While the subject matter isn't necessarily rocket surgery it
is a bit challenging at first to wrap your brain around. For most, the subject of bending light isn't terribly intuitive and when you introduce compound lenses and varying film/sensor sizes it gets very muddy in short order.
After months of wrestling with the subject(s) via various sources I finally (I think) got my brain wrapped around most of it after finishing up with Ansel Adams' Book 1 "The Camera"...
For any given photographic subject from a fixed viewing position and constant aperture:
+ The perspective remains constant regardless of focal length and film/sensor size. Perspective being defined as the relative size and position of each object in proportion to each other within the frame of the shot. Perspective
only changes by changing your viewing position with respect to the subject - be it nearer, farther, higher, lower, to the left or to the right.
+ The field of view (how many degrees from zero, or straight ahead) captured will vary with changes in focal length and film/sensor size. If the film/sensor size remains constant, the field of view will
decrease as the focal length increases. If the focal length remains constant, the field of view will
increase as the film/sensor size increases.
+ The depth of field will vary with changes in focal length and film/sensor size. With constant film/sensor size the depth of field will
decrease as the focal length increases. With constant focal length the depth of field will
increase as the film/sensor size increases.
Where's the teacher, and do I get a gold star or a dunce cap?