incorporated in a single circular piece of glass (fig.
1-16).
CHARACTERISTICS OF LENSES
There are several factors that must be included when
you are considering the characteristics of lenses. To
perform well as a Navy photographer, you must
recognize the effect of these lens characteristics. Realize
also that it is the recognition and use of these various
lens features and/or qualities that can make the
difference between good and poor photography. You
must learn to recognize the photographic effect of these
characteristics and be able to apply them to produce top
quality photography. Finally, you must learn how some
of the lens characteristics may limit photographic
quality or operational capability.
Lens Focal Length
In photography, lens focal length is the distance
between the optical center of a lens and the focal plane
(film plane) of the camera when the lens is focused at
infinity (fig. 1-17). To understand this definition, you
must fully understand the terms focal plane, optical
center, and infinity.
Figure 1-17.–Focal plane and optical center.
Figure 1-18.–Effects of lens-to-subject distance on light rays.
Focal plane-The surface (plane) on which an
image transmitted by a lens is brought to sharp focus;
the surface or area at the back of the camera occupied
by the film.
Optical center-The optical center of a lens is a
point, usually (although not always) within a lens, at
which the rays of light from two different sources
entering the lens are assumed to cross.
Infinity-This term is not easily described. When
light is reflected from the point of an object, the closer
the point is to the lens, the greater is the angle of the
spread of light rays from the object (fig. 1-18). As the
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