Lenses with changeable focal lengths are called variable or zoom lenses. The
photographer pulls, pushes, or twists a sleeve around the barrel of the lens.
The aperture and focusing rings are coordinated and move in unison with the
focal-length sleeve. This lens is indispensable when shooting in conditions
that change too rapidly to manually change lenses.
Focal length is a determining factor in the coverage of the lens.
maximum image coverage at the focal plane of a lens is expressed in degrees
and known as the angle of field. The angle of field is the widest angle at
which light entering a lens produces a usable portion of illumination at the
focal plane. Beyond the angle of field, light becomes less intense and image
sharpness decreases. The angle of field is similar but not the same as the
angle of view. The angle of view (also in degrees) is the amount of image
coverage of a lens on the focal plane within a particular film size.
of view should not exceed the angle of field.
Figure 7-19 illustrates the different angles of view.
Figure 7-19.Angles of view
available with focal lengths shown.
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