Focal plane shutter
The focal plane shutter (engl. Focal-plane shutter ) is - in addition to the central closure - one of the two photo Apparatus conventional principles of construction for the closure . The focal plane shutter is located directly in front of the film or image sensor module in the camera housing .
The focal plane shutter is formed by two blinds, also known as shutter curtains. The curtains move either horizontally or both vertically, depending on the construction. After the release, the first curtain opens and releases the sensor or film for exposure. Once the desired exposure time has been reached, the second curtain covers the film again by following the first curtain in motion. In the case of short exposure times, the second curtain follows the first curtain so quickly that the entire sensor or film is never released for exposure . Rather, a slot formed from both curtains moves across the sensor or film. Different areas of the sensor or film are exposed at different times.
Very different materials are used for the blinds, such as rubberized fabric, titanium foil (each referred to as a cloth closure). In some closures, the respective curtain is formed by several overlapping metal slats, whereby the movement can be rolled up in the case of connected slats or linear in modern designs. In the first versions of the EXA , the mirror had the function of the first curtain (flap closure). This limited the exposure time to 1/175 s.
The exposure time is determined either purely mechanically or electromechanically with the aid of electronic timers.
The focal plane shutter, initially called "moment shutter", was developed by the photographer Ottomar Anschütz and patented in 1888. In 1890, the Berlin optical institute CP Goerz presented the “Goerz-Anschütz-Moment-Camera”, the world's first focal plane shutter camera.
Advantages and disadvantages
Compared to the central shutter , the focal plane shutter is better suited to achieve very short exposure times. Single lens reflex cameras achieve shutter speeds of 1/16000 seconds and less. The film is evenly exposed by the curtain running down close to the film plane, while with the central shutter, incorrect positioning in the beam path or incorrect control can result in lower exposure at the edge of the image.
If a central shutter integrated in the lens is used with the SLR camera , the equipment becomes considerably more expensive when using several lenses. The film must also remain covered by the mirror or an auxiliary shutter during the lens change, which means a certain degree of care and additional effort. A focal plane shutter, on the other hand, can easily be integrated into the camera housing without restricting the focal length range at the lower end.
In the Voigtlander Bessa Matic ( mm film SLR camera with shutter and interchangeable lenses, built 1958 - 69 ) or in the Pentina , built at Pentacon Dresden 1961-1965, they chose another way: The central lock was integrated into the camera body and has the shortest focal length of the Lenses limited to 35mm. This construction is also called a rear lens shutter.
With other products ( Zeiss Ikon Contaflex , early Kodak Retina Reflex ) even the rear part of the optics including the lock remained in or on the housing, only the front lens group could be replaced. Thanks to various combinations of the basic lens remaining in the housing with the various attachments, different focal lengths were possible despite the somewhat restricted optical construction, up to 135 mm in the telephoto range. This type of construction is called a sentence lens.
Some panorama cameras use the slit diaphragm, which can be viewed as a special form of a focal plane shutter.
In the case of moving objects, the fact that not the entire image area is exposed at once, but rather the slit moves across the image, leads to geometric distortions due to the rolling shutter effect . The strength of this distortion depends on the angular velocity, on the direction of movement relative to the direction of movement of the slit, on the exposure time and on the width of the slit. The effect can be both disturbing and, in rare cases, used artistically.
The sequential exposure raises the problem that the aperture or the exposure time changes during the exposure. If the technology fails, the first shutter curtain starts running before the spring diaphragm has reached the working diaphragm : the image is overexposed in this part. This error can be completely eliminated by manually closing the aperture before taking the picture. Likewise, the first curtain can run away from the second (increasing overexposure), or the second can catch up with the first (increasing underexposure). The three error effects become more noticeable as the shutter speed decreases.
Use of flash light
The focal plane shutter proves to be problematic in flash photos with short exposure times . Every camera with a focal plane shutter has a certain shortest flash sync time , which is typically between 1/60 and 1/250 seconds in 35mm format. If this camera or shutter-dependent value is not reached, there is no point in time at which the entire image is exposed at once.
Since the lighting time of a modern flash light is significantly shorter, it is not possible to uniformly illuminate an image with a single flash if the shutter speed falls below the shortest flash sync time. The result is uneven illumination of the image with dark stripes.
Some special flash units are capable of stroboscopically emitting a rapid sequence of flashes so that shorter exposure times with flash are also possible. In the case of old, slowly burning pyrotechnic flashes (flash light bulbs), a shorter time can be used under certain circumstances if the entire passage through the slot is covered by the effective burning time. However, the lightning strength is reduced and must be calculated separately.
Exposure times that are longer than the shortest flash sync time do not cause any problems when using the flash.
With many cameras, you can choose whether the flash is triggered synchronized with the first or the second shutter curtain (no camera currently offers a trigger exactly in the middle). The synchronization with the second shutter curtain creates a tail when photographing moving objects (for example the rear lights of a car at night), which emphasizes the movement, as the object is only frozen and properly visible at the end of the exposure by the flash. Synchronization with the first shutter curtain also creates a tail, which, however, seems to be pointing in the wrong direction - the object was frozen at the beginning of the exposure time. With the P-TTL flash method with pre-flash , the synchronization to the second shutter curtain extends the time between pre-flash and main flash by the exposure time and thus intensifies the blinking due to the glare effect of the pre-flash. With classic TTL OTF flashes without a pre-flash, the reaction of the subject (for example in animal photography) does not affect the resulting picture, even when the flash is synchronized with the second shutter curtain, because the picture is already completed immediately after the flash.