It's the angle from the extreme left side to the extreme right measured in degrees. Also considered field of view. Most are measured using a full 35mm frame unless stated "DX" of cropped chip, something similar.
That all depends on what you want to do. "Normal" is 50mm on a full frame (35mm) sensor - about 35mm on a crop sensor (most dSLRs). "Normal" is approximately the field of view that your eye has. Telephoto is anything longer than normal, wide angle is anything shorter than normal.what is a good mm on a lens to choose? is there any sites like the one above that tells you more in dept about these things. i always want to know which mm is a good distance so when i shoot my photos they look very natural and same proportion as how i saw it.
sorry i'm not trying to steal your thread "plastii". i been trying to search around and your thread is the closest i got.