>>3350703>No, photographer artfags never were good about making things straight-forward. Else they'd have just used angular field of view...There are decent reasons for talking about lenses by focal length, based on both optics and history.
In optical terms, focal length is a property of a lens. Together with the aperture it defines the depth of field, which can be useful when switching between formats as you can know what kind of dof you're getting from, say, a 100mm lens at a given aperture regardless of what format you're shooting.
Conversely, angular field of view depends on both the lens and on what it's projecting an image onto. When you mount a FF lens on an APS-C camera it's the same lens, same focal length, same aperture, but you've changed the field of view. If the lens had its fov engraved on it then it would not be correct, which would get very confusing - particularly as lenses designed for APS-C would presumably be labelled with their fov on that format. So two lenses both marked as 45° would actually be different.
Finally, the way in which lenses are described goes back to the early days of photography, when there were still many different formats and little standardisation on which people used. A manufacturer could release a lens and have it bought by photographers shooting half plate, 4x5, 5x7, metric centimetre LF formats in Europe, and a range of frames on 120/220.
By the late pre-digital days almost everyone was shooting 35mm with 35mm lenses, MF with MF lenses designed for their camera mounts, 4x5, or possibly 8x10. Each lens would have been used almost exclusively on one format, so it would have been possible to mark them with an angular field of view. However, it would have been hard to introduce a change like that as everyone was used to doing it the way it had always been done, plus anyone buying MF or LF lenses would already be familiar with the standard focal length and aperture way of designating them
