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