The focal plane shutter (English focal -plane shutter) - in addition to the central locking system - one of the two commonly used in photo apparatus design principles for the closure. The focal plane shutter is located directly in front of the film or image sensor module in the camera body.
The focal plane shutter is called by two blinds, shutter curtains also formed. The curtains move depending on the design either both horizontal or both vertical. After the release of the first curtain opens and plays the movie free for exposure. If the desired exposure time is reached, the second curtain covers the movie again, by following the first curtain in the movement. For short exposure times of the second curtain follows the first curtain so fast, that at no time the entire image is released for exposure. Rather, an educated of two curtains slit moves across the image. Different regions of the image are exposed at different times.
For the blinds very different material is used, such as rubberized fabric, titanium foil ( each, a cloth cap). In some closures of each curtain is formed by a plurality of overlapping metal blades, whereby the movement can be linear in consecutive lamellae furling or in modern constructions. In the first versions of the EXA, the mirror had the function of the first curtain ( snap closure ). This, however, the exposure time to 1/175 s was limited.
The formation of the exposure time is by mechanical or electro-mechanically using electronic timers.
The focal plane shutter, initially called " torque lock", was developed by the photographer Ottomar Anschütz and patented in 1888. Berlin Optical Institute CP Goerz presented in 1890 with the " Goerz -Anschütz Moment Camera" the world's first focal plane shutter camera.
Pros and Cons
Compared with the central locking system of the focal plane shutter is better suited to achieve short exposure times. SLR achieve shutter speeds of 1/16000 seconds and less. The film is thereby uniformly illuminated by the close of the film plane trailing curtain, while it may come on the screen when central locking at the wrong position in the beam path or faulty drive to lower exposure.
For SLR cameras with interchangeable lenses, the focal plane shutter has prevailed due to the maximum design simplification in various forms.
If in the SLR a central locking, integrated in the lens used, the equipment (several lenses) considerably more expensive. The film must be covered, which means a certain amount of care or an extra effort also during lens exchange by the mirror or an auxiliary closure. A focal plane shutter, however, can be easily integrated into the camera body, without restricting the focal range at the lower end.
In the Voigtländer Bessamatic ( miniature reflex central locking and interchangeable lenses, built 1958-69 ) or even at the Pentina, built in Pentacon Dresden from 1961 to 1965, was chosen another way: The central locking system was integrated into the camera body and has the shortest focal length of the lenses limited to 35 mm. This design is also called rear lens cap.
For other products (Zeiss icon Contaflex, early Retina reflex) remained even the rear portion of the optics including the closure in or on the housing, only the front lens group has to be replaced. Through various combinations of remaining in the housing base lens with different intentions different focal lengths were possible despite the somewhat limited optical construction, in the telephoto range up to 135 mm. This design is called sentence lens.
Some panoramic cameras use the slit diaphragm, which can be regarded as a special form of a focal plane shutter.
With a moving subject, the fact that not the entire image area is exposed at once, but the slit moves across the image by the rolling shutter effect to geometric distortions. The strength of this distortion depends on the angular speed of the direction of movement relative to the moving direction of the slit of the exposure time and the width of the slit from. The effect can be both disruptive and, in rare cases, be used artistically.
The succession of the exposure gives rise to a problem that it is changed during recording, the aperture or shutter speed. When disturbed technique of first shutter curtain starts running before the automatic diaphragm has reached the working aperture: the image is overexposed in this part. By manually close the aperture before recording can completely eliminate this error. Likewise, the first shutter curtain can the second run away (increasing exposure), or the second one can obtain the first (increasing exposure). The three error effects are apparent with decreasing shutter speed.
Use of flash
Problem, the slot closure proves for shooting with a short exposure time. Each focal plane shutter camera has a certain shortest flash sync speed, which is in small picture format is typically between 1/60 and 1/ 250 seconds. If this camera or shutter -dependent value is exceeded, there is no time at which the entire image is exposed at once.
As the burn time of a modern flash light is much shorter, it is thus not possible to evenly illuminate with a single flash an image when the shutter speed is less than the fastest flash sync speed. The consequence is a non-uniform illumination of the image with dark stripes.
Some special flash units are able stroboscopically make a quick series of flashes, so that faster shutter speeds with flash are possible. In old, slow burning pyrotechnic flashes ( flash bulbs ) a shorter time can be used under certain circumstances, if the entire slot cycle is covered by the effective focal length. The flash intensity is thereby reduced, however, and must be calculated separately.
Exposure times that are longer than the shortest flash sync speed, cause no problems with the flash light use.
Many cameras can choose whether the flash fires synchronized with the first or with the second shutter curtain (activated right in the middle provides no camera). The synchronization with the second shutter curtain produced during the illumination of moving objects a tail ( for example car with headlights at night), which emphasizes the movement, because the object is frozen until the end of the exposure by the flash and is really visible. Synchronization with the first shutter curtain also generates a tail but subjectively seems to point in the wrong direction - the object was already frozen at the beginning of the exposure time. One problem with the synchronization to the second shutter curtain is that in P -TTL pre-flash methods with the time between pre-flash and the main flash is extended by the exposure time and thus the wink is enhanced by the dazzling effect of the pre-flash. In the classical TTL - OTF flash without pre-flash, the reaction of the subject (for example, in wildlife photography ) affects the resulting recording is not when a flash synchronization at the second shutter curtain is because the recording is completed immediately after the flash.