Pentax DA 12-24 f4.0 | Lens Reviews | The Northcoast Photographer
This guy pretty much single handedly convinced me that the 12-24 was worth the moolah. He also has good reviews of the 15mm and 16-45. When I found one used for 629 at B&H, I jumped on it and I have no regrets. I got the sigma 10-20 f4-5.6 but I was disappointed in the results. Maybe I got a bad copy, but the edges were poorly resolved and this was shot at f8 at reasonable shutter speeds. Even if I got a perfectly sharp copy, there are other issues that isn't just about QC. One, the color are 'off', even the 18-55 kit lens has better colors than the sigma. Northcoast mentions this in his article. Two, the sigma has significant mustache distortion, this leads to a weird focus zone where the edges and center are sharp but the middle ring is slightly off. Minor focusing errors can be very bad on a ultra wide because you can not see the lens misfocused in your viewfinder, but will see it once you get back to your computer. Also mustache distortion needs to be corrected with advanced optical correction to truly straighten out the lines. Finally, the sigma is weaker wide open and weaker at its long range. The pentax 12-24 while its not perfect, its great wide open (so awesome wide open, that I was taking nighttime city shots handheld thanks to combining 12mm focal length with SR), and its a well-behaved lens through out the zoom range (which means you can use any focal length of the lens to compose your shots, what is the point of a zoom otherwise?). The SMC coating of course gives the lens supremely wonderful colors. The only weakness of the 12-24 is controlling flare. When you get flare you get nasty polygonal flare that can ruin a shot rather than a controlled flare that can be worked on in post.
That being said, any focal length is good for landscape photography, not just wide angles. Good landscape photography is seeing a vision, picking a good composition, and processing your shot to match that vision. Just because you can include everything in your field of view, doesn't mean that you should. I would recommend the 16-45mm if you are starting out, its wide enough to give you a taste of wide compositions, but long enough be flexible. Also the 16-45 has excellent flare control, great corner to corner resolution.
This was taken with the 12-24.
This with the 16-45mm