Beach Bum skrev:

You can find tests on nikonians and kenrockwell. According to Ken, the
Canon is far superior.

Sadly, The Canon lens is not an L series. Neither is the Sigma A EX, DG
either. It (the Sigma) has a problem with flair where I would rather it
didn't. At about 15mm it is quite sharp but any wider and you get the
distortion/compression thing which tends to make people at the edge of the
frame look rather dwarf like. With either of these lenses, be prepared to
invoke some distortion and perspective correction. Quite a lot of it at wide
settings. Have you considered the possibility of using a 50mm lens and a
pano head, stitching the images together to get the coverage? It is almost
as much work fixing a 10mm Sigma shot as doing this.

If you can you should look through these lenses with your camera, focus with
them, look at light sources, etc. There are some lenses, OEM and third
party, that are not at all what many reviewers and advertisers would have
you believe.
If you had a Nikon I would recommend you look through the 55-200 for an
example of a lens that has obvious issues the minute you try to look through
and focus. It is a very good primer for what can be wrong with lenses that
have been favorably reviewed in mass market publications.
While OEM lenses genaerally, but not at all always, tend to be technically
superior  the price differential may be impossible to justify to the person
who is putting down the credit card.
For most amateur users the price difference between the OEM and third party
leneses like this can be a real deal breaker as qualitative differences, for
the purposes for which the lenses will actually be used, are not very
significant.
By current technical standards think about how lousy the lenses were that
were used in the 1930s, 40s and 50s to create the movies and still images
that changed the world and fill your head in ways you do not even realize.
At some point aesthetic ends trump minor consumer technical considerations.

It's almost surely designed for nikon compatibility.

I got the Sigma 12-24 instead of the Nikon. It's 'OK' but I do wish I'd
gone for the OEM now. Flare is a big problem with wide angle shooting
and that effects contrast quite a bit.

How about a prime of some sort? Check out the pbase samples to see what
that FOV looks like and the amount of distortion, flare & contrast, it's
lots of fun but I can't imagine needing wider than 12mm.

If it's anything like comparing the Sigma 15mm fisheye to the Canon 15mm
fisheye, the Canon wins easily while only about 25%(?) more money. IOW, I
wish I had bought the Canon. On these hi-res DSLR sensors, the difference is
easy to spot, not to mention the funky focus performance/motor issues and
general compatibility problems Sigma has sometimes when new cameras come
out. Not worth the trouble for a professional or serious amateur, IMO.

I have one for a Nikon and am very happy with it. Excellent sharpness right
to the corners and very good correction of chromatic aberration. Build
quality seems very good, good contrast and excellent flare control for a
lens like this. The big downside is light falloff towards the edges near max
aperture, but it is quite smooth and not too objectionable unless you are
shooting something that is obviously monotonic (like a snow scene). I have
had various Sigma lenses in the past and was not terribly impressed with
most of them, but this one is a winner IMO.

Toby

Update:

After reading the comments here and other related comments about sigma
lenses and reading some consumer reviews of both cameras, I decided to go
with the Canon.  It should be here before the end of the week.

I'll post my opinion of the lens after scooping up a few photons with it.