Quote Originally Posted by Zeva View Post
So... You mean like for the 17-70 sigma... its F/2.8-4.5 which means at 17ish its maxed at 2.8 and 70 its maxed at 4.5? so it cant go below 4.5 at 70 but like the 70-200 F/4 COULD go to f/4 at 70 AND 200? so its just the max? and its not fixed and can be shrunk? thanks BTW those were fast replies!
Quote Originally Posted by Razr View Post
Only if you are in "P" (PROGRAM) or "A" (AUTOMATIC) modes.
The aperture stays the same in "Av" and "M" (MANUAL) mode, no matter what focal length you choose, making any f/2.8-f4.5 etc. lens a "fast"-constant apeture lens throughout the zoom range.
Quote Originally Posted by zmikers View Post
Maybe I misread your post here but I think you are wrong. If you are in Av or M mode with a f/2.8-f4.5 lens on your camera and your Aperture is set at f/2.8, as you zoom the aperture will not stay at f/2.8, it cannot. It will change to match the max aperture to that specific focal length. So the sigma 17-70 f/2.8-4.5 has a max aperture of f/2.8 only at 17mm and a max aperture of f/4.5 at 70mm.
Zmikers is correct here. If you have a f/2.8-4.5 lens, you cannot change it to a f/2.8 lens just by putting it in Av or M mode. The max aperture changes throughout the focal range and the aperture value will change accordingly in both Av and M modes.

Ray.