Sigma's 10-20mm F4-5.6 EX DC HSM lens allows enjoyment of super wide-angle photography and it is a very powerful tool for indoor shooting and landscape photography with APS-C size image sensors of digital SLR cameras. The wide angle of view ranges between 102.4-degrees at 10mm and 63.8-degrees at 20mm, which offers the photographer greater freedom of expression. It has a minimum focusing distance of 24cm (9.4 inches) at all focal lengths, and is able to use 77mm filters.
Three SLD glass elements are employed for effective compensation of color aberration, which is a common problem with super-wide angle lenses. This lens has almost no visible CA at f/8 and above at 12mm. At 10mm, there is some CA in the corners, but this can be fixed in Photoshop. One piece of glass mold and two hybrid aspherical lenses, offer excellent correction for distortion, as well as all types of aberration. With that said, it has slight barrel distortion at 10mm and slight pincushion distortion above 15mm, which can also be fixed in Photoshop. This lens is equipped with an inner focusing system, and the models which are equipped with HSM system provide quiet, high speed auto-focus shooting and also offer full time manual focusing. The non-rotating lens barrel perfectly suits the petal shaped lens hood.
The HyperSonic Motor enables virtually silent, highly responsive auto and manual focusing, as well as manual focus only override.
This is a DC lens, which means that it's designed so that the image circle matches the smaller size of the image sensor of most digital SLR cameras. This can cause the corners to be slightly darker, although this problem goes away when you use an aperture of around f/8 or smaller. Since most people will be using this for landscapes, a smaller aperture is usually desired to maximize depth of field anyway.
Since this lens costs half what the Nikkor 12-24mm super wide angle does, it's definitely a viable solution for the Nikon landscape photographer. Sigma has a 12-24mm super wide lens as well, but you can't use a filter on it. Without a polarizer filter, lens flare becomes a huge problem when shooting wide angle. I believe this is the case for their 15-30mm lens as well.