Excerpt: The Canon EF 20mm f/2.8 USM Lens fits in a nice wide angle focal length space below the ultra-common 24-something and 28-something zoom lenses. There is a noticeable FOV (field of view) difference between 20mm and 24/28mm.
Canon EF 20mm f/2.8 USM - Review / Lab Test Report
11 May 2008
Conclusion: 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.