The Sigma 17-50mm f/2.8 EX DC OS HSM Lens is available in Canon (reviewed), Nikon, Pentax, Sony/Minolta and Sigma mounts. I find a lens with this size and weight to be easy to take with you, comfortable to use and to have enough mass to be stable.

I didn't see any significant differentiators from a CA standpoint - these lenses are all similar in this regard.There are, however, many 17 to 50mm (or 55mm) f/2.8 lenses available from other manufacturers - I'll reference some of these later in this review.

Following the formula, input the values of a and b in the corresponding fields. These can be integers, decimals, etc. Sigma USA's 4-year warranty is far superior to Canon's standard 1 year warranty (Sigma's international warranty is also 1 year).

Any clouds in the sky, even if they appear to be even, can change the lighting between shots and cause one lens to appear better or worse than it is. due to lens focal length marking discrepancies and my guess at the focal length setting on lenses not having 28mm marked. The good news is that only the edges of the zoom and focus rings retain the conventional Sigma finish.