Originally posted by fromunderthebridge Umm. Sorry, I don't want to turn this into an equivalency 'discussion', --but-- isn't the apparent magnification more a question of pixel pitch on each format than the focal length of the lens (higher density pixels -> 'more magnification')? I mean otherwise you could just crop from the FF sensor to achieve the 'cropped omg magnification!' It is a 'cropped sensor' after all...
Ofc. to cover the larger image circle on larger formats the optics need to be larger, but focal length is focal length is focal length.
If there's a 6 inch tall bird sitting in a bush 20 feet away, a photographer is most likely to wonder: what focal length do I need to have the bird fill 2/3rds of the frame height? For FF, the answer is 640mm. For APS-C, it's 427 mm. For M43, it's 320 mm.
Pixel pitch would matter if the photographer asks a slightly different question: what focal length do I need to have the bird be 3000 pixels high in the final image? In that case, the K-1 and the K-5 would need the same focal length (about 587 mm) and the K-3 would need only a 480 mm lens. (Note that a photographer might ask this question if they want the bird to be 15 inches tall in the final print when printed at 200 PPI.)
Although it's true that focal length is focal length is focal length, the goals of the composition (object size relative to frame size) or goals for printing (final print object size and preferred PPI print resolution) imply that different cameras require different focal lengths to accomplish the same image creation goals. Equivalency is just a short-hand way to say that one focal length on one camera format has the same photographic composition effects (angle-of-view, perspective, and distance compression) as some other focal length on some other camera format.