formed directly on the ground glass or focusing screen,
the lens forms the image on a mirror that reflects the
image to the focusing screen or ground glass.
­The twin-lens reflex
(TLR) system uses a matched set of lenses for focusing
and viewing. One lens is the viewing lens; the other is
the picture-taking lens. The viewing lens is always wide
open. That makes focusing and viewing easy, but depth
of field cannot be viewed. Depth of field must be
determined by a scale that is provided on the lens or
camera body.
An advantage of the twin-lens reflex system is that
the image is visible on the focusing screen, before,
during, and after exposure. A disadvantage of twin-lens
systems is that parallax errors occur. Parallax refers to
the difference between the image seen through the
viewing lens and the image transmitted to the
picture-taking lens (fig. 4-8). For distant subjects the
difference is not very great or noticeable; however,
when your subject is close to the camera, parallax is
much more noticeable. You see a different image area
through the viewing lens than what is being transmitted
through the picture-taking lens. Some twin-lens reflex
cameras have an indicator in the viewing lens, so you
can compensate for parallax. Another disadvantage of
the twin-lens reflex camera is that it takes practice to
follow action and compose the subject. The image seen
on the focusing screen is backwards from the actual
image. Twin-lens reflex cameras are no longer
commonly used in Navy imaging, but they are still
­Single-lens reflex
(SLR) cameras have a focusing and viewing system that
shows you the image formed by the picture-taking lens.
SLR cameras are designed so the distance between the
focusing screen and the lens is exactly the same as the
distance between the lens and the film; therefore,
whatever appears in focus on the focusing screen will
also be recorded in focus on the film. With an SLR
camera, there is no parallax error.
Sometimes two small prisms or a split screen is
included in the central area of an SLR camera viewing
screen. When the image is out of focus, it appears split
in this area (fig. 4-9). Some screens have a central grid
of minute prisms that produce a shimmering effect when
the image is out of focus.
An SLR camera is focused by rotating the focusing
ring on the lens until the image seen through the
viewfinder is in sharp focus. SLR cameras are the most
commonly used camera in Navy imaging today.
Direct-Vision Range Finder Focusing
Cameras that use direct-vision range finder
focusing produce a double image in the viewfinder until
the subject is in focus on the film plane. This system has
a coupled range finder optical device that is linked to the
focusing ring. To focus a direct-vision coincidence or
split-image range finder camera, you must align two
separate images of the subject. When looking through
the camera viewfinder, you see a pale or tinted area in
the center of the viewing window. This area shows the
double image. To set the correct focus, you aim the
camera so the subject you want in sharpest focus is in
the pale area. You then turn the lens focus ring, or camera

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