Lens angle of views query

I'm looking at buying a super wide angle zoom for my Canon 10D to take interior shots. I had been looking at the Sigma 12-24mm F4-5.6 EX DG lens but held back on purchasing since they announced the 10-20mm f4-5.6 EX DC HSM. Studying the specs I have to admit I'm rather confused, I'm after the widest angle of view possible, and according to Sigmas website the 12-24 has an angle of view of 122° - 84.1°, whereas the 10-20 only has 102.4° - 63.8°. Now this doesn't make sense to me, unless it's to do with the fact that 12-24 is a full size 35mm compatible lens whilst the 10-20 is designed purely for APS sized digital. If this is the case does anyone know what angle of view I would get out of the 12-24 on an APS sized digital?
Also I know the 10-20 has not yet been released but has anyone heard any reports on it?
many thanks

That's a really good question!

The multiplication factor of the camera is definitely going to have an impact on the FOV of a lens. If a manufacturer markets the specs of a lens based on cameras with greater than a x1.0 crop factor then it's hard to compare apples to apples when each camera manufacturer sells cameras with slightly different multiplication factors...x1.6, x1.7, x1.3, etc. I think your best bet is to call Sigma and ask them what the FOV would be for your camera if that's the deciding purchase point for you. It would be nice if they told you at what crop factor they get their numbers for on that 10-20 lens so you can do some quick math and answer your own question. Maybe it's only designed for their Sigma cameras?

The 122-84 is for full-frame. On the 10D it's approx. 97.3-59.2 (those are diagonal FOVs)

Yes - Sigma's 12-24 angle of view is based on a 35mm camera or full-frame sensor on something like a Canon 1DS.

Check out the Tokina 12-24 too if it will work on your camera (only for smaller sensor DSLR's, not full frame). I'm not a Canon shooter so I'm not sure about the EF vs. EFS thing. I've used Nikon's 12-24 which is very impressive, and Tokina's lens looks like a copy. It's in really short supply because it just came out but it's pretty reasonably priced (less than the Sigma even).

Oh, and one other thing - while they're quoting diagonal FOVs for the 12-24, they're quoting horizontal FOVs for the 10-20. The 10-20 spec is based on an APS-C sized sensor. On the 10D you would get the following:

10-20 HFOV: 97-59 VFOV: 74-41 DFOV: 107-69
12-24 HFOV: 87-51 VFOV: 64-35 DFOV: 97-59

Danielle

Thanks to all of you for your answers, you've really helped. I've found out that the 10-20 is now not coming out at least until the end of this month and unfortunately I need a lens for a job before this so the 12-24 it is. Can't wait to try it out.
thanks again
Andy