LV - Near Low Vision Aids II - Week 5

Question 1

Briefly describe how to prescribe near magnfication and considerations you will take into account (6).

Answer 1

Establish patient's reading goals
Define print size and reading rate
Determine goal near acuity
-2 lines spare
Decide on importance of lighting
Determine magnification required
-can also use equivalent viewing distance
Demonstrate that magnifiers are appropriate for your patient

Question 2

Do stand magnifiers provide illumination?

Answer 2

There is an illuminated option alongside non-illuminated

Question 3

List two advantages and one disadvantage of stand magnifiers.

Answer 3

Advantages
-greater magnification
-stable
Disadvantages
-shorter working distance

Question 4

Describe stand magnifiers. Is the focus variable or fixed? What is generally required for stand magnifiers?

Answer 4

Convex lens mounted at a fixed distance from reading material, supported on a stand
Variable or fixed focus
Add/accommodation usually required

Question 5

What is the usual distance from the reading material to the lens?

Answer 5

Slightly less than fm

Question 6

What age populations are visolettes for?

Answer 6

Children

Question 7

List three advantages and one disadvantage of visolettes.

Answer 7

Advantages
-stable
-good light gathering properties
-simple
Disadvantages
-low magnification

Question 8

What magnification can visolettes provide up to?

Answer 8

Only up to 2x

Question 9

Describe how visolettes are used. What kind of lens is it?

Answer 9

It is a simple magnifier placed onto the page directly

-planoconvex lens

Question 10

Must patients wear their prescription in order to use stand magnifiers?

Answer 10

Yes

Question 11

List the instructions for using stand magnifiers (5).

Answer 11

Rest stand on reading material
Line of sight perpendicular to magnifier
Move eyes and magnifier as a unit
Use appropriate lighting
Maintain good posture

Question 12

Patients with what kind of disorders particularly benefit from stand magnifiers?

Answer 12

Tremors/poor motor control

Question 13

Describe the condition required for apparent magnification. When raytraced, where does the chief ray travel through and does it deviate? How do all other rays from the object travel relative to this chief ray and what does this mean for how the object is seen?

Answer 13

The object is at the focal plane of the lens
-image is formed at infinity
Chief ray from the object passes through the lens optical centre and passes through without deviating
All other emerging rays will be parallel to the chief ray
-therefore the image is seen at optical infinity

Question 14

Consider the optics behind apparent magnification. Is the angular subtense of the image dependent or independent to the distance between the lens and the eye? Explain why this is the case.

Answer 14

It is independent

• all emerging rays are parallel to the chief ray, which passes through the optical centre with no deviation
• if the distance changes, the angular subtense is the same

Question 15

Consider the optics behind apparent magnification. What happens as the eye moves back away from the lens and what happens to the image as a result? What happens to field of view and the magnifier’s resolving capacity?

Answer 15

The angular dimension of the lens becomes smaller
The image appears to be enlarging
-hence ‘apparent’ magnification, patients will coment on it
-image size actually remains constant, but field of view becomes smaller
-resolving capacity is unaltered
-image size will only appear larger to the patient, it remains the same

Question 16

What is the better way to specify magnification and why (3)?

Answer 16

Use equivalent powwer (Fe)
Dioptral power (F) does not acknowledge effects of variation in z, Fspec add/accom, doesnt consider mag/eye system

Question 17

What is the formula for equivalent power (Fe)? Specify each variable.

Answer 17

Fe = Fm + Fa - z * Fm * Fa
Fe - equivalent power
Fm - power of the magnifier
Fa - accommodation or spec add required
z - eye-magnifier distance

Question 18

What is equivalent power if an object is placed at fm (not the same as Fm)? Describe the optics behind why this is the case. Explain the importance of z.

Answer 18

Parallel light leaves, the image is at infinity and Fa is not required

• therefore Fe = Fm
• z is not important and remains constant

Question 19

What is equivalent power if an object is placed closer than fm (not the same as Fm)? Are the rays leaving the lens divergent or convergent? Where does this move the image of the reading material relative to the object itself (closer or futher)? In this case, what is required to see the image clearly?

Answer 19

Divergent light leaves the magnifier and either accommodation or an add is required to see the image clearly
The image plane is shifted forward behind the object plane

Question 20

Describe the effect of zfm on Fe and equivalent power.

Answer 20

zfm (aid is away from the eye) - decrease in dioptral power from the system

Question 21

Comparing z to fm, in what cases should add or distance Rx be used with magnifiers (3)?

Answer 21

If 0 < z < fm - use add
If z = fm - add not required
If z > fm - use distance Rx

Question 22

Describe the formula for field of view when an object is at the focal point. Name all the variables.

Answer 22

W = A / (z * Fe)
W is the width of the field (mm)
A is the width of the magnifying lens (mm)
z is eye to lens distance (m)
F is power of lens (D)

Question 23

Describe the relationship between magnification to field of view.

Answer 23

Inversely related

-as one gets bigger, the other gets smaller

Question 24

Describe the relationship between z to field of view.

Answer 24

Inversely related

-small z means larger field of view

Question 25

Define equivalent viewing distance.

Answer 25

The distance at which the object would subtend an angle that is equal to the angle subtended by the image

Question 26

Describe the formula for equivalent viewing distance.

Answer 26

Required EVD = (required PS/current PS) x current EVD
PS = print size
i.e. required PS N4, current PS N12, WD = 40cm
4/12x40=12cm
-EVD is 12cm

Question 27

Define equivalent viewing power using equivalent viewing distance.

Answer 27

EVP = 1/EVD
i.e. 12cm = 8D
Select aids that had an EVP of 8D

Question 28

Describe how to verify a magnifer using a vertometer and why it is done this way.

Answer 28

Fit onto a vertometer backwards to approximate Fe

-magnifiers are thick lenses and FVP best approximates Fe

Question 29

Describe how to accurately verify a magnifier (4). What variable is an approximation of Fe in this case?

Answer 29

Set up an optical pench with an object of known h position >3m from the magnifier
Capture image on ground glass screen and measure h’ and calculate l’
-l’ is an approximation to Fe

Question 30

Describe how to triangulate to verify magnifiers (4).

Answer 30

Set up an optical pench with two penlights of known h position >3m from the magnifier
Focus onto ground glass screen and measure h’ and calculate l’

Question 31

Describe how a reading card can be used to verify magnifiers (4).

Answer 31

Place magnifier on a reading card
Observe print with relaxed accommodation a distance away
Add plus lens on top of the magnifier until image is blurred
The highest plus where image is clear is L’

Question 32

Describe how field of view can be used to verify a magnifier.

Answer 32

Measure A directly with a mm ruler on a magnifier then use the equation
W = A/zFe
W is the width of the field (mm)
A is the width of the magnifying lens (mm)
z is eye to lens distance (m)
F is power of lens (D)