Sensor size and focal length
This page is intended to clarify the impact of using full frame lenses on a cropped sensor camera. To emphasize the impact I am using images taking with a 50mm and a 135mm lens rather than with a 50mm and 75mm lens which would match the relation between the APS-C and the full frame sensors.
The focal length of a lens is specified in the same units of mm regardless of whether the lens is intended for a full frame DSLR/ mirror-less camera or a camera with a smaller image sensor such as an APS-C sensor. This means, for example, that the image produced using a “normal” 50mm lens on a APS-C sensor camera will be cropped to a size resembling a 75mm lens used on a full frame camera . However, the two 75mm images will not be quite identical. The latter, or the actual 75mm lens, will have shallower depth-of-field. A full frame 50mm lens used on a cropped sensor camera will also produce a slightly less sharp image than a 75mm lens on a full frame camera (assuming the two sensors have the same pixel density and the lenses used are of approximately the same quality.) There is one benefit of using a full frame lens on a cropped sensor camera. Unsharp edges and corners produced by the lens will be cropped out. Unsharp edges are common with most lenses particularly when using large aperture settings such as f/1.4-f/3.6.