Wilt wrote in post #2158684
...So a 15mm on APS-C would be great for interiors, 17mm would be fine for wedding groups and candids with APS-C camera, making the 17-55mm the equivalent of the 28-70mm on 5D.
The 15mm lenses that I know of (excepting the Sigma 15-30 and the Voigtlander 15) are fisheye lenses, which wouldn't do for most interiors. Much as I like fisheye projection for lots of things, it usually doesn't work for architecture.
To the OP: If you want a wide that is 35% of normal, you'll need a lens in the 17mm range. For most people, that's almost more wide than they can tolerate.
But if you want obscene rectilinear wide-angle lenses such as the lens used in this shot:
Then you'll need what I call an extreme or ultra-wide lens. For full frame, there are few choices that will work. One of them is the Canon 14/2.8L, but I think the price/performance ratio on that lens does not recommend it. A cheaper alternative is either the Sigma or Tokina 14mm primes. I made the image above on Velvia (full frame, of course) with a 14mm Sigma.
The only ultra-wide zoom that will cover full frame is the Sigma 12-24, which is an excellent general-purpose extreme wide for those of us who think extreme wide is routine.
If 17mm is wide enough, the 17-40/4L is mighty hard to beat for the money. I think you'll find that the 20-35 ain't shabby, either, though it isn't as wide. But the L is cheap enough to make it worth spending more.
Rick "a fan of extreme wides" Denney