Originally posted by chhayanat Do we take it then that in a lens of a given focal length, say 50mm, the size of the image remains the same for an APS-C sensor frame when compared with a 35mm film frame or a 35mm digital sensor frame ; only the amount of the image visible, or the field-of-view (FOV), is less in the APS-C sensor?
Correct. The sensor can't magically make the lens project a different image; the lens doesn't know if its projecting onto APS-C or 35mm. But when the image is projected onto the smaller "screen" of APS-C, the edges of the images are lost. If you then take this smaller/cropped image from an APS-C and the larger image from 35mm and blow them up to the same size, then effect is very similar to what would happen had you shot with a 1.5X longer lens on 35mm, which is where the idea of using a "crop factor" to compare focal lengths comes from. It works very well as it accurately captures the change in FOV. It does not, however, account for the difference in DOF - the 1.5X longer focal length lens on 35mm would give less DOF than the shorter lens on APS-C, all else being equal. But "most" people are more concerned about FOV than DOF "most" of the time.