Camera shutter
Camera shutters can be fitted in several positions: * Leaf shutters are usually fitted within a lens assembly (''central shutter''), or more rarely immediately behind (''behind-the-lens shutter'') or, even more rarely, in front of a lens, and shut off the beam of light where it is narrow. * Focal-plane shutters are mounted near the focal plane and move to uncover the film or sensor. Behind-the-lens shutters were used in some cameras with limited lens interchangeability. Shutters in front of the lens, sometimes simply a lens cap that is removed and replaced for the long exposures required, were used in the early days of photography. 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 (exposure time, often called "shutter speed") is determined by a timing mechanism. These were originally pneumatic (Compound shutter) orFocal-plane shutter
A focal-plane shutter is positioned just in front of the film, in the focal plane, and moves an aperture across the film until the full frame has been exposed. Focal-plane shutters are usually implemented as a pair of light-tight cloth, metal, or plastic curtains. For shutter speeds slower than a certain point (known as the X-sync speed of the shutter), which depends on the camera, one curtain of the shutter opens, and the other closes after the correct exposure time. At shutter speeds faster than the X-sync speed, the top curtain of the shutter travels across the focal plane, with the second curtain following behind, effectively moving a slit across the focal plane until each part of the film or sensor has been exposed for the correct time. The effective exposure time can be much shorter than for central shutters, at the cost of some distortion of fast-moving subjects. Focal plane shutters have the advantage over central leaf shutters of allowing the use of interchangeable lenses without requiring a separate shutter for each lens. (Leaf shutters behind the lens also allow interchanging the lens using a single shutter.) They have several disadvantages as well: * Distortion of fast-moving subjects: 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. * They are noisier, which is a detriment to candid and nature photography. * Their more complex mechanical structure causes a shorter life-span than other shutter designs. * If a focal-plane shutter camera is left with sunlight falling on the lens (and the mirror up for an SLR), it is possible to burn a hole in the closed curtain of a non-metal shutter. * Camera shake due to the impact of the larger curtains starting and stopping rapidly. Camera designers have learned to overcome SLR mirror-slap by including a mirror lock-up feature in some cameras. This removes the camera-shake from the large slapping mirror inside the camera, but does not prevent camera-shake caused by the shutter mechanism itself. Mirror-lock-up introduces yet another problem: with the mirror locked-up out of the way the optical viewfinder cannot be used for focussing, framing, or exposure metering. Newer DSLR cameras include a " live preview" where the image from the main imaging sensor is displayed directly on an LCD display, so it is still possible to focus (manually or in newer models by contrast detection) and frame. This prevents most camera shake from the focal-plane shutter, as instead of a first curtain an electronic shutter is used.Simple leaf shutter
image:leaf shutter.svg, 250px, Simple leaf shutterGuillotine shutter
In the simplest version of Guillotine shutter a plate with an aperture slides across the lens opening. Simple versions from the 1880s and 1890s were often known as Drop shutters. They worked vertically and were usually powered by a rubber band, a spring or just gravity. Later they were fitted to run horizontally in hand cameras where they were spring powered with spring tension or pneumatic regulation.Rotating shutter
Hemispheric
The shutter plate consisted of a part of a sphere integrated to rotate behind the lens. It was a type of shutter found on the Photosphere and other cameras.Diaphragm shutter
A ''diaphragm'' or ''leaf'' shutter (as distinct from the simple leaf shutter above) consists of a number of thin blades which briefly uncover the camera aperture to make the exposure. The blades slide over each other in a way which creates a circular aperture which enlarges as quickly as possible to uncover the whole lens, stays open for the required time, then closes in the same way. The larger the number of blades, the more accurately circular is the aperture. Flash synchronization is easily achieved with a pair of contacts that close when the shutter is fully open. Ideally the shutter opens instantaneously, remains open as long as required, and closes instantaneously. This is essentially the case at slower speeds, but as speeds approach their maximum the shutter is far from fully open for a significant part of the exposure time. Effectively the shutter acts as an additional aperture, and may cause an increased depth of field, undesirable if shallow focus is being used creatively. Or it may cause mechanical vignetting if the diaphragm is outside of the lens (like a focal plane shutter or apodization filter). The term ''diaphragm shutter'' has also been used to describe an optical stop with a slit, near the focal plane of a moving-film high-speed camera. A few types and makers of leaf shutters became very well known. The earlyCentral shutter
A ''central shutter'' is not a type of shutter as such, but describes the position of the shutter: it is typically a leaf shutter (or simple leaf shutter), and located within the lens assembly where a relatively small opening allows light to cover the entire image. Leaf shutters can also be located behind, but near, the lens, allowing lens interchangeability. The alternative to a central or behind-the-lens shutter is a focal-plane shutter. Interchangeable-lens cameras with a central shutter within the lens body require that each lens has a shutter built into it. In practice most cameras with interchangeable lenses use a single focal plane shutter in the camera body for all lenses, while cameras with a fixed lens use a central shutter. Many medium-format and most large-format cameras, however, have interchangeable lenses each fitted with a central shutter. A few interchangeable-lens cameras have a behind-the-lens leaf shutter. Large-format press cameras often had a focal-plane shutter. Some had both a focal-plane shutter (for lens interchangeability) and a lens with central shutter (for flash synchronisation); one shutter would be locked open. Film cameras, but not digital cameras, with a central shutter and interchangeable lenses often have a secondary shutter or darkslide to cover the film and allow changing lens in mid-roll without fogging the film. The main advantages of central and behind-the-lens leaf shutters compared to a focal-plane shutter are: * Flash synchronization is possible at all speeds because the shutter opens fully, unlike a focal-plane shutter sliding a slit relatively slowly across the film for a short effective exposure. * Small size is possible as the shutter is placed where the bundle of rays is narrow, either inside or just behind the lens. * Many versions have no connection between the cocking mechanism and the film advance mechanism, making multiple exposures possible (this can be a disadvantage, as multiple exposures can be produced accidentally if the photographer forgets to advance the film). * Usually much quieter. * More realistic photographs in high speed panning—lateral focal plane shutters compress or elongate the image in such cases. * Longer shutter life. Some disadvantages of the central shutter are: * For an interchangeable lens system, each lens has to have a shutter built into it. * All leaf shutter speeds are limited by the speed at which the leaves can move: typically 1/500th of a second for a high-specification diaphragm shutter and 1/125th of a second for a simple leaf shutter. * Some versions may have no connection between the cocking mechanism and the film advance mechanism, making accidental multiple exposures a common problem, although this is a feature of camera manufacture rather than the shutter itself.Electronic shutter
Digital image sensors (both CMOS and CCD image sensors) can be constructed to give a shutter equivalent function by transferring many pixel cellShutter lag
Shutter lag is the time between the shutter release being pressed and the exposure starting. While this delay was insignificant on most film and some digital cameras, many digital cameras have significant delay, which can be a problem with fast-moving subjects as in sports and other action photography. Release lag of a bridge camera such as the 2010 Pentax X90 is a relatively short 1/50 s, or 21 milliseconds (ms). The Canon 50d dSLR is specified at 131 ms lag. In many cases autofocus (AF) lag is the root cause of shutter lag. Lower-cost cameras and low-light or low-contrast situations will make the effect more pronounced and it is in these cases that AF lag is more noticed. Most AF systems use contrast to determine focus; in situations where contrast is low, the speed at which the camera can determine the best focus can be quite noticeable. Since most modern cameras will not activate the shutter until autofocus is complete, the result is shutter lag. In these cases, the photographer can switch to manual focus to avoid the delay that is attributable to the AF function.Shutter cycle
A shutter cycle is the process of the shutter opening, closing, and resetting to where it is ready to open again. The life-expectancy of a mechanical shutter is often expressed as a number of shutter cycles.Projector shutter
In movie projection, the shutter admits light from the lamphouse to illuminate the film across to the projection screen. To avoid brightness flicker, a double-bladed rotary disc shutter admits light two times per frame of film in 24 fps projection, resulting in 24 * 2 = 48 Hz, which is the lower brightness flicker fusion threshold. For 16fps (most silent films and Regular 8mm) and 18fps ( Super 8), a triple-bladed shutter is used instead, as 16 * 3 = 48 Hz and 18 * 3 = 54 Hz. Shutters are also used simply to regulate pulses of light, with no film being used, as in a signal lamp.See also
* Photographic lens * Kerr cell shutter *References
Weblinks
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