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Optics Of The Eye Block 6 > Formation Of Retinal Image > Flashcards

Formation Of Retinal Image Flashcards

1
Q

Refractive index of cornea

A

1.376

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2
Q

Where is there a one to one correspondence of photoreceptors to retinal ganglion cells

A

Fovea

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3
Q

Where is there not a one to one correspondence of photoreceptors to retinal ganglion cells

A

Periphery

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4
Q

How is vision in the periphery compared to central

A

Worse

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5
Q

In the fovea, one ganglion cell is connected to ________ photoreceptors

A

1

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6
Q

What is the visual angle subtended by a foveal cone

A

0.5 min of arc

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7
Q

What is the approximate width of a foveal cone?

A

2.5 microns

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8
Q

Approximately how many cone photoreceptors end-to-end would be used to see the entire length of a 20/200 snellen E?

A

100

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9
Q

Which photoreceptors absorb light with a wavelength of approximately 560nm?

A

L cones

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10
Q

Where are there more rods

A

Periphery

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11
Q

Where are there more cones

A

Fovea

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12
Q

Where are there no cones

A

Optic disk

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13
Q

1 foveal cone equals

A

2.5 microns (0.5 min of arc)

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14
Q

How many cones can fit into a 20/20 letter

A

10 cones

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15
Q

How many cones for a 20/10 E?

A

5 cones

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16
Q

How many degrees is the ONH?

A

5 degrees

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17
Q

What are the dimensions of the ONH?

A
  1. 76mm H

1. 92mm V

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18
Q

How many arc min for moon

A

30

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19
Q

How big is the blind spot?

A

5x the size of the anglular size of the moon

Moon=0.5 degree*5=2.5 degree blind spot

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20
Q

L cones

A

558 wavelength

Called red cones but they are actually greenish

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21
Q

M cones

A

531 wavelength

Green cones, but actually yellowish

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22
Q

S cones

A 
419 wavelength 
Blue cones (actually violet)
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23
Q

Rods peak sensitivity

A

500 wavelength (sky blue)

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24
Q

What is RG colorblind actually

A

Shift of L and M cones, not so red

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25
Q

What is the visible wavelength

A

400 (violet) to 700 (red)

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26
Q

What is the difference in appearance of the cones in the fovea vs the periphery

A

In the fovea, they are smaller and tightly compressed, in the periphery, they are more spread out and appear larger due to them not being compressed together. Rods are between them in the periphery

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27
Q

Retinal sampling limits, Nyquist frequency

A

Need sample size of at least 1:1
-need to be able to sample various features of the E to tell what it is. If its only the size of 1 photoreceptor, wont be able to tell what it is

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28
Q

Does the eye stay still when you are looking at an image?

A

No, eye does not sit still, brain takes in signals from all the photoreceptors moving around and interprets it.
-this is why if you hold your eye still on an object, it seems to fade. Movement is what keeps the image in sight

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29
Q

Sampling not frequent enough that you can tell what it is. Assuming its a different shape than what it is.

A

Aliasing

The stimulation is too frequent for your eye to pick up every piece of it so it only picked up certain points and connects those points together, creating a different shape

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30
Q

A plus spectacle lens causes

A

Magnification

31
Q

A minus spectacle lens causes

A

Minification

32
Q

A contact lens causes

A

No magnification (or minification)

33
Q

Axial myopia

A

Eye is too long

34
Q

Refractive myopia

A

Lens is too strong

35
Q

Axial hyperopia

A

Eye is too short

36
Q

Refractive hyperopia

A

Eye too weak, not enough power

37
Q

What type of ametropia do we use keratometry for

A

Refractive myopia/hyperopia. Checks to see if the curvature of the cornea is the cause of the refractive error

38
Q

Uncorrected image in axial myopia

A

Larger

39
Q

Image corrected with spectacles in axial myopia

A

Same size

40
Q

Image when corrected with CL in axial myopia

A

Larger

41
Q

Uncorrected image in axial hyperopia

A

Smaller

42
Q

Image corrected with spectacles in axial hyperopia

A

Same size

43
Q

Image corrected with contacts in axial hyperopia

A

Smaller

44
Q

Uncorrected image in refractive hyperopia

A

Same

45
Q

Image corrected with spectacles in refractive hyperopia

A

Larger

46
Q

Inmage corrected with CL in refractive hyperopia

A

Same

47
Q

Image in uncorrected refractive hyperopia

A

Same

48
Q

Image corrected with spectacles in refractive hyperopia

A

Larger

49
Q

Image corrected with CL in refractive hyperopia

A

Same

50
Q

Uncorrected image in refractive myopia

A

Same

51
Q

Image corrected with spectacles in refractive myopia

A

Smaller

52
Q

Image corrected with CL in refractive myopia

A

Same

53
Q

What is the best type of correction for someone with refractive problems?

A

Contact lenses

54
Q

What is the best type of correction for someone with axial problems

A

Spectacles

55
Q

When people come in with the complaint that they have HA with their glasses on and it goes away when they take them off, what should tou probably do?

A

Switch them to CL

56
Q

Knapp’s law if the ametropia is axial

A

Retinal image will be the same as emmetropic if corrected with spectacles. Uncorrected is larger/smaller than emmetropia

57
Q

Knapp’s law if the ametropia is refractive

A

Uncorrected image size is same as emmetropia. CL lens will correct the blur and not change image size

58
Q

According to Knapp’s law, we use _______ to correct axial aniseikonia

A

Specs

59
Q

According to Knapp’s law, we use ___________ to correct for refractive aniseikonia

A

Contact lenses

60
Q

Why may Knapp’s law not always work

A

Due to photoreceptor spacing

61
Q

Are CL or specs better for anisometropia?

A

CL

62
Q

Why are CL better for anisometropic

A
  • better appearance
  • prismatic and centraction problems in specs
  • better image quality
63
Q

In axial myopia, the size of the image on the retina is ______ than in emmetropia

A

Larger

64
Q

In axial hyperopia, the size of the image on the retina is _______ than in emmetropia

A

Smaller

65
Q

In refractive myopia and hyperopia, the retinal image is __________ as in emmetropia (when corrected with CL)

A

Same size

66
Q

If the eye is the same size (as in if there is any error, it is refractive and not axial) then the image is

A

The same size

67
Q

A defect of binocular vision in whihc the two retinal images of an object differ in size

A

Aniseikonia

68
Q

What are the top complaints of people with aniseikonia

A
  • headaches are number one

- asthenopia

69
Q

At what values does aniseikonia becaimse clinically significant

A

When the magnification value difference between the eyes is 3-5%

70
Q

What can differing spacing of photoreceptors and other retinal elements do

A

Effect perceived retinal image sizes

71
Q

Longer eyes can result in what

A

Stretched retina and wider spacing

-this compensates for differences in retinal image sizes present in axial anisometropia

72
Q

How do you find spectacle magnification?

A

(Mpower)*(Mshape)

Mpower= 1/(1-(d*back vertex power))

Mshape= 1/(1-(t/n)*front surface power)

73
Q

A lens clock reads +5D on the front surface of the polycarbonate lens (n=1.6) with a 6.0mm center thickness. The lens clock is calibrated for a 1.53 refractive index. What is the approximate mag due to the shape factor?

A

Divide the two RI and then multiply that by 5=5.66D

Then do the Mshape formula and get the percentage from that

2%

74
Q

A patient has the following refractive error and K readings:
MRx: OD -2.00, OS -5.00
Kreadings: OD 43.25, OS 43.25

Is this anisometropia axial or refractive? Which eye has a larger image? If our goal is to equalize retinal image size, should we Rx specs or CL?

A

Axial
OS larger
Specs

Optics Of The Eye Block 6 (7 decks)

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