Shutter (photography)
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In photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to the right amount of light to create a permanent image of a view. A shutter can also be used to allow pulses of light to pass outwards, as in a movie projector or signal lamp.
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[edit] Camera shutters
Camera shutters are normally of two basic types:
- Central shutters, shutters mounted within a lens, or more rarely behind or even in front of a lens. One such common shutter is the leaf shutter.
- Focal plane shutters, shutters mounted near the focal plane
Central shutters usually have a diaphragm mechanism which progressively dilate to a circular opening the size of the lens, then stay open as long as is required, and finally close. Ideally the opening and closing are instantaneous; in reality this cannot be so. The time taken to dilate, and then to contract, places a lower limit on the exposure time. A less obvious property is that at the highest speeds the shutter is fully open for only a fraction of the exposure; the effective aperture is less, and the depth of field greater, than at lower speeds.
Shutters immediately behind the lens were used in some cameras with limited lens interchangeability. Shutters in front of the lens were used in the early days of photography.
Focal-plane shutters are usually implemented as a pair of cloth, metal, or plastic curtains which shield the film from light. For exposures of, typically, 1/30th of a second or more, one curtain opens, and the second one later closes. For shorter exposures, the two curtains move simultaneously, but leaving a slit-shaped opening through which light can pass. The speed of motion of the curtains and the width of the slit are adjusted so that each part of the film is exposed to light for the required time (the effective exposure), although the assembly may take an appreciable time (typically 1/30") to traverse the film. The effective exposure time can be much shorter than for central shutters.
Focal plane shutters have the advantages of enabling much shorter exposures, and allowing the use of interchangeable lenses without requiring the expense of a separate shutter for each lens. They have the disadvantage of distorting the images of fast-moving objects: although no part of the film is exposed for longer than the time set on the dial, one edge of the film is exposed an appreciable time after the other, so that a horizontally moving shutter will, for example, elongate or shorten the image of a car speeding in the same or the opposite direction to the shutter movement.
Other mechanisms than the dilating aperture and the sliding curtains have been used; anything which exposes the film to light for a specified time will suffice.
The time for which a shutter remains open, the exposure time, is determined by a timing mechanism. These were originally mechanical, but since the late twentieth century are mostly electronic.
The exposure time and the effective aperture of the lens must together be such as to allow the right amount to reach the film or sensor. Additionally, the exposure time must be suitable to handle any motion of the subject. Usually it must be fast enough to "freeze" rapid motion; sometimes a controlled degree of blur is desired, to give a sensation of movement.
Most shutters generate a signal to trigger a flash, if connected. This was quite a complicated matter with mechanical shutters and flashbulbs which took an appreciable time to reach full brightness, but is simple with electronic timers and electronic flash units which fire virtually instantaneously.
Cinematography uses a rotary disc shutter in movie cameras, a continuously spinning disc which conceals the image with a reflex mirror during the intermittent motion between frame exposure. The disc then spins to an open section that exposes the next frame of film while it is held by the registration pin.
[edit] Shutter lag
Shutter lag is the time between pressing the shutter release and the camera responding by taking the picture. Ironically, while this delay was insignificant on most film cameras, some digital cameras have shutter lag times on the order of hundreds of milliseconds, which may be a minor annoyance to the user.
[edit] Projector shutters
In movie projection, the shutter admits light from the lamphouse to illuminate the film across to the projection screen. To avoid flicker, a double-bladed rotary disc shutter admits light two times per frame of film. There are also some models which are triple-bladed, and thus admit light three times per frame (see Persistence of vision).
Shutters can also be used simply to regulate pulses of light, with no film being used, as in a signal lamp.
