Exposure control
The size of the aperture and the
brightness of the scene control the amount of light that enters the camera
during a period of time, and the shutter controls the length of time that the
light hits the recording surface. Equivalent exposures can be made with a
larger aperture and a faster shutter speed or a corresponding smaller aperture
and with the shutter speed slowed down.
Shutters
Although
a range of different shutter devices have been used during the development of
the camera only two types have been widely used and remain in use today.
The Leaf
shutter or more precisely the
in-lens shutter is a shutter contained within the lens structure, often close
to the diaphragm consisting of a number of metal leaves which are maintained
under spring tension and which are opened and then closed when the shutter is
released. The exposure time is determined by the interval between opening and
closing. In this shutter design, the whole film frame is exposed at one time.
This makes flash synchronisation much simpler as the flash only needs to fire
once the shutter is fully open. Disadvantages of such shutters are their
inability to reliably produce very fast shutter speeds (faster than 1/500th
second or so) and the additional cost and weight of having to include a shutter
mechanism for every lens.
The focal-plane
shutter operates as close to the
film plane as possible and consists of cloth curtains that are pulled across
the film plane with a carefully determined gap between the two curtains
(typically running horizontally) or consisting of a series of metal plates
(typically moving vertically) just in front of the film plane. The focal-plane
shutter is primarily associated with the single lens reflex type of cameras,
since covering the film rather than blocking light passing through the lens
allows the photographer to view through the lens at all times except during the exposure itself. Covering
the film also facilitates removing the lens from a loaded camera (many SLRs
have interchangeable lenses).
Complexities
Professional medium format SLR cameras (typically using 120/220 roll film) use a hybrid solution, since such a large focal-plane shutter would be difficult to make and/or may run slowly. A manually inserted blade known as a dark slide allows the film to be covered when changing lenses or film backs. A blind inside the camera covers the film prior to and after the exposure (but is not designed to be able to give accurately controlled exposure times) and a leaf shutter that is normally open is installed in the lens. To take a picture, the leaf shutter closes, the blind opens, the leaf shutter opens then closes again, and finally the blind closes and the leaf shutter re-opens (the last step may only occur when the shutter is re-cocked).
Using a focal-plane shutter, exposing
the whole film plane can take much longer than the exposure time. The exposure
time does not depend on the time taken to make the exposure over all, only on
the difference between the time a specific point on the film is uncovered and
then covered up again. For example an exposure of 1/1000 second may be achieved
by the shutter curtains moving across the film plane in 1/50th of a second but
with the two curtains only separated by 1/20th of the frame width. In fact in
practice the curtains do not run at a constant speed as they would in an ideal
design, obtaining an even exposure time depends mainly on being able to make
the two curtains accelerate in a similar manner.
When photographing rapidly moving
objects, the use of a focal-plane shutter can produce some unexpected effects,
since the film closest to the start position of the curtains is exposed earlier
than the film closest to the end position. Typically this can result in a
moving object leaving a slanting image. The direction of the slant depends on
the direction the shutter curtains run in (noting also that as in all cameras
the image is inverted and reversed by the lens, i.e. "top-left" is at
the bottom right of the sensor as seen by a photographer behind the camera).
Focal-plane shutters are also difficult
to synchronise with flash bulbs and electronic
flash and it is often only
possible to use flash at shutter speeds where the curtain that opens to reveal
the film completes its run and the film is fully uncovered, before the second
curtain starts to travel and cover it up again. Typically 35mm film SLRs could
sync flash at only up to 1/60th second if the camera has horizontal run cloth
curtains, and 1/125th if using a vertical run metal shutter.