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Eyes with
normal vision see images at a close distance as well as images at a
far distance clearly.
The anatomy of the normal human eye, seen from the side, is shown in
Figure 1. The foremost part of the eye is transparent and is called
the cornea. The iris with the pupil lies to the right of the cornea
as shown in the figure. The lens is located directly behind the iris
and pupil, and the retina is shown on the very right of the figure.
Figure 1: Anatomy of an Eye With Normal Vision
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The cornea, pupil,
and lens together produce a focused image of the environment on the
retina. In the same manner, the lens of a camera produces a sharp image
of the environment on the film.
Before taking a picture, the lens of the camera neeeds to be focused
to produce a sharply focused image (modern cameras have a so-called
"autofocus", a function with which focusing is done automatically).
The normal human eye focuses on images by automatically changing the
shape of the eye lens. A normal eye always has sharply focused distant
vision. For close-up vision, for example for reading, the eye lens changes
shape automatically so that sharply focused vision is also possible
for close-up images. The lens's ability to change shape decreases with
age (approximately after the age of 40).
Figure 3: The cornea, pupil, and lens together produce
a focused image of the environment on the retina.
If the different components of the eye are not precisely in tune, this
condition is referred to as refractive error. This is a collective term
for nearsightedness (myopia), farsightedness
(hyperopia) and astigmatism.
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