Hi,
Any lens has a sweet spot. It's upto your needs. It all depends
your needs and depth of pocket. And think resale/trade-in value.

As a Nikon owner I had no choice, but I read the reviews very carefully,
including a number of good tests in Japanese mags. The Sigma is rated
equally to the Canon in those. It is actually a bit sharper overall, with
better containment of chromatic aberration at the edges, but it suffers more
from light falloff off axis.

My experiences with the Sigma have been extremely positive. Mine is *very*
sharp, all the way to the edges, although there is a bit of loss at the very
extreme edges, but not noticeable until you examine closely at 100%. I have
been pleasantly surprised at the flare resistance - even shooting into the
sun, one only gets a well contained green comet. Light falloff is noticeable
at max apertures, but is really only a problem when shooting continuous tone
objects like a field in the snow. Build quality is excellent - much better
than the cheaper Sigma lenses. I have heard that the QC on Sigmas is not as
good as with OEM lenses, but both my 10-20 and 17-70 are fine.

My experiences mirror those of the Nikonians:

http://www.nikonians.org/nikon/nikkor-12-24mm/super-wide_shootout_3.html

Obviously the Canon is also excellent, and gives you the advantage of an
extra 2 mm at the long end and 1/2 stop across the range. But the Sigma is
absolutely fine IME.

Toby

Canon, Nikon and Sigma lenses in this focal range all suffer from visible
barrel distortion. Fortunately this is the least noticeable and most easily
corrected, via Photoshop or other methods, type of distortion. Chromatic
aberration is not as easy to correct as some Photoshop and plugin gurus
would have you believe and low sharpness, low contrast, flare and vignetting
at open apertures are an inherent problem with all lenses of this type. I do
not believe there is anything to gain from the higher priced Canon/Nikon
lenses in this group compared to Sigma et al. Materials, design and
manufacturing tolerances are critical to produce high quality lenses in this
focal range and Canon and Nikon are not providing that level of quality to
justify their prices. This is a true caveat emptor market sector. If you
believe PopPhoto the Pentax is the best lens in this class, but if you
believe PopPhoto then the Nikon 55-200 zoom is an excellent lens.

Try http://www.kenrockwell.com/tech/digital-wide-zooms/comparison.htm.
Not a big KR fan but this is an unbiased review. A noted Nikon advocate
he likes the Canon lens the best out of the 4 or 5 available choices.
It seems the only lens that you wouldn't want is the Tamron, softest of
the bumch but also the lightest. A friend shot a 20D with the Canon
10-22 on Easter Island, very neet photos.
The major problem with the Sigma 10-20 is Sigma's usual absence of QC.
Some are great some are awful, there seem to be a lot of complaints
with this lens with half the frame being soft. If I were buying this
lens I'd buy it from a local dealer or a place that has good return
policies. In searching for a WA lens I have seem a lot of samples from
the Sigma and some spectacular photos, so there are good copies out
there.

Tom

Tom

Hi Mark.

Sigma has a bit of an adverse reputuation because they do not have a licence
for the EOS mount.
They reverse engineer their lenses so that they work with current bodies.
These lenses may not work with future bodies. A friend's Sigma 500mm lens
works well with an old EOS body but does not work in all modes with her more
recent EOS 30 body.
You will also have seen postings which query the quality control at Sigma.
Postings have made comments about the mechanical and optical quality of
Sigma lenses.
My guess is that Sigma's "acceptable" quality is a lot wider than (for
example) Canon.

Finally, my friend and I have directly compared only one Canon and Sigma
lens - the 100mm macro lens (100mm in Canon, 105mm in Sigma).
One shop's salesman said the Sigma was optically and mechanically the equal
of Canon. Wedidn't find this to be true. The Sigma was slower to focus and
could all-too-easily be put partially into manual focus mode, something its
instruction leaflet warns against.

Hope this helps.

Regards,  Ian.

The core argument in any Canon/Sigma comparison is this: what do you
intend to do with the lens when you next change body?

If you answer "Keep it of course!" then go Canon.

The Sigma lens is always cheaper, but it is also likely to stop working
with some future body. At that point a re-chip may or may not be
available to fix it.

So if you're happy to sell the Sigma at the same time as you sell the
body and wait to be sure that the next one you buy will work with the
new camera, than fair enough.