Originally posted by: vi_edit
The smaller number is how wide the lens is(how much you can frame at once), the big one is how far it can zoom.
well, no, not really. zoom is merely the number that you divide the telephoto end by to get the wide end.
both numbers are focal lengths. the smaller is the shortest focal length, the larger number is the longest focal length. both focal lengths could be wide, or telephoto, or a combination. sigma's 10-20mm is extremely wide at it's longest, and super ridiculously wide at it's shortest. 'normal' for a 35 mm camera is about 40 to 70mm. 35 is a mild wide angle, 28 is where wide angle really gets started, and 20 mm is dramatic. 85 is a short telephoto, and on up you narrow the field of view.