I wish I could love this lens, maybe i'll try another copy and give it another chance.
On full frame a 15mm lens is very wide, and gives a great range- a surprising amount wider than the 17-40.
People say it's a bit slow at f3.5 to f4.5, but realistically it's no slower than the 17-40 L, and when you're shooting stopped down it doesn't really matter.
There are 4 major problems with this lens:
1- you can't use filters, which limits your creative freedom- apparently you can use real mounted filters but I have never seen them for sale or heard of anyone using them. The nikon 14-24 has the same problem but it is popular enough that someone came up with a solution
2- it's not sharp, especially in the corners where it's not sharp at all. The 17-40 gets a bad rep for corner sharpness but compared to the canon the sigma looks broken- maybe I had a bad copy as I've read good and bad reviews about the optical performance of this lens
3- the AF is loud, and slow. I use my 17-40 for photojournalism, and quick silent Af is useful- less so if you shoot landscapes
4- there is no AF override. With the 17-40 you just grab the focus ring and go, the 15-30 has some weird clutch system where you pull the tab forward and then you can manual focus, it's slightly intrusive and can be annoying in situations where you need to respond fast.
if it helps the lens looks amazing, and feels very well made- but when I dropped it I could see some cost cutting inside- the canon lenses are much more sturdy, although I have seen a 17-40 break in the same way my 15-30 did.
Despite the mostly negative review I did enjoy this lens, and despite yearning for the 17-40 from canon I didn't upgrade until I knocked over my tripod and smashed the lens, I then realised the although the lens is mostly metal, it's actually plastic where it matters, it was made to look tougher than it actually was.
for the price though it's pretty hard to beat- the 17-40 was more than double the price
Okay.....before I start, I think I should explain that I am somewhat of a camera snob. In the same way that some people like to buy designer clothes, I like camera equipment that has a "name". I am generally not satisfied buying any brand I deem to be sub-standard or simply not the best. However, unfortunately, I have to be a realist and sometimes I want / need something which I can't afford. Where camera equipment is concerned, this is where Sigma fits in to bridge the gap between the giants of Canon and Nikon and the lesser mortals of the camera world!
So, I needed a wide angle lens for my Canon 5D and, having looked at the Canon wide angle lenses on offer, I simply couldn't stretch to the asking price at that specific period in time. And so, after attending the In Focus show at the NEC where I got to try out the Sigma 15 - 30 mm F3.5 - 4.5 EX DG, I decided to purchase this as it was at a much more affordable price. I managed to get the lens from the In Focus exhibition from the Jessops stand for the reasonable price of £200 (including VAT) which, compared to what I have seen since, appears to be very reasonable.
My first impression of the lens is that it didn't have the feel that you generally get from the better quality lens. The casing all felt very plasticky and without the sturdiness that I have come to expect from quality lenses. When I am holding it, it feels okay, but it also feels like if I was to drop the lens then it would break which, as my experience has born out on many occasions, this is not the case with canon products which are so well protected that they can be bashed around pretty much without suffering any ill-effects. However, you'll be surprised to know that I haven't dropped this from a great height to test out my belief and, as of yet, two years on, it is still in one piece and in good working order! Although the lens feels a bit bulky and cumbersome, it is not heavy and this is something that it a real benefit for me because I'm a bit weedy and carrying my camera equipment round with me can be a bit of a chore because of its weight.
I am generally quite impressed with its performance, which is why I haven't felt the need to replace it. I have to admit that I don't do a great deal of photography that benefits from a wide-angled lens and so its not one of those lenses I use every day, but having said that, it has become a very necessary part of my kit and something that I use from time to time.
The images produced using this lens are generally clear and focused and of a good quality. I do sometimes find the zoom (15-30mm) and the aperture range (3.5 - 4.5) to be a bit limited and its a shame that these ranges weren't somewhat widened, but having said that, I generally use this camera for landscaped photography and for this, the ranges are not too much of an issue.
There is a switch for Automatic Focusing and Manual Focusing which can be used interchangably. Compared to my L series lenses, I do find using the AF on this lens to be quite noisy and an little sluggish, but again, the majority of the time I am using this lens in conditions where speed or noise are not really issues and so its something that doesn't really bother me and so its more of an observation. I do miss....call me lazy.... having an image stabilizer on this lens, but I did know there was no IS on the lens when I bought it and so there are no complaints regarding that.
I'm not a big fan of the fact that the lens has a petal type hood on it which in unremovable. I generally don't like to use hoods in my photography and I usually don't have a need for even using external non-fixed ones, and so it can be a bit irritating having the hood in situ when I simply don't want it. I find having the hood in position tends to add to the risk of vignetting - again something I don't want. I also don't like that it means that I cannot add a UV filter or a polarising filter to the lens. However, the use of the hood does tend to result in images with a deeper saturation and a less imposing contrast - and this seems to be a good combination for landscape photography. The hood is also a protecting factor of the lens as it is virtually impossible to scratch the lens accidentally.
The lens works out to provide a very wide angle on full frame sensor cameras and distortion is kept to a minimum.
Overall, I like this lens but I don't love it. For the money it is a good value lens and it gets the job done. However, I'm afraid I will be investing in a Canon wide angle lens at some time in the near future and I will loose this loyal yet sub-optimal lens to Ebay! I would recommend this lens for people who don't want to fork out loads of money for a wide-angled lens and, in its price bracket, I think this is the best that there is.