essential skills: digital photography in available light
Duration
Light duration is controlled by the shutter. The time the image sensor is exposed to light is
measured in whole and fractions of a second. This time is regulated by the shutter speed. Until the
invention of the focal plane shutter, exposure time had been controlled by devices either attached
to or within the lens itself. These shutters regulated the length of exposure. Early cameras had
no shutter at all and relied upon the photographer removing and replacing a lens cap to facilitate
correct exposure times. Other rudimentary shutters, very similar in appearance to miniature roller
blinds, were tried but it was not until the invention of a reliable mechanical shutter that exposure
times could be relied upon. As fi lm emulsions became faster (increased sensitivity to light) so did
the opportunity to make shorter exposures. Within a relatively brief period exposures were no
longer in minutes but in fractions of a second.
The shutter
The main types of shutter systems used in photography are the digital shutter, the leaf shutter and
the focal plane shutter. The primary function of all systems is exposure. When the shutter is released
it opens for the amount of time set on the shutter speed dial or LCD. These fi gures are in whole and
fractions of seconds. The length of time the shutter is open controls the amount of light reaching the
image plane. Each shutter speed doubles or halves the amount of light reaching the image sensor.
Leaving the shutter open for a greater length of time will result in a slower shutter speed. Shutter
speeds slower than 1/60 second, using a standard lens, can cause motion blur or camera shake
unless you brace the camera, mount it on a tripod or use some form of image stabilization. Using a
shutter speed faster than 1/250 or 1/500 second usually requires a wide aperture, more light or an
image sensor of increased sensitivity (raised ISO). These measures are necessary to compensate
for the reduced amount of light passing through a shutter open for a short period of time.
The same exposure or level of illumination can be achieved using different
combinations of aperture and shutter speed. For example, an exposure of f11
@ 1/125 second = f8 @ 1/250 second = f16 @ 1/60 second and so forth.Digital shutter
The duration of the exposure can be controlled entirely by electronic means by switching on and
switching off the image sensor for the prescribed amount of time. Many digital SLR cameras,
however, use a hybrid system of digital and focal plane shutters.
Mark Galer