Camera
lens
[Film Technology]
No lens
works perfectly. It is far from simple to grind glass to the ideal shapes, a
number of problems arises like chromatic aberrations
and the images of object
not directly in front of the lens are distorted and light
from even a centrally placed object is only moderately well refocused to a
point. Many of these defects can be reduced by replacing the simple convex lens by a system consisting of a number of lens elements, made of different type of glass. Such
one correcting the faults in the others. A typical camera lens may consists of
between four and eight such elements. The lens is as important a part of a
camera as the camera body. Lenses are referred
to in generic terms as wide-angle, normal, and telephoto.
The three terms refer to the focal length of the
lens, which is customarily measured in millimetres.
Focal length is defined as the distance from
the centre of the lens to the image it forms
when the lens is set at infinity. In practice, focal length affects the field of view, magnification,
and depth of field of a lens. Cameras used by professional photographers
and serious amateur photographers are designed to accept all three lens types inter
changeably. In 35-mm photography, lenses with focal lengths from 20 to 35 mm are considered
wide-angle lenses.
They provide greater depth of field and encompass a larger field of view(or angle) but provide relatively low magnification. Extreme wide-angle, or fisheye, lenses provide fields of
view of 180° or more. A 6-mm fisheye lens made by Nikon has a 220° field of view that produces a circular image on film,
rather than the normal rectangular or square image. Lenses with focal lengths of 45 to 55 mm are referred to
as normal lenses
because they produce an image that approximates the perspective perceived by
the human eye. Lenses with longer focal lengths, called telephoto lenses, constrict
the field of view and decrease the depth of field while greatly magnifying the image. For a 35-mm camera, lenses with
focal lengths of 85 mm or more are considered telephoto. A fourth generic lens
type, the zoom lens, is designed to have a variable focal length, which can be
adjusted continuously between two fixed limits. Zoom lenses are especially
useful in conjunction with single-lens reflex
cameras, for which they allow continuous
control of image scale.
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