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Get information and user reviews for this lens at Amazon: Canon EF 20mm f/2.8 USM Wide Angle Lens for Canon SLR Cameras 
Photo Zone:
Since the release of the EOS system we have seen several updates to Canon ultra-wide zoom lenses but they didn’t really care about their fix-focals in this range ever since their initial release back in the early 90s. The Canon EF 20mm f/2.8 USM feels a little dated by now – less so regarding its mechanical qualities but optically it isn’t really something special anymore. Most good quality zooms can match its quality by now. The fix focal lens can still convince regarding distortions and CAs. Resolution-wise it is still a good to very good lens for sure – just not more than that. Vignetting is a little on the high side at f/2.8 which is a little disappointing for a full format lens tested on an APS-C DSLR. So unless you require an affordable, large aperture lens in this range there’re few reasons left to favor this lens over e.g. the Canon EF 17-40mm f/4 USM L. READ FULL REVIEW
Other Canon 20mm f/2.8 USM EF Reviews
SLRGear
For full-frame digital SLR or film users, the 20mm Æ’/2.8 shows off its poorer performance issues, especially for sharpness and light falloff. Whether this performance is poor however, depends on your point of view; if you’re looking for a lens which isolates a centrally-located subject by means of sharpness and light falloff, the lens does it spectacularly. If you need even light distribution and even sharpness, the 20mm prime will disappoint, especially at Æ’/2.8. At other apertures, you have a better chance of getting what you need. For subframe digital SLR users, the lens performs a bit better, but at Æ’/2.8 image sharpness is still poorer than we’d like. At Æ’/4 and smaller, the lens performs wonderfully, so if you don’t mind using it at this setting, you really can’t go wrong. READ FULL REVIEW