object point gets farther away from the lens, the angle
of spread becomes less and less until a distance is
reached at which the rays from a single point, for all
practical purposes, can be considered parallel. This
distance is known by the term infinity. For all practical
purposes, light rays from a distant object or an object at
600 or more feet away may be considered to be parallel.
But this is only for practical purposes. When very long
focal-length lenses or telephoto lenses are being
considered, the distance of 600 feet may be much less
than infinity. In other words, infinity is a distance so far
removed from the camera lens that the rays of light
reflected to the lens from a point at that distance may be
regarded as parallel. Infinity is expressed by the
symbol
and is a setting on a camera focusing scale.
The manner in which light rays are refracted by a
lens determines the focal length. This refraction, in turn,
depends on the nature of the glass used in the elements,
the curvatures of the element surfaces, and the
separation of the elements. The first two factors are fixed
quantities once the lens is manufactured, but the third
factor may be changed individually in certain lenses.
In zoom lenses the distance separating the lens
elements can be changed. In convertible lenses, portions
or elements of the lens can be used by themselves. In
either method, the focal length of the lens can be
changed. When one of these two conditions cannot be
met, the focal length is fixed and constant.
Photographic lenses are measured according to their
focal length which is normally imprinted somewhere on
the lens mounting (usually the front surface of the lens
barrel). This focal length information is sometimes
given in inches, sometimes in millimeters, and
occasionally in both systems. Focal length is frequently
used to indicate the size of a lens. Thus, a lens labeled
as an 8-inch lens indicates that when it is focused on a
point at infinity, the distance from its optical center to
the focal plane is 8 inches.
The focal length of a photographic lens dictates the
size of the image produced by the lens at a given
lens-to-subject distance. Focal length also determines
the minimum distance between the lens and the focal
plane. The normal focal length of a lens (normal lens)
for a camera is approximately equal to the diagonal
dimension of the film being used. Since the diagonal
dimension of a 4x5 film is 6.4 inches, a lens about
6 inches is a normal lens for such film.
Lenses with a longer than normal focal length may
be used on a camera, provided the distance from the lens
to the film can be increased sufficiently to accommodate
the increase in focal length. Lenses shorter than the
normal focal length may also be used, provided they are
designed to meet the constraints of the camera and film
size.
FOCAL LENGTH AND IMAGE SIZE.
­When
you photograph the same object at the same distance, a
lens with a long-focal length produces a larger image
than one with a short-focal length. In effect, the longer
focal-length lens magnifies or brings the subject closer
to the camera without changing the camera-to-subject
at a distance of 25 feet from three cameras, one equipped
with a 6-inch lens, one with a 12-inch lens, and one with
a 24-inch lens. The 6-inch lens produces a 1 1/2-inch
image of the man. The 12-inch lens produces an image
that is 3 inches high. The 24-inch lens produces a 6-inch
image. From this example, it is obvious that the longer
the focal length of the lens, the larger the image size of
a given object from a given lens-to-subject distance.
FOCAL LENGTH AND SUBJECT COVER-
AGE.
­Focal length and subject coverage go hand in
hand-just as do focal length and image size. But,
whereas image size increases with increased focal
length, coverage decreases with increased focal length.
We can consider coverage as the amount of subject
matter included in a given format film size from a given
lens-to-subject distance. With two cameras-each with a
different focal-length lens-at the same distance from the
same subject, the camera with the shortest focal-length
lens includes the greatest subject area-the camera with
the longest focal-length lens the least subject area
(fig. 1-20).
Angle of Field.
­The focal length of a lens is a
determining factor in the coverage of that lens. The
maximum coverage at the focal plane of a lens is
expressed in degrees as the angle of field. Angle of field
is the widest angle at which light entering a lens
produces a usable portion of the circle of illumination
at the focal plane. Light around the edges of the entire
circle falls off in intensity before disappearing
completely. The usable portion of this circle is called the
circle of good definition.
The maximum size of film you can use with a lens
depends on the angle of field because any part of the
film outside the circle of good definition produces an
indistinct image.
Angle of field is a basic optical condition that is
approximately equal for all normal focal-length lenses.
A normal lens, as it is called, has an angle of field of
about 45 degrees to 55 degrees. This angle of field
1-15

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