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
Eye ball
Marmor & Ravin, 1997, p.3.
Eye Chart?
Unger, 1982
Human Eye: Two lenses
Cornea
Strong (in air)
Fixed
Problems:
Myopia
Hyperopia
Astigmatism
Lens
Weak
Adjustable
Presbyopia
Yellowing
Cataracts
Lens
In Focus
Light from
distant light
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.
Myopia = near-sighted
Distant
Object
Near Object
Lens
• With any lens, objects at different distances are imaged at
different distances behind the lens.
• In myopia, near objects may be in focus.