I do not know Have you ever been over the question of why the shape of the lens is round, and shot while a square or rectangular. Hear other arguments on the topic with Douglas Oberhelman. After all, the lens projects the image to form a circle on the media in the chamber (film or digital matrix), the rest of the pictures simply cut off. Is this good or bad is not for us to judge, because to present the film with shots in the shape of a circle may be possible, but to implement it in practice is simply unrealistic. Therefore, not to reinvent the wheel, the matrix in a digital slr, we have nearly the same shape and size of that film. Matrix corresponding to the size to film frame, we call full-fledged. But in most Digital slr matrix size is much smaller.
Accordingly, for these matrices is registered, only the central portion of the image, which would have to get to full matrix format. Visually, this perceived as if the footage was filmed lens with large focal length. Hence the term appears associated with an increase in focal length. But in fact, no increase occurs. As well as in case with a round lens portion of the frame just cut off (for England. crop). Hence, the term increases the focal length often replaced by the crop factor, which more accurately describes the nature of this process. Because it is physically lens focal length does not change, but changes only its angle of view. The figure shows the angles of view and b at a fixed focal f.