Focal-plane shutter
Figure 3: After the required amount of exposure the second shutter curtain moves to the left to cover the frame aperture.
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.
Because the exposure requires a very fast shutter speed, the second curtain begins to move across at a set distance from the first one.
[5]While the concept of a travelling slit shutter is simple, a modern FP shutter is a computerised microsecond accurate timer,[6] governing sub-gram masses of exotic materials,[7] subjected to hundreds of gs acceleration,[8] moving with micron precision,[9] choreographed with other camera systems[10] for 100,000+ cycles.
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.
[15][16] 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.
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.
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.[19] 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.
[21] 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).
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.
[29] Revolving shutters that do not rotate smoothly may create uneven exposure that will result in vertical banding in the image.
[31] 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.
[34] This was an accessory guillotine-like device—a wooden panel with a slit cutout mounted on rails in front of the camera lens that gravity dropped at a controlled rate.
[37] In 1883, Ottomar Anschütz (Germany) patented a camera with an internal roller blind shutter mechanism, just in front of the photographic plate.
[40] A drop shutter-like mechanism with an adjustable slit was used at the focal plane of an apparently one-off William England camera in 1861 and this is considered the first FP shutter of any kind.
[41] Folmer and Schwing (US) were the most famous proponents of single curtain FP shutters, with their large format sheet film Graflex single-lens reflex and Graphic press cameras using them from 1905 to 1973.
As revised in the 1954 Leica M3 (West Germany),[49][50] a typical Leica-type horizontal FP shutter for 35 mm cameras is pre-tensioned to traverse the 36 millimeter wide film gate in 18 milliseconds (at 2 meters per second) and supports slit widths for a speed range of 1 to 1/1000 s. A minimum 2 mm wide slit produces a maximum 1/1000 s effective shutter speed.
Their maximum flash synchronization speed is also limited because the slit is fully open only to the film gate (36 mm wide or wider) and able to be flash exposed up to 1/60 s X-synchronization (nominal; 18 ms = 1/55 s actual maximum; in reality, a 40 mm slit to allow for variance gives 1/50 s ⅓ stop slow).
Some horizontal FP shutters exceeded these limits by narrowing the slit or increasing curtain velocity beyond the norm; however, these tended to be ultra-high-precision models used in expensive professional-level cameras.
Copal collaborated with Nippon Kogaku to change the Compact Square shutter for the Nikon FM2 (Japan) of 1982 to using a honeycomb pattern-etched titanium foil for its blade sheaves.
This permitted cutting shutter-curtain travel time by nearly half to 3.6 ms (at 6.7 m/s) and allowed 1/200 s flash X-sync speed.
[65] The fastest focal-plane shutter ever used in a film camera was the 1.8 ms curtain travel time (at 13.3 m/s) duralumin and carbon fiber bladed one introduced by the Minolta Maxxum 9xi (named Dynax 9xi in Europe, α-9xi in Japan) in 1992.
[69][70] The traditional 1/1000 s and 1/2000 s top speeds of horizontal and vertical FP shutters are often 1⁄4 stop too slow, even in ultra-high-quality models.
If the curtains are not properly braked after crossing the film gate, they might crash and bounce; reopening the shutter and causing double exposure ghosting bands on the image edge.
[74] At first, electromagnets controlled by analogue resistor/capacitor timers were used to govern the release of the second shutter curtain (though still operated by spring power).
[77] Electric "coreless" micromotors, with near instantaneous on/off capability and relatively high power for their size, would drive both curtains and other camera systems replacing springs in the late 1980s.
[80] A spring-wound clockwork escapement must completely unwind fairly quickly and limit the longest speed—generally to one full second,[81] although the Kine Exakta (Germany) offered 12 s in 1936.
[93] However, with very limited need for such extremely fast speeds, FP shutters retreated to 1/8000 s in 2003 (and 1/250 s X-sync in 2006)—even in professional level cameras.
