LV - Near Low Vision Aids I - Week 4 Flashcards

1
Q

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

A

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

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2
Q

What print size should be aimed for when prescribing magnification?

A

Two lines spare

-if N8 required, aim for N5

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3
Q

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

A

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

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4
Q

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

A

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

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5
Q

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

A

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

  • relative size
  • relative distance
  • angular
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6
Q

Define relative distance magnification.

A

Distance between the eye and object is reduced

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7
Q

Define angular magnification.

A

Optical devices used to increase visual angle

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8
Q

When combining magnification, is it additive or multiplicative?

A

Multiplicative

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9
Q

List the two types of magnification.

A

Relative/effective

Rated/conventional

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10
Q

Which type of magnification is most used by clinicians?

A

Relative

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11
Q

Define relative magnification.

A

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

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12
Q

What is usually the standard reference distance?

A

25cm

-sometimes 40cm

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13
Q

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

A

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

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14
Q

List 6 near magnification options.

A
High additions
Hand magnifiers
Stand magnifiers
Near telescopes
Handheld electronics
Desk electronics
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15
Q

Name two advantages and disadvantage of high addition spectacles.

A

A - hands free, large field of view

D - short working distance

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16
Q

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

A

No, it acts as a converging system

  • as reading material brought closer, retinal image size increases
  • lens produces negligible amount of magnification
17
Q

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?

A

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

18
Q

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

A

Combination of two convex lenses separated by an airspace

Provides high mag without peripheral aberrations

19
Q

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

A

Power of the lens + 2

20
Q

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

A

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

21
Q

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

A

Correct working distance

22
Q

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

A
Fatigue - arms/neck
Nausea/dizziness
Eye fatigue
Headaches
No mobility
23
Q

List 7 advantages of spectacles for magnification.

A
Acceptance?
Cosmesis
Largest field of view
May be used binocularly
Hands free
Better for prolonged reading
Astigmatism corrected
24
Q

List 6 disadvantages of spectacles for magnification.

A

With increasing power:

  • working distance decreases
  • depth focus decreases
  • lighting more critical
  • aberrations increase
  • lens size decreases
  • reduced reading speed
25
Q

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

A

Longer working distances

Hands not free

26
Q

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

A

Aspheric

27
Q

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?

A

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

28
Q

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

A

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

29
Q

List 8 advantages of simple handheld magnifiers.

A
Portability
Relatively inexpensive
Familiarity/acceptance
Easy to prescribe
Extended wdist
Allows head movement
Quick spotting
Illumination available
30
Q

List 4 disadvantages of simple handheld magnifiers.

A

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