Focal-plane shutter
In camera design, a focal-plane shutter is a type of photographic shutter that is positioned immediately in front of the focal plane of the camera, that is, right in front of the photographic film or image sensor.
Two-curtain shutters
The traditional type of focal-plane shutter in 35 mm cameras, pioneered by Leitz for use in its Leica cameras, uses two shutter curtains, made of opaque rubberised fabric, that run horizontally across the film plane. For slower shutter speeds, the first curtain opens from right to left, and after the required time with the shutter open, the second curtain closes the aperture in the same direction. When the shutter is cocked again the shutter curtains are moved back to their starting positions, ready to be released.Focal-plane shutter at low speed
Figure 1: The black rectangle represents the frame aperture through which the exposure is made. It is currently covered by the first shutter curtain, shown in red. The second shutter curtain shown in green is on the right side.Figure 2: The first shutter curtain moves fully to the left allowing the exposure to be made. At this point, the flash is made to fire if one is attached and ready to do so.
Figure 3: After the required amount of exposure the second shutter curtain moves to the left to cover the frame aperture. When the shutter is recocked the shutter curtains are wound back to the right-hand side ready for the next exposure.
This is a graphical representation only; the actual mechanisms are much more complex. For example, the shutter curtains actually roll on and off spools at either side of the frame aperture so as to use as little space as possible.
Faster shutter speeds are achieved by the second curtain closing before the first one has fully opened; this results in a vertical slit that travels horizontally across the film. Faster shutter speeds simply require a narrower slit, as the speed of travel of the shutter curtains is not normally varied.
Focal-plane shutter at high speed
Figure 1: The black rectangle represents the frame aperture through which the exposure is made. It is currently covered by the first shutter curtain, shown in red. The second shutter curtain shown in green is on the right side.Figure 2: The first shutter curtain begins to move to the left allowing the exposure to be made. Because the exposure requires a very fast shutter speed, the second curtain begins to move across at a set distance from the first one.
Figure 3: The first shutter curtain continues to travel across the frame aperture followed by the second curtain. It would be pointless to use an electronic flash with this shutter speed as the short duration flash would expose only a very small amount of the frame as the rest is covered by either the first or second shutter curtain.
Figure 4: The first shutter curtain finishes moving, followed closely by the second curtain which is now covering the frame aperture completely. When the shutter is recocked both shutter curtains are wound back to the right-hand side ready for the next exposure.
Vertical-travel shutters
Most modern 35 mm and digital SLR cameras now use vertical travel metal blade shutters. These work in the same way as the horizontal shutters, with a shorter distance for the shutter blades to travel, only 24 mm as opposed to 36 mm.Features
Focal-plane shutters can be built into the body of a camera that accepts interchangeable lenses, eliminating the need for each lens to have a central shutter built into it. Their fastest speeds are either 1/4000 second, 1/8000 second, or 1/12000 second; much higher than the 1/500 second of the typical .File:Lightning rolling shutter.jpg|thumb|right|Two sections of the frame are exposed differently due to a lightning strike that occurred during the exposure. A similar effect occurs if an electronic flash is used when the shutter is set faster than X-sync.
While the concept of a travelling slit shutter is simple, a modern FP shutter is a computerised microsecond accurate timer, governing sub-gram masses of exotic materials, subjected to hundreds of gs acceleration, moving with micron precision, choreographed with other camera systems for 100,000+ cycles. This is why FP shutters are seldom seen in compact or point-and-shoot cameras. In addition, the typical focal-plane shutter has flash synchronization speeds that are slower than the typical leaf shutter's 1/500 s, because the first curtain has to open fully and the second curtain must not start to close until the flash has fired. In other words, the very narrow slits of fast speeds will not be properly flash exposed.
The fastest X-sync speed on a 35 mm camera is traditionally 1/60 s for horizontal Leica-type FP shutters and 1/125 s for vertical Square-type FP shutters.
Rolling shutter
Focal-plane shutters may also produce image distortion of very fast-moving objects or when panned rapidly, as described in the Rolling shutter article. A large relative difference between a slow wipe speed and a narrow curtain slit results in distortion because one side of the frame is exposed at a noticeably later instant than the other and the object's interim movement is imaged.For a horizontal Leica-type FP shutter, the image is stretched if the object moves in the same direction as the shutter curtains, and compressed if travelling in the opposite direction of them. For a downward-firing vertical Square-type FP shutter, the top of the image leans forward. The use of leaning to give the impression of speed in illustration is a caricature of the distortion caused by the slow-wiping vertical FP shutters of large format cameras from the first half of the 20th century.
Electro-optical shutters
Instead of using relatively slow-moving mechanical shutter curtains, electro-optic devices such as Pockels cells can be employed as shutters. While not commonly used, they avoid the problems associated with travelling-curtain shutters such as flash synchronisation limitations and image distortions when the object is moving.Rotary focal-plane shutter
Besides the horizontal Leica and vertical Square FP shutters, other types of FP shutters exist. The most prominent is the rotary or sector FP shutter. The rotary disc shutter is common in film and movie cameras, but rare in still cameras. These spin a round metal plate with a sector cutout in front of the film. In theory, rotary shutters can control their speeds by narrowing or widening the sector cutout and/or by spinning the plate faster or slower. However, most cameras' rotary shutters have fixed cutouts and can be varied in their spinning speed. The Olympus Pen F and Pen FT half-frame 35 mm SLRs spun a semicircular titanium plate to 1/500 s.Semicircular rotary shutters have unlimited X-sync speed, but all rotary FP shutters have the bulk required for the plate spin. The Univex Mercury half-frame 35 mm camera had a very large dome protruding out the top of the main body to accommodate its 1/1000 s rotary shutter. They also produce unusual distortion at very high speed because of the angular sweep of the exposure wipe. Bulk can be reduced by substituting blade sheaves for the plate, but then the rotary FP shutter essentially becomes a regular bladed FP shutter.
Revolving drum focal-plane shutter
The revolving drum is an unusual FP shutter that has been used in several specialised panoramic cameras such as the Panon Widelux and KMZ Horizont. Instead of using an extremely short focal length lens to achieve an extra-wide field of view, these cameras have a medium-wide lens encapsulated in a drum with a rear vertical slit. As the entire drum is horizontally pivoted on the lens's rear nodal point, the slit wipes an extra-wide-aspect image onto film held against a curved focal plane. The Widelux produced a 140° wide image in a 24×59 mm frame on 135 film with a Lux 26 mm f/2.8 lens and controlled shutter speed by varying rotation speed on a fixed slit width.In the Kodak Cirkut and Globus Globuscope cameras, the entire camera and lens revolved as the film was pulled past the slit in the opposite direction. The Globuscope produced a 360° angle of view image in a 24×160 mm frame on 135 film with a 25 mm lens and had an adjustable slit width with a constant rotation speed.
Revolving FP shutters produce images with unusual distortion where the image center seems to bulge toward the viewer, while the periphery appears to curve away because the lens's field of view changes as it swivels. This distortion will disappear if the photograph is mounted on a circularly curved support and viewed with the eye at the center. Revolving shutters that do not rotate smoothly may create uneven exposure that will result in vertical banding in the image. Using the flash will also interfere.
These cameras are often used for photographing large groups of people. The subjects may be arranged in a shortened semicircle with the camera at the centre such that all the subjects are the same distance from the camera and facing the camera. Once the exposure is made and processed, the panoramic print shows everyone in a straight line facing in the same direction. The distortion present in the background betrays the technique.
History and technical development
The earliest daguerreotype, invented in 1839, did not have shutters, because the lack of sensitivity of the process and the small apertures of available lenses meant that exposure times were measured in many minutes. A photographer could easily control exposure time by removing and returning the camera lens' lens cap or plug.However, during the 19th century, as one increased-sensitivity process replaced another and larger aperture lenses became available, exposure times shortened to seconds and then to fractions of a second. Exposure timing control mechanisms became a necessary accessory and then a standard camera feature.