In digital photography, a crop factor is related to the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital cameras, relative to 35 mm film format as a reference. In the case of digital cameras, the imaging device would be a digital sensor. The most commonly used definition of crop factor is the ratio of a 35 mm frame's diagonal (43.3 mm) to the diagonal of the image sensor in question; that is, CF=diag35mm / diagsensor. Given the same 3:2 aspect ratio as 35mm's 36mm x 24mm area, this is equivalent to the ratio of heights or ratio of widths; the ratio of sensor areas is the square of the crop factor.
This ratio is also commonly – but wrongly – referred to as a focal length multiplier ("FLM") since multiplying a lens focal length by the crop factor or FLM gives the focal length of a lens that would yield the same field of view if used on the reference format. For example, a lens with a 50 mm focal length on an imaging area with a crop factor of 1.5 with respect to the reference format (usually 35 mm) will yield the same field of view that a lens with a 75 mm focal length will yield on the reference format. It is important to note that the focal length of the lens does not change by using a smaller imaging area; the field of view is correspondingly smaller because a smaller area of the image circle cast by the lens is used by the smaller imaging area.
The term format factor is sometimes also used, and is a more neutral term that corresponds to the German word for this concept, Formatfaktor.
Read more about Crop Factor: Introduction, Estimating Sensor Performance, Digital Lenses, Crop Factor of Point-and-shoot Cameras, Magnification Factor, Secondary Effects, Common Crop Factors
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