As a longtime lurker I thought I'd get my first post in just to see if the assembled experience of the GetDpi crowd might be able to help, or possibly just confirm a decision I've got to make. I use a 5D III and tend towards the tele end. My standard walk-around kit being a 70-200 f4 IS and and 35 f1.4. Just occasionally I find myself needing something wider and stitching isn't an option. This happens rarely enough that it's only now starting to bother me. I already have the 17-40 f4 but the quality of my other lenses leaves me wholly unsatisfied with this zoom at any aperture, particularly in the corners. It's also a little too large to be carrying with me to be used for one shot in 100, if that. So I thought that I'd try to find a small prime in the 20-24mm range that I could hide in the corner of my bag. I've been looking through the options and here they are:
Sigma/Tamron/Tokina: Have various 17/20/24mm primes available but none review very well, certainly nothing that would completely outclass my 17-40. The one lens that does get a lot of attention is the Tokina 16-28 but that is far too large and heavy.
Zeiss 21mm: Too heavy, too expensive
Voigtlander 20mm f3,5: When I first heard about this I thought it would be perfect but again reviews are mediocre.
Canon 20mm f2.8: Poor reviews, no better than the 17-40
Canon 24mm f2.8 IS: The favourite so far. Good quality, not too large and IS is a bonus.
So the result is that the new Canon 24mm IS is the favourite by quite a way. Could others with some experience with any of these lenses confirm or dispute my findings? I realize that they're mostly based on a few select review sites.