spec plane)

Question 6

SM produce what type of image

Answer 6

Virtual, upright, magnified

Question 7

How to calculate angular mag of a SM

Answer 7

Angular mag = lateral mag x distance

Question 8

How to calculate magnification of telescopes

Answer 8

Mag = - F ocular/ F objective 
Mag = entrance pupil/ exit pupil 

*ent pupil is commonly the objective lens, exit pupil is the ocular lens.

Question 9

Telescopes are labeled AXB

What do these indicate

Answer 9

A is mag

B is entrance pupil/objective lens diameter

Question 10

The FOV of a telescope is dependent on what

Answer 10

FOV is dependent on the size of the exit pupil of the telescope and the size of the entrance pupil of the patient

The FOV will continue to increase until the diameter of the exit pupil matches the diameter of the patient’s pupil. Then the FOV will be limited by the patient’s pupil.

Question 11

Characteristics of a Keplarian telescope

Answer 11

+ Objective and ocular lenses
Image will be inverted unless you add an inverting lens. This will make the tube longer and heavier.
Exit pupil is outside the telescope, making the FOV larger.
However, the exit pupil is SMALL, making the image dim.
high mags

Question 12

Characteristics of a galilean telescope

Answer 12

+ objective, - ocular lens
Image is upright
Tube is small and low weight
Exit pupil is located inside the telescope, making the FOV smaller
Exit pupil is large, making the image bright
Up to 4x mag due to limitations of the tube length.

Question 13

A reading cap acts as a

Answer 13

HHM

Question 14

A telescope can be changed to view near targets by

Answer 14

1. Adjusting the tube length
2. Adding a reading cap
3. Viewing through an add power

Question 15

2 types of spectacle mounted telescopes

Answer 15

1. Center fit

2. Bioptic fit

Question 16

2 primary types of peripheral VF loss and 2 primary optical devices that can relocate or expand the field

Answer 16

1. Mirrors
   - Usually for patients with temporal VF loss
   - Always put the mirror nasal on the eye with the defect.
   - Will induce a scotoma, may cause nausea.
2. Prism.
   - Placed with base in the direction of the defect
   - 2 pd cause 1 degree shift

Question 17

__million americans are legally blind

__ million are visually impaired

Answer 17

1. 1

4. 3

Question 18

When patients require reading glasses greater than +4.00D, prescribe what type of prism? How much prism per eye?

Answer 18

Prescribe BI prism to move the image further away.

The amount per eye is 2 + D

Question 19

Qualifications for legally blind

Answer 19

1. Patient cannot read any letters on the 20/100 line in the better seeing eye
2. Visual field diameter is 20 degrees or less in the better seeing eye.

Question 20

1M is defined as

how tall is it in mm

Answer 20

The size of a symbol that subtends 5 arc min at 1 meter and is equivalent to a Snellen acuity of 20/20

It is 1.45mm tall.

Most newspaper print at 40cm is 1M. Equivalent to Snellen 20/50

Question 21

How to calculate JND

Answer 21

The denominator/100

ex: pt can see 20/400? JND is 4.00D. Begin the trial frame refraction by showing the patient +/- 2.00D sphere lenses.

Question 22

How to determine the amount of magnification required for a patient to view a distant object

Answer 22

Distance mag = Current distance BCVA/goal BCVA

Question 23

2 formulas to determine a near add

Answer 23

1. Kestenbaum’s rule: flip the distance BCVA

2. Lighthouse method = (current near VA/ goal near VA) (1/working distance)

Question 24

3 ways contrast sensivity can be measured

Answer 24

Pelli Robson chart- each line reduces in contrast
Vistech- uses a series of sine wave gratings.
Bailey-Lovie: Consists of 3 charts, each at a different level of contrast

Question 25

SM uses which 2 forms of magnification

Answer 25

Lateral

RDM