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, 1/8000 second, or 1/12000 second; much higher than the 1/500 second of the typical leaf shutter.
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.
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.
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