Lens come with 82mm filter holder. Attaching a UV filter is possible but vignetting will be inevitable for the first half of the range.
If you are using a 1.5x or 1.6x it is quite ok but you need to get an expensive slim UV filter like the hoya pro 1.
On the performance - it is a very good lens. Provides a very interesting wide perspective, good colours (for such a wide lens) and overall sharpness. But need to be careful on the edges when taking people close as some distortion is noticable.
all lenses will produce distortion, more or less, if the subject is too close to the lens itself. just that it is more noticeable on ultra wides, because the photographer tend to move in closer in order to frame the subject to fill the frame. use the lens on an ultra far subject like a mountain range, and it should look fine. vignette is another thing though...
I am using a Kenko 82mm filter for my 12-24... standard thickness... Personally I find that it is no point to buy a super thin filter cos when the filter adapter is on, the allowable zooming I can get on my D100 w/o vignetting is 15-24mm... If I want to shoot at 12mm, just remove the adapter... which can be removed quite easily...
FYI, a 82mm super thin filter can cost a bomb....and breaks easily!!! My 82mm Kenko filter only cost around $40 (can't remember actual price)
When shooting with super wide angle lens, try to maintain a level between the ground and the lens axis, this will prevent too much distortion.
However, a slight tilt of the axis can result in quite a lot of distortion for a 12mm lens.
I have heard of the new tokina 12-14 wif 77mm filter, if you can wait, why not wait for that to see if it is better than the sigma one...
Oh yes, one more thing....when shooting with the lens w/o the filter, I quite often get some fingerprints on the lens surface as it is rounded and popped out.....
These are just my personal opions about the lens.... hope it can help you with your decision making...
I guess amount of 'acceptable' distortion for a given focal length is quite subjective. Really depends on the one behind the lens. I have attached some sample pics (both at 12mm end on 1.6x) - hope it helps in coming to your own conclusion over the distortion aspect I brought up. When I mentioned 'some' distortion it is relative to such a wide focal length.
Subject at close range
Subject at lens center
Some distortion towards bottom to exaggerate the gown
Shooting people on wide or tele it's a matter of personal style. For me, I try not to limit myself only on tele or a portrait lens at 85mm. Sometimes I find that a little distortion helps bring some interest to a photo.
Agreed that all wide lenses will have some degree of distortion. For a given focal length some lenses have it better controlled than others. IMO it's precisely that since it's a characteristic that's inherent in ultra-wides/wides, it becomes an important factor in considering which wide/ultra lens to get apart from the usuals (sharpness, colour rendition, contrast, OOF etc.)
This lens is currently my only non-Canon lens but it is the lens that changed my perception over 3rd party lenses. Build-quality is very good. Built-in hood provides a good deal of protection over the curved lens - hence a UV filter isn't really necessary if you hold the lens properly. It is like how you would use ultra-wide primes such as 14mm etc.
I have found that on a 1.6x, vignetting at 12mm with a slim UV filter (specifically Hoya Pro 1) is barely noticable. Whereas when compared with the normal UV-Guard, the first 1/4 to 1/3 of the range is affected. Heard that the Hoya Pro 1 on a 1.5x DSLR is also usable at 12mm. I personally do not use a filter for my 12-24 as I also use it on cameras with smaller crop factors. Best for you to actually test it out before buying a filter. As pointed out, 82mm filters are not cheap!