Originally Posted by jimi_hoffa
... Some lenses are just sharper than others too. I am a Nikon guy myself so I don't know all the lens lines from Cannon, but there are certain lenses that Nikon users know that are tack ass sharp. I'm sure there are some Cannon favorites out there as well. Do some research of the available lenses on bhphotovideo.com and adorama.com. Make sure to read user reviews, some crap in there, but you can usually tell the folks that know what they are talking about.....
1. I knew you meant depth of field.
2. Other than that, all good and accurate info here, but leaves out one very important thing - no matter how sharp the lens CAN be, you need to keep it in the optimal f/ stop range to maximize the sharpness. My 12-24 f/4 and 17-35 f/2.8 are both excellent Nikkor lenses, but they are sharpest in the range of f/8. This applies to all lenses, and has to do with how the light passes through the lens elements and the aperature blades. At the wide-open setting, you are using the outer fringes of the lens, and it is not as sharp. Stopped down to f/22, the light actually bends around the blades of the diaphram, and you lose sharpness here as well. Most lenses do best 1-2 stops on either side of f/8.