Digital camera back
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A digital camera back is an item of high-end digital photography equipment. Most digital cameras are built to operate as a self-contained unit. This is especially so at the lower end, where cameras usually include zoom lens and flashes that cannot be changed. However, at the highest end, the equipment is completely modular, with interchangeable lenses, bodies, and film backs. A digital camera back essentially replaces a film back in such a system. Experienced photographers attach digital camera backs to their professional medium format SLR cameras, such as a Hasselblad, or a view camera, such as a Sinar.
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[edit] History
The first digital camera back was introduced by Leaf Photography (now part of Kodak) in 1991. The Leaf DCBI, nicknamed 'The Brick', offered resolution of 4 million pixels (MP). It took another 4 years of development to then introduce the 2nd digital camera back; the DCBII, which included a live-video view. Two years later, in 1996, Leaf introduced the first 6 MP digital camera back; the Leaf Volare. From 2002, new models were introduced yearly, with rising resolution and sensor sizes, achieving today resolutions of 33MP for the new Leaf Aptus family of digital camera backs, and 39MP for Phase One and Hasselblad.
[edit] Uses
Early digital camera backs were only used in a studio to take pictures of still objects. Because they created huge amounts of data relative to the available storage mediums at the time, they had to be tethered to a computer during capture. They were using linear array sensors which could take seconds or even minutes for a complete high-resolution scan. The linear array sensor acts like its counterpart used in a flatbed image scanner by moving vertically to digitize the image. The latest one-shot digital backs, however, can keep pace with the speediest motorized medium format cameras, and they are for all practical purposes without limitations to the available shutter speeds. The availability of high-speed, high capacity memory cards have made modern digital backs into self-contained units, which makes them practical for use outside the studio as well.
Many of these cameras could originally only capture grayscale images. To take a color picture, it requires three separate scans done with a rotating colored filter. These are called multi-shot backs. Some other camera backs are using CCD arrays similar to typical cameras. These are called single-shot backs.
[edit] Technical features
Since it is much easier to manufacture a high-quality linear CCD array that has only a few thousand pixels than a CCD matrix that has millions of them, very high resolution linear CCD camera backs were available much earlier than their CCD matrix counterparts. For example, you could buy an expensive camera back with an over 7,000-pixel horizontal resolution in the mid-1990s. As of 2006, you can buy a comparable CCD matrix camera of similar resolution, in the Phase One P45, and in the Leaf Aptus 75 digital camera backs.
Many modern digital camera backs are using very large CCD matrices. This eliminated the need of scanning. For example, Fujifilm produces a 20-million-pixel digital camera back with a 52 mm x 37 mm (2.04" x 1.45") CCD in 2003. This CCD array is a little smaller than a frame of 120 film and much larger than a 35 mm frame (36 mm x 24 mm). In comparison, a compact digital camera usually uses a so-called 1/1.8" or 7.176 mm x 5.319 mm CCD sensor. The 1/1.8" diagonal value is based on an archaic measurement originally used for TV picture tubes and doesn't even accurately identify the physical size of the sensor, hence the actual photo-sensitive area is much smaller.
A digital camera back is a good idea to smooth the transition from film to digital. A photographer can reuse his or her SLR camera and lens collection without too much trouble or expense. To some medium format camera users, the convenience of a bellows has no substitute.
[edit] Sensor size and angle of view
Only a few very expensive high-end digital cameras or camera backs use film-sized sensors. Since a camera back's sensor is seldom as large as the film it replaces, either one has to use a smaller sensor, or to use multiple smaller sensors that form a larger piece. For example, various cameras of the late 1990s based on the "building block" sensors of Philips use of four to six 12 x 12 mm 1-megapixel sensors to form larger light sensors.
If a sensor smaller than the camera's original film format is used, such as the use of APS-C-sized digital sensors in 35mm format digital SLRs, then the image's field of view is cropped by the sensor giving the impression that the focal length of the lens has changed. In reality, the effect of this is the same as taking a 35mm negative and cutting it down to the size of the camera's sensor before producing the final print. The focal length and perspective in the digital image haven't changed, but if printed at the same size as an equivalent 35mm image it appears magnified. This can be useful if extra telephoto reach is desired, as a certain lens on an APS sensor will produce an equivalent image to a significantly longer lens on a 35mm film camera shot at the same distance from the subject, the equivalent length of which depends on the camera's field of view crop. This is sometimes referred to as the focal length multiplier, but the focal length is a physical attribute of the lens and not the camera system itself. The downside to this is that wide angle photography is made somewhat more difficult, as the smaller sensor effectively and undesirably reduces the captured field of view. Some methods of compensating for this or otherwise producing much wider digital photographs involve using a fisheye lens and "defishing" the image in post processing to simulate a rectilinear wide angle lens.
Common values for field of view crop in digital SLRs include 1.3x for some Canon sensors, 1.5x for Sony APS-C sensors used by Nikon, Pentax and Konica Minolta, 1.6 (APS-C) for most Canon sensors and Fujifilm sensors, ~1.7x for Sigma's Foveon sensors and 2x for Kodak 4/3" sensors currently used by Olympus. Canon and Kodak also produce 35mm digital cameras using full size sensors that introduce no field of view crop and behave as if they were loaded with 35mm film.
This fact also allows compact digital camera designers to produce small and inexpensive lenses that have large focal ranges and high "equivalent" focal lengths. For example, a camera with a 1/1.8" sensor has a 5.0x field of view crop, and so a hypothetical 5-50mm zoom lens will produce images that look similar (again the differences mentioned above are important) to those produced by a 35mm film camera with a 25-250mm lens, while being much more compact than such a lens for a 35mm camera since the imaging circle is much smaller.
