Focal-plane shutter

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.

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 rubberized 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 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. Using this technique, modern SLR cameras are capable of shutter speeds of up to 1/2000, 1/4000 or even 1/8000 of a second.

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.

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Wikipedia