The mirror sets the image right side-up, but like
any mirrored image, the image is reversed left-to-
right. The image is projected on a ground glass
surface in the viewfinder for focusing and composi-
tion. Since the two lenses are mounted as one piece,
any focusing that takes place for the viewing lens is
mimicked in the photographic lens. The viewfinder
in a twin-lens reflex camera is most often a waist-
level finder, meaning the image is projected to the
ground glass mounted to the top of the camera. In
order to easily see the image in the ground glass,
the photographer must hold the camera at waist-
level. The ground glass image is typically shielded
for viewing by a set of metal leaves that form a box
around the ground glass.
Twin lens reflex cameras are typically made for
medium format roll films.
Single Lens Reflex
The most common modern camera type is the sin-
gle lens reflex camera. In a single lens reflex cam-
era, both viewing and photography takes place
through a single lens. A mirror deflects the image
coming through the lens and sends it to the prism
for viewing and composition. The prism of a single-
lens reflex camera is made up of a series of mirrors
and lenses to brighten the image and orient it prop-
erly for the viewer. When the shutter button is
depressed, the mirror swings out of the way to
allow the image to travel back to the film. Al-
though the most popular example of a single lens
reflex camera is 35-mm roll film format, most pro-
fessional grade medium format cameras today are
also single lens reflex.
Light Control
Exposure
Exposure is the amount of light needed to form
an image on film or other light sensitive materials.
In the early stages of photography, it was not
uncommon to require several minutes of exposure
to form an image. Technological advances in film
sensitivity and characteristics have reduced this
time to fractions of a second.
There are many different levels of sensitivity
available in today’s modern films. Film sensitivity,
often referred to as film speed, is measured in terms
of ISO (International Standards Organization).
The higher the ISO number, the more sensitive
the film is, and the less light it will need to make a
proper exposure. The lower the film speed, the
more light it will need to obtain a proper exposure.
For example, a film with an ISO of 800 will allow
the photographer to capture images with shorter
exposures than will a film with a speed of 400. ISO
numbers correspond to their sensitivity; ISO 400
film needs twice as much light to make the same
exposure as ISO 800 film.Light Meters
In order to determine how much light is needed
for an exposure, a photographer must use a light
meter. Light meters are most often integrated into
the camera system, either informing the photogra-
pher of the proper exposure for that film and
scene, or setting that exposure automatically. In
order for the light meter in the camera to function
properly, the meter must know the sensitivity, or
ISO, of the film being used. Many modern film
manufacturers mark the film canister with an
electronically read code, called a DX code, so
that a DX-code enabled camera can automatically
set the meter to the proper ISO.Shutter and Aperture
Once the amount of exposure has been deter-
mined, cameras employ two controls to obtain
that exposure, shutter and aperture. The shutter is
a device inside the camera that controls the amount
of time that the film is exposed to light. Because of
today’s high film speeds, shutters must travel very
quickly, exposing the film in fractions of a second.
A typical shutter speed range could be from^1 = 2000 of
a second all the way up to one second. Many
cameras set shutter speed in increments so that
each speed is twice or half the one next to it. For
example, the next speed up from^1 = 500 of a second
would be^1 = 1000 (half the amount of time), and the
next down would be^1 = 250 (twice the amount of time).
These increments are commonly referred to as
stops. Most commonly, shutters are located inside
the camera body directly in front of the film. These
types of shutters are called focal plane shutters, and
are usually constructed of a series of cloth curtains
or metal leaves. Less common is the leaf shutter,
typically found in view cameras and professional
grade medium format cameras. A leaf shutter is
constructed of lightweight metal in a circular, iris-
like fashion.
The other camera control used to control expo-
sure is the aperture. Located in the lens, the aperture
is an iris-like device that is set to permit a certain
amount of light. When the blades of the aperture are
closed down, a very small hole is created for light to
pass through. When the blades of the aperture are
opened up, a larger hole is created, therefore allow-
ing more light to pass through. Openings of apertureCAMERA: AN OVERVIEW