Let me start by saying, this Sigma is a very unique piece of kit indeed. In fact, if you are looking at this review there is a strong possibility your probably strongly contemplating adding one to your own kit yourself. Well I will cut to the chase - two very good lab tests with all the technical data can be found...
I'll start with probably this lenses archilles heel - Sample variation
Well, I didn't get very good luck. Like alot of samples out there my first 8-16mm was strongly decentered, so much so that stopping down to f/11 still showed noticable corner variation (soft left frame, especially lower corner). However I am happy to report my second copy is much much better! Unfortunitly I have heard that alot of these share similar problems with decentering so if you decide to purchase one, I strongly recommend trying it instore first or having a very good return policy. Here in New Zealand these are extremely hard to get instore so I got mine from HongKong.
Having owned a Tamron 10-24mm and had time with the Sigma 10-20mm and Canon 10-22mm, I can report that the build quality is about on par with the previous Sigma and Canon. The Tamron was a little too much plastic feeling for my liking but was still and well made lens. I can't remember much about the Sigma 10-20mm, but unusually the zoom ring rotates clockwise (my Sigma 17-70mm OS is the opposite here).
The HSM motor allows for full time manual focusing which is a nice touch, though rather pointless on a wide angle in my opinion. Of course it lacks any form of optical stabilisation. There will always be a debate on whether wide angles need this feature. I'm not going to start one here, but at 1/10 sec and sometimes less at 8mm I can comfortably hand hold sharp images.
There is one switch which is the AF on/off. One thing I was suprised about was the balance when mounted on my 550D. I expected it to be lens heavy at 555g, but suprisingly it sits rather well balanced. Haven't had a chance to test it on a 50D or 7D yet but I suspect due to the length of the lens it will feel right at home there aswell. I feel this is a big tick in the Sigma 8-16mm life box.
At f/4.5-5.6 not a fast lens by any means but for this field of photography I feel that speed is the least of the worries.
8mm vs 10mm:
Because this is a real world review I feel this is important. Many say 'just step back a few steps with your 10mm and you will have an 8mm' - this statement is rather lacking in vision. I feel (apart from the extra 10 or so degrees of wideness) that the extra 2mm offers so much more on the creative side. As a rather specialized lens, you will find yourself having to 'work' a shot alot more than a standard kit range lens. Sure the results are amazing but be prepared to spend time to realize it's full potential.
I would go as far as to say that the extra 2mm will place you right back at the start of what you thought you knew about wide angle photography and perspective if you own a 10mm lens. It did for me, but hey that's half the fun of something new.
Sublime, needs no more. Corners are a little soft at f/4.5 at 8mm but stopping down to f/5.6 sharpens them up nicely. I would say that this is on par or better even with the Canon 10-22mm or indeed any wideangle I have ever used in the real world. The major downside to such wide angle is ofcourse vignetting and this will require stopping down to at least f/8 to reach good levels. I see this as a rather small problem though as this can be fixed rather well using software. Yes distortion will be noticable at 8mm if you have photographing straight lines, but no more than the competition. Flare is also rather hard to avoid, and it's not exactly pretty, but that's simply the price you pay for this sort of lens.
I think the new FLD glass deserves a mention aswell. Whilst it is supposed to minimise chromatic aberrations, it is still visible on harsh contrast borders. Levels are very acceptable though, and you'd really have to have a bee up your backside to consider it a worry. What it would be like without this glass I don't know, but in all honesty it is still much better than my old Canon 15-85mm was (and that's a very good lens).
Is hard, and it has a coating on it of some description which may put you off touching it with lens tissue or microfibre. I have done it, and the coating is resistant to both these, albeit extremely hard to get evey last piece of dust off it.
I love it, to the point where I will probably keep it on my camera 50% of the time. For a lens like this that is alot, but the amount of creative flare it can unleash knows no bounds. With impressive sharpness and that beautiful 8mm end I can only highly recommend it - If you can find a good copy! I have focused mainly on the 8-12mm section of the range here, but I think this is the section that will be used most of the time for most people.
Price I paid - $660 USD incl shipping
* Photo's will be here shortly as my internet is capped at 5 kb/s