Focal plane shutter

Focal-plane shutter

[foh-kuhl pleyn]

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.

Advantages

One of the main advantages of focal-plane shutters is that the shutter can be built into the body of a camera which accepts interchangeable lenses, eliminating the need for each lens to have a central shutter built into it. The focal-plane shutter is also a fairly simple mechanism which is capable of quite fast and accurate shutter speeds.

Two-curtain shutters

The traditional type of focal-plane shutter in 35 mm cameras, pioneered by Leitz, the camera company that makes the Leica, uses two shutter curtains, made of opaque rubberized fabric, that run horizontally across the film plane. For slower shutter speeds, the first curtain opens from (usually) 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.

See figure at right:

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.

See figure at right:

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 only expose 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.

One limitation of the focal-plane shutter is that the flash synchronisation speeds are generally quite slow. This is because the first curtain has to open fully and the second curtain must not start to close until the flash has fired, the fastest speed that can be used for flash sychronisation on a 35 mm camera is normally 1/60 or 1/125 of a second. Many modern cameras have increased this to 1/250 of a second at the expense of reliability and greatly increased cost.

Cameras with focal-plane shutters also produce image distortion when photographing fast moving objects or panning rapidly. Depending on the direction of travel, the recorded image can be seen to be elongated if motion is in the direction of the shutter blades, or compressed if travelling in the opposite direction to the shutter blades.

Vertical-travel shutters

Most modern 35 mm and digital SLR cameras now use vertical travel metal blade shutters. These work in precisely the same way as the horizontal shutters, but because of the shorter distance the shutter blades must travel (24 mm as opposed to 36 mm) and the faster metal construction, the shutter blades can travel across the film plane in less time. This can result in faster flash synchronisation speeds than are possible with the horizontal curtain focal-plane shutter, and the shutter can reliably provide higher speeds (up to 1/8000 of a second).

Rotating shutters

Some focal-plane shutters are simpler, such as the rotary focal-plane shutter of the Olympus Pen F half-frame 35-mm SLR, which has a single-piece curtain mechanism, similar to the rotary disc shutter mechanisms of movie cameras.

See also

References

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