optical conditions in the eye. marmor & ravin, 1997, p.3. eye ball
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Optical Conditions in the Eye
Marmor & Ravin, 1997, p.3.
Eye ball
Unger, 1982
Eye Chart?
Human Eye: Two lenses
Cornea LensStrong (in air) WeakFixed Adjustable
Problems:Myopia PresbyopiaHyperopia YellowingAstigmatism Cataracts
Lens
Lens
Light fromdistant light
In Focus
Image Size
• A positive lens collects light over a large area
• Cones of light.
• Point of cone: focused
• Cone diameter varies from point
Light Refraction in the Eye
Cornea -chief refractive surface of the eye
Lens -shape is changed by accommodation
to focus images on the retina.
Emmetropia (Normal)
When the eye is emmetropic, the eye is exactly the right size so that the image formed by the optics in the front falls on the retina.
Myopia (Nearsighted)
When the eye is myopic, the eye is too long and the point of focus is in front of the retina. A blurry image falls on the retina.
Myopia
When the eye is myopic, objects close to the eye will be in focus.
Lens
Near Object
DistantObject
• With any lens, objects at different distances are imaged at different distances behind the lens.
• In myopia, near objects may be in focus.
Myopia = near-sighted
Negative Lens: Spreads Light Out.
Myopia corrected
Myopia is corrected with a negative lens that compensates for the excess power of the cornea and lens. The focus point is moved backwards.
Surgical Corrections• RK: small cuts around edge of cornea
• PRK: laser reshapes surface of cornea
• LASIK: laser reshapes internal layers of cornea
• Implantable lens and rings
Hyperopia (Farsighted)
When the eye is hyperopic, the eye is too short and the point of focus is in behind of the retina. A blurry image falls on the retina.
Positive Lens: Focuses Light; Magnifier
Hyperopia corrected
Hyperopia is corrected by placing an additional, positive lens in front of the eye. The stronger the lens, the more the focus point is moved forward.
Hyperopia: Corrected
Spherical Aberration
• Human cornea is “aspherical”
• Human lens has progressive index of refraction
Current Research Question:• Why are some eyes myopic or hyperopic?
• Why are so many eyes emmetropic?
• Not random as after birth % emmetropic increases.
• Most cases of myopia start later, in teens. Associate with “close-work”.
• Two theories:– Extra muscular forces for accommodation etc.– Misguided growth
Astigmatism
Christman, 1971, p.150.
Zakia, 1997, p.220.
Radial
Accommodation
Marmor & Ravin, 1997, p.16
The Lens & Accommodation
The lens changes shape to focus objects at different distances.
Can also compensate for some myopia, hyperopia
Marmor & Ravin, 1997, p.17
PresbyopiaPresbyopia
Gregory, 1997, p.39.
Accommodation vs Age
• Fixed Focus by 50
• “Presbyopia”
Hyperopia and Accommodation
When young, hyperopia may compensated by making the lens more round.
Hyperopia & Accommodation
Myopia and Accommodation
When young, myopia may not be obvious as it can be compensated by flattening the lens.
Myopia & Accommodation
• Presbyopia
• Yellowing
• Opaque (Cataracts)
The Lens and Aging
.
Cataract: young
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