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Ophthalmic Optics > 10 - Spectacle Magnification (Exam 3) > Flashcards

10 - Spectacle Magnification (Exam 3) Flashcards

1
Q

How are anisometropia and antimetropia different?

A

Anisometropia is a difference in lens power between the two lenses
Antimetropia is a difference in sign of lens power between two lenses

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2
Q

What challenge does a large difference in lens power between eyes bring?

A

Difference in spectacle magnification

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3
Q

What is the formula for spectacle magnification?

A

Retinal image size (corrected) divided by Retinal image size (uncorrected) for the same eye

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4
Q

How is spectacle magnification expressed?

A

As a ratio (1.05 is equal to 5% magnification)

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5
Q

How is the normal image size taken?

A

The image size for an emmetropic eye with +60D refractive power

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6
Q

What 2 factors contribute to magnification (or minification) of an image?

A
  1. Power of the lens

2. Shape of the lens

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7
Q

What 2 parameters are included in the power factor of a lens?

A
  1. Vertex distance

2. Back vertex power

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8
Q

What 3 parameters are included in the shape factor of a lens?

A
  1. Lens thickness
  2. Index of refraction
  3. Front base curve
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9
Q

What is the formula for shape factor?

A

1 divided by 1-((t/n)f)

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10
Q

What is the formula for power factor?

A

1 divided by 1-dP

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11
Q

What are the units for lens thickness in the shape factor equation?

A

Meters

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12
Q

What are the units for vertex distance in the power factor equation?

A

Meters

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13
Q

What is spectacle magnification equal to?

A

Product of shape factor and power factor

shape factor)(power factor

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14
Q

What is the formula for relative spectacle magnification?

A

Image size for corrected ametropic eye divided by image size for standard emmetropic eye

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15
Q

What does Knapp’s Law state?

A

Refractive error caused by axial ametropia can be corrected by thin lenses positioned 15mm in front of the eyes

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16
Q

Do spectacle lenses follow Knapp’s Law?

A

No

They are not thin, flat lenses

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17
Q

What is the best way to correct patients with refractive ametropia without changing retinal image size?

A

Contact lenses

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18
Q

What is the normal magnification of an uncorrected emmetropic eye?

A

1.00

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19
Q

What do plus lenses give in terms of magnification?

A

Gain in magnification (>1.00)

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20
Q

What do minus lenses give in terms of magnification?

A

Loss in magnification (<1.00)

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21
Q

Where is the entrance pupil for the eye located?

A

3 mm behind the cornea

22
Q

How is the ‘d’ value found for the power factor of a lens?

A

vertex distance plus 3 mm

23
Q

For a myope, will CLs or spectacles result in a smaller loss of magnification?

A

Contact lenses

24
Q

For a hyperope, will CLs or spectacles result in a smaller gain of magnification?

A

Contact lenses

25
Q

What other problems result in symptoms similar to aneiseikonia?

A

Refractive error and oculomotor imbalance

26
Q

What are 4 indications of clinically significant aneiseikonia?

A
  1. High anisometropia/ high astigmatism
  2. Physical alterations to the eye (IOL, LASIK, etc.)
  3. Complaints of spatial distortion
  4. Monocular vision is more comfortable
27
Q

What is aneiseikonia?

A

Dissimilar retinal image size

28
Q

What are the 4 types of aneseikonia?

A
  1. Physiological aniseikonia
  2. Symmetrical aniseikonia
  3. Meridional aniseikonia
  4. Asymmetrical aniseikonia
29
Q

What is significant about physiological aniseikonia?

A

Allows for the perception of depth

30
Q

What is symmetrical aniseikonia?

A

Images equally larger or smaller in every meridian

31
Q

What is meridional aniseikonia?

A

Symmetrical, but meridional size difference in the eyes

Can be along any meridian

32
Q

What is noticed with asymmetrical aniseikonia?

A

Progressive increase or decrease across the visual field

33
Q

What are the 2 types of ametropia?

A
  1. Axial ametropia

2. Refractive ametropia

34
Q

What is the best way to correct ametropes with refractive ametropia to prevent aniseikonia?

A

Contact lenses

35
Q

How can you tell clinically whether a patient’s ametropia is axial or refractive?

A

Keratometry findings

Significant differences signal refractive ametropia

36
Q

What is the best way to correct anisometropes with high astigmatism?

A

Contact lenses

37
Q

What are 4 methods of reducing aniseikonia with spectacles?

A
  1. Short vertex distance
  2. Small eyesize frame
  3. Aspheric lenses
  4. High index lenses
38
Q

What may be necessary to correct with presbyopes that have aniseikonia?

A

Vertical imbalance

via Slab off

39
Q

What is another term for iseikonic lens?

A

Size lens

40
Q

When correcting aniseikonia, should aspheric lenses be used in just the right or left lens?

A

No

Aspheric lenses should be used OU

41
Q

What is the minimum center thickness for finished poly/trivex lenses?

A

1.0 mm thick

42
Q

What is the minimum center thickness for semi-finished poly/trivex lenses?

A

1.5 mm thick

43
Q

What is the expected base curve for a minus lens?

A

Flatter than +6.00D

44
Q

What is the expected base curve for a plus lens?

A

Steeper than +6.00D

45
Q

What is the minimum vertex distance for spectacles, and what does a smaller distance result in?

A

11 mm

Lash crash

46
Q

What is the industry standard for minimum center thickness of a lens?

A

2.0 mm

47
Q

At what percent of retinal image size difference can patients reliably fuse 95% of the time?

A

3.90 to 3.99%

48
Q

If you can’t get retinal image size difference to just below 4%, should you force it and risk poor cosmetics?

A

No

Patients are likely already monocular at this point

49
Q

What 4 components of a lens can be altered in iseikonic lens design?

A
  1. Front base curve
  2. Index of refraction
  3. Vertex distance
  4. Center thickness
50
Q

When dealing with aniseikonia, should you add magnification or minification?

A

Always add magnification to the eye that has less

51
Q

What are 3 ways to increase magnification in any lens?

A
  1. Steepen the front base curve
  2. Increase center thickness
  3. Increase vertex distance

Ophthalmic Optics (10 decks)

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