Crop factor

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Crop factor is a commonly used term to describe the difference between the way a lens of a particular focal length behaves on a DX camera from how it behaves on a film or FX camera.

Because the sensor size on a DX camera is smaller, conforming to the APS-C format, it is equivalent to taking a crop of an image around its centre. This, in turn, for the DX format, means that the angle of view changes, so that lenses behave as if they were 1.5x longer. However, there is also greater depth of field, so a 75mm on FX format has a shallower depth of field than a 50mm lens on DX format, even though they have the same angle of view.