LV - Near Low Vision Aids I - Week 4

Question 1

How would you estimate the magnification a patient may require for near?

Answer 1

Measure near acuity at 25cm with a +4 addition over the distance prescription

Question 2

What print size should be aimed for when prescribing magnification?

Answer 2

Two lines spare
-if N8 required, aim for N5

Question 3

Give the number of reserve lines for the following intensities of reading the patient intends to do:
Spot reading
Fluent reading
Maximum/near maximum

Answer 3

Spot reading - 1 line reserve
Fluent reading - 3 lines reserve
Maximum/near maximum - 4 to 5 lines reserve

Question 4

Give the formula for the magnification ratio. Give the approximation based on logMAR lines.

Answer 4

mag ratio = size letters read / size letters wish to read
-i.e. reading N20, wanting to read N5 = 20/5 = 4x
Every 3 lines is 2x mag

Question 5

Describe the purpose of magnification and how it achieves this. List three ways this can be achieved.

Answer 5

To increase retinal image size by increasing angular subtense of the object
-relative size
-relative distance
-angular

Question 6

Define relative distance magnification.

Answer 6

Distance between the eye and object is reduced

Question 7

Define angular magnification.

Answer 7

Optical devices used to increase visual angle

Question 8

When combining magnification, is it additive or multiplicative?

Answer 8

Multiplicative

Question 9

List the two types of magnification.

Answer 9

Relative/effective
Rated/conventional

Question 10

Which type of magnification is most used by clinicians?

Answer 10

Relative

Question 11

Define relative magnification.

Answer 11

Angular size image through the magnifier / angular size of the object at a standard reference distance

Question 12

What is usually the standard reference distance?

Answer 12

25cm
-sometimes 40cm

Question 13

What is the formula for relative magnification and how is rated magnification different? What does it assume?

Answer 13

Relative: M = F/4
Rated: 1 + (F/4)
-assumes lens will be held close to the eye and the eye will exert 4D accommodation

Question 14

List 6 near magnification options.

Answer 14

High additions
Hand magnifiers
Stand magnifiers
Near telescopes
Handheld electronics
Desk electronics

Question 15

Name two advantages and disadvantage of high addition spectacles.

Answer 15

A - hands free, large field of view
D - short working distance

Question 16

Does the lens itself produce increased retinal image in high addition lens? Explain.

Answer 16

No, it acts as a converging system
-as reading material brought closer, retinal image size increases
-lens produces negligible amount of magnification

Question 17

What is a problem with high addition spectacles and up to what power D is it generally ok? What can be done to minimise this?

Answer 17

Increased peripheral aberrations with increasing power
Up to +8D is ok
Aspheric designs minimise them

Question 18

What is a doublet spectacle? Does it have the same problems as regular high addition spectacles?

Answer 18

Combination of two convex lenses separated by an airspace
Provides high mag without peripheral aberrations

Question 19

For high addition spectacles with half-eye lookover designs, what amount of prism is used?

Answer 19

Power of the lens + 2

Question 20

What is a loupes design for high addition spectacles? What is the tradeoff with these designs?

Answer 20

Allows for slightly extended working distance by extending lens in front of the spectacle plane
-as extension increases field of view decreases

Question 21

What should you emphasise when a patient is trialling high addition spectacle designs?

Answer 21

Correct working distance

Question 22

List 5 things the patient must be aware of when high addition spectacles are being considered.

Answer 22

Fatigue - arms/neck
Nausea/dizziness
Eye fatigue
Headaches
No mobility

Question 23

List 7 advantages of spectacles for magnification.

Answer 23

Acceptance?
Cosmesis
Largest field of view
May be used binocularly
Hands free
Better for prolonged reading
Astigmatism corrected

Question 24

List 6 disadvantages of spectacles for magnification.

Answer 24

With increasing power:
-working distance decreases
-depth focus decreases
-lighting more critical
-aberrations increase
-lens size decreases
-reduced reading speed

Question 25

Give an advantage and disadvantage of simple hand-held magnifiers.

Answer 25

Longer working distances
Hands not free

Question 26

What design of lenses are typically used in hand-held magnifiers?

Answer 26

Aspheric

Question 27

What is an aplanatic design of lenses? What design of magnification are they used in and what is an advantage and disadvantage of this design?

Answer 27

Two planoconvex lenses with the convex surfaces in contact with each other
Least amount of image distortion
Expensive

Question 28

Describe how a patient should use a simple handheld magnifier (2).

Answer 28

Lay magnifier on page and slowly lift up until clearest image found
Lens parallel to page, most convex side toward the patient

Question 29

List 8 advantages of simple handheld magnifiers.

Answer 29

Portability
Relatively inexpensive
Familiarity/acceptance
Easy to prescribe
Extended wdist
Allows head movement
Quick spotting
Illumination available

Question 30

List 4 disadvantages of simple handheld magnifiers.

Answer 30

Steady hand and coordination
Decreased field of view with increased working distance
Need to replace bulbs/batteries if illuminated
Decreased reading speed