It's perhaps easier to think of the lens/camera combination as providing an "angle of view".
The human eye has an "angle of view" which is a little better than 40 degrees (maybe 42... 45). It's not exact and we don't have well-defined edges on our field of view. We don't see a rectangular box. It varies from person to person. This is "roughly" the area that you'd see if you look straight ahead without moving your head and without moving your eyes.
Any lens/body combination which provides a similar angle of view will seem "normal" to you. That means the image will neither seem magnified with compressed depth... nor will it seem wide with expanded depth.
A 50mm lens is only "normal" on a full-frame DSLR body (one with a sensor size approximately 36mm x 24mm -- the size of a 35mm film negative.) With that combination, the angular size of the field of view approximately 40 degrees wide by 27 degrees tall (and about 48 degrees on the diagonal).
That same 50mm on a body with an APS-C crop-frame sensor only provides an angular field of view which is about 25 x 17 (degrees) and about 30 degrees diagonally. That's too narrow to be considered "normal" -- that's a slight telephoto focal length. To get a "normal" focal length on an APS-C crop-frame sensor you'd need a 31mm lens (which nobody makes) but a 28mm or a 35mm lens would be pretty close.
On a medium format camera (an image size of about 6cm square or even 6 x 4.5cm format) the 80mm lens provides a "normal" angle of view.
If the lens has a noticeably shorter focal length, then it will seem wider than natural, with some wide-angle distortion and the depth will appear expanded (like the warning they stamp on your car's mirror "Objects in mirror are closer than they appear"). That's the "wide angle" side.
If the lens has a noticeably longer focal length, then the sense of depth will be compressed and objects appear magnified and closer. That's the "telephoto" side (not telephonic, btw... "phonic" is sound. "photo" is light.)
Some zooms are referred to as "telephoto zooms", or "wide angle zooms", or "standard zooms". The "kit" lens that comes with most camera bodies when you buy it as a body with a lens is a "standard zoom". That lens will have some point which provides a "normal" focal length and will also provide focal lengths which are wider as well as longer. A "telephoto zoom" would provide focal lengths which are "longer than normal" to "much longer than normal" (so there is "normal" focal length anywhere in the range.)
The point is, "normal" is whatever angle of view will approximately match what your human eye can see without having to "look around".
Tim Campbell
5D III, 5D IV, 60Da
