Depth of Field• Only one distance is

in “perfect “ focus.• But all sensors have

finite sizes• Can not tell whether

in perfect focus and a small circle of confusion

• Therefore always a range of distances in measurable focus.

.

Small pupil and Lens

• Narrower cones of light.• Smaller circles of confusion• Larger depth of field.

Retinal Surface

Optic Disk

Optical Conditions in the Eye

Marmor & Ravin, 1997, p.3.

Eye ball

Unger, 1982

Eye Chart?

Human Eye: Two lenses

CorneaLensStrong (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