1)angle of view-diagonal
2)I'm not a big fan of 35mm lenses, to me they are too close to a 50 and so, I
prefer a 28. 21mm is a pretty wide lens if it's the only one you've got and it's
possible to get pretty tired of the distortion all the time.
3)Don't ask why 'cus I don't know, but although I don't care for the 35mm fl. I
just really like the 40mm!