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See Crop factor.
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Describes how much an imaging sensor has been cropped in relation to its full-frame equivalent. It always describes how many times larger the full-frame is in relation to the cropped sensor. Take an APS-C sensor with a crop factor of 1.6, for instance. This indicates the sensor is 60% of the size of a frame of 35mm film. The crop factor is used to calculate how much of the equivalent of the full-frame field of view the cropped sensor will have with a lens. In order to calculate this, one multiplies the focal length of the lens by the crop factor. A 1.6 crop-factor, for instance, will give a 100mm lens the same field of view as a 160mm lens on a full-frame camera.
Extreme wide-angle lens that gives 180 degrees coverage and is uncorrected for curvilinear distortion so you produce a heavily distorted photograph. If the photo turns out to be rectangular or circular, and how distorted it looks, depends on the lens and the camera. There are basically two kinds of fish-eye lenses for photography: 1. Lenses that produce a circular image, typically 8 or 10mm lenses, used on 35mm cameras or full-frame digital cameras. They produce a circular image with dark corners (due to most photos being rectangular). A similar lens is available for digital cameras with a so-called "cropped sensor". To achieve the same circular effect, it's a 4.5mm lens. 2. Lenses that produce a rectangular image. These lenses enlarge the image to cover the whole rectangular frame of a photograph. They are typically 15 or 16mm lenses. On a 35mm or digital full-frame camera they produce rectangular but still heavily distorted images without dark corners. The same lenses on a digital camera with a cropped sensor will still have a distorting effect, but less so. Fish-eye lenses used to be prime lenses only, but one company has gone so far as to produce a 10-17mm fish-eye zoom lens. This lens still produces distorted, curvy images, but zooming in lessens the distortion.
Advanced Photo System type-C should not be confused with the Advanced Photographic System (APS) camera/film system, since the former is digital and the latter is analogue. APS-C is a type of image sensor found in certain digital single lens reflex (DSLR) cameras. These sensors are smaller than the ones used for the conventional 36 mm x 24 mm (35 mm) sensor SLR cameras (also called full frame DSLR cameras). Being smaller causes a 1.x multiplier for the effect of the focal distance of the lenses, in comparison with the effect of those lenses on conventional 35 mm film SLR cameras or full frame digital SLR cameras. Several manufacturers produce cameras with an APS-C sensor and some even make lenses especially for them. These lenses are found in the Canon EF-S, Nikon DX, Pentax DA and Sigma DC ranges, to name the most popular ones. The most common multiplier ratios are 1.6 (Canon), 1.5 (Nikon, Fuji, Sony, etc.), 1.3 (Canon). (1.3 crop sensors are also sometimes called APS-H.) Most manufacturers not only produce cameras with these cropped sensors, but also offer DSLR cameras with full frame sensors.