Formation Of Retinal Image Flashcards
Refractive index of cornea
1.376
Where is there a one to one correspondence of photoreceptors to retinal ganglion cells
Fovea
Where is there not a one to one correspondence of photoreceptors to retinal ganglion cells
Periphery
How is vision in the periphery compared to central
Worse
In the fovea, one ganglion cell is connected to ________ photoreceptors
1
What is the visual angle subtended by a foveal cone
0.5 min of arc
What is the approximate width of a foveal cone?
2.5 microns
Approximately how many cone photoreceptors end-to-end would be used to see the entire length of a 20/200 snellen E?
100
Which photoreceptors absorb light with a wavelength of approximately 560nm?
L cones
Where are there more rods
Periphery
Where are there more cones
Fovea
Where are there no cones
Optic disk
1 foveal cone equals
2.5 microns (0.5 min of arc)
How many cones can fit into a 20/20 letter
10 cones
How many cones for a 20/10 E?
5 cones
How many degrees is the ONH?
5 degrees
What are the dimensions of the ONH?
- 76mm H
1. 92mm V
How many arc min for moon
30
How big is the blind spot?
5x the size of the anglular size of the moon
Moon=0.5 degree*5=2.5 degree blind spot
L cones
558 wavelength
Called red cones but they are actually greenish
M cones
531 wavelength
Green cones, but actually yellowish
S cones
419 wavelength Blue cones (actually violet)
Rods peak sensitivity
500 wavelength (sky blue)
What is RG colorblind actually
Shift of L and M cones, not so red
What is the visible wavelength
400 (violet) to 700 (red)
What is the difference in appearance of the cones in the fovea vs the periphery
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
Retinal sampling limits, Nyquist frequency
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
Does the eye stay still when you are looking at an image?
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
Sampling not frequent enough that you can tell what it is. Assuming its a different shape than what it is.
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
A plus spectacle lens causes
Magnification
A minus spectacle lens causes
Minification
A contact lens causes
No magnification (or minification)
Axial myopia
Eye is too long
Refractive myopia
Lens is too strong
Axial hyperopia
Eye is too short
Refractive hyperopia
Eye too weak, not enough power
What type of ametropia do we use keratometry for
Refractive myopia/hyperopia. Checks to see if the curvature of the cornea is the cause of the refractive error
Uncorrected image in axial myopia
Larger
Image corrected with spectacles in axial myopia
Same size
Image when corrected with CL in axial myopia
Larger
Uncorrected image in axial hyperopia
Smaller
Image corrected with spectacles in axial hyperopia
Same size
Image corrected with contacts in axial hyperopia
Smaller
Uncorrected image in refractive hyperopia
Same
Image corrected with spectacles in refractive hyperopia
Larger
Inmage corrected with CL in refractive hyperopia
Same
Image in uncorrected refractive hyperopia
Same
Image corrected with spectacles in refractive hyperopia
Larger
Image corrected with CL in refractive hyperopia
Same
Uncorrected image in refractive myopia
Same
Image corrected with spectacles in refractive myopia
Smaller
Image corrected with CL in refractive myopia
Same
What is the best type of correction for someone with refractive problems?
Contact lenses
What is the best type of correction for someone with axial problems
Spectacles
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?
Switch them to CL
Knapp’s law if the ametropia is axial
Retinal image will be the same as emmetropic if corrected with spectacles. Uncorrected is larger/smaller than emmetropia
Knapp’s law if the ametropia is refractive
Uncorrected image size is same as emmetropia. CL lens will correct the blur and not change image size
According to Knapp’s law, we use _______ to correct axial aniseikonia
Specs
According to Knapp’s law, we use ___________ to correct for refractive aniseikonia
Contact lenses
Why may Knapp’s law not always work
Due to photoreceptor spacing
Are CL or specs better for anisometropia?
CL
Why are CL better for anisometropic
- better appearance
- prismatic and centraction problems in specs
- better image quality
In axial myopia, the size of the image on the retina is ______ than in emmetropia
Larger
In axial hyperopia, the size of the image on the retina is _______ than in emmetropia
Smaller
In refractive myopia and hyperopia, the retinal image is __________ as in emmetropia (when corrected with CL)
Same size
If the eye is the same size (as in if there is any error, it is refractive and not axial) then the image is
The same size
A defect of binocular vision in whihc the two retinal images of an object differ in size
Aniseikonia
What are the top complaints of people with aniseikonia
- headaches are number one
- asthenopia
At what values does aniseikonia becaimse clinically significant
When the magnification value difference between the eyes is 3-5%
What can differing spacing of photoreceptors and other retinal elements do
Effect perceived retinal image sizes
Longer eyes can result in what
Stretched retina and wider spacing
-this compensates for differences in retinal image sizes present in axial anisometropia
How do you find spectacle magnification?
(Mpower)*(Mshape)
Mpower= 1/(1-(d*back vertex power))
Mshape= 1/(1-(t/n)*front surface power)
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?
Divide the two RI and then multiply that by 5=5.66D
Then do the Mshape formula and get the percentage from that
2%
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?
Axial
OS larger
Specs
