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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.
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.
Small pupil and Lens
• Narrower cones of light.• Smaller circles of confusion• Larger depth of field.
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Retinal Surface
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Optic Disk
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Optical Conditions in the Eye
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Marmor & Ravin, 1997, p.3.
Eye ball
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Unger, 1982
Eye Chart?
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Human Eye: Two lenses
CorneaLensStrong (in air) WeakFixed Adjustable
Problems:Myopia PresbyopiaHyperopia YellowingAstigmatism Cataracts
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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
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Light Refraction in the Eye
Cornea -chief refractive surface of the eye
Lens -shape is changed by accommodation to focus images on the retina.
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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.
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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.
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Myopia
When the eye is myopic, objects close to the eye will be in focus.
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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