In camera design, a focal-plane shutter (FPS) 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 (usually) 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 overlapped and 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,Anonymous, K200D/K20D: Pentax. no city of publication: Pentax Corp., 2008. pp. 33–34. 1/8000 second,Anonymous, Canon EOS System Spring 2008. Lake Success, NY: Canon USA, 2008. pp. 18–20.Anonymous, Nikon Digital SLR Comparison Guide: Fall Collection 2008. Melville, NY: Nikon Inc., 2008. p. 10. or 1/12000 second; much higher than the 1/500 second of the typical leaf shutter.Norman Goldberg, Camera Technology: The Dark Side of the Lens. San Diego, CA: Academic Press, 1992. . pp. 65–66.
While the concept of a travelling slit shutter is simple, a modern FP shutter is a computerised microsecond accurate timer,Goldberg, Camera Technology p. 78 governing sub-gram masses of exotic materials,Anonymous, "Modern Tests: Nikon FM2: Fastest Shutter and Sync" pp. 98–101, 112. Modern Photography, Volume 46, Number 9; September 1982. . subjected to hundreds of gs acceleration,Tony Gioia, "SLR Notebook: Window on a Shutter". p. 32. Modern Photography, Volume 52, Number 8; August 1988. . moving with micron precision,Anonymous. "Test: Nikon F5: Simply the quickest shooting, most advanced, safety-loaded pro AF SLR ever". pp. 70–79. Popular Photography, Volume 61 Number 5; May 1997. . choreographed with other camera systemsMichael J. Langford, Basic Photography. Fifth Edition. London, UK: Focal Press/Butterworth, 1986. . pp. 71–73. for 100,000+ cycles.Michael J. McNamara, "Test: Nikon D3: Best Ever: Believe the buzz. It's all true. Really". pp. 80–83. Popular Photography & Imaging, Volume 72 Number 3; March 2008. . 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,Peter Kolonia, "The War Continues: 35 mm Vs. 2¼: Does moving to 2¼ from 35 mm really pay back in quality what you lose in convenience?" pp. 76–83. Popular Photography, Volume 59 Number 11; November 1995. . p 78. 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.Goldberg, Camera Technology. pp. 221–223.Alan Horder; editor, The Manual of Photography. (formerly The Ilford Manual of Photography) Sixth edition. Philadelphia, PA: Chilton Book Company/Focal Press Limited, 1971. . pp. 174, 197–199.
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.Michael J. Langford, Basic Photography: A Primer for Professionals. Third Edition. Garden City, NY: Amphoto/Focal Press Limited, 1973. . pp. 109–111.Goldberg, Camera Technology. pp. 80–86, 115–117.
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 (by using two overlapping plates and varying the overlap) and/or by spinning the plate faster or slower.Goldberg, Camera Technology pp. 86–87. However, most cameras' rotary shutters have fixed cutouts and can be varied in their spinning speed. The Olympus Pen F and Pen FT (1963 and 1966, both from Japan) half-frame 35 mm SLRs spun a semicircular titanium plate to 1/500 s.Jason Schneider, "The Camera Collector: Half-frame 35's of the 60's, Part 3. Wherein collectors immortalize the only SLRs of their kind" pp. 64, 75. Modern Photography, Volume 39, Number 2; February 1975. .
Semicircular rotary shutters have unlimited X-sync speed, but all rotary FP shutters have the bulk required for the plate spin. The Univex Mercury (1938, US) 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.S. F. Spira with Eaton S. Lothrop, Jr and Jonathan R. Spira. The History of Photography as Seen Through the Spira Collection. New York, NY: Aperture, 2001 . pp. 154, 159–160. 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.Norman Goldberg, "3 new shutters: how they work" pp. 74–77, 124. Popular Photography, Volume 82, Number 3; March 1975. .
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 (1959, Japan) and KMZ Horizont (1968, Soviet Union).John Wade, The Collector's Guide to Classic Cameras: 1945–1985. Small Dole, UK: Hove Books, 1999. . pp. 113–117. Instead of using an extremely short focal length (wide-angle) 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.Kraszna-Krausz, A.; chairman of the editorial board, The Focal Encyclopedia of Photography. Revised Desk Edition, 1973 reprint. New York, NY: McGraw-Hill Book Co., 1969. no ISBN. p. 1048. 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.Anonymous. "Modern Photography's Annual Guide to 47 Top Cameras: Widelux F-7" p 158. Modern Photography, Volume 38, Number 12; December 1974. ."Modern Photography's Annual Guide '84: 48 Top Cameras: Widelux F7" p 118. Modern Photography, Volume 47, Number 12; December 1983. .
In the Kodak Cirkut (1907, US) and Globus Globuscope (1981, US) 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.John Owens, "Whirled Tour: Lessons from an all-around photographer", pp. 12–13. Popular Photography, Volume 72 Number 9; September 2008. .Harold Martin, "Time Exposure: 25 Years Ago: Cover: July 1981", p. 112. Popular Photography & Imaging, Volume 70 Number 7; July 2006. .Wade, Collector's Guide pp. 117–118.