Originally posted by KevChandler 
what ultra wide angle would anyone recomend for my K-5, I'm currently looking at Sigma 8-16mm f4.5-5.6, Sigma 10-20mm (both f3.5 and f4-5.6) and Pentax 10-17mm. I would be using it for both landscape and for "impact" wide angle shots of just about anything.
You are looking at two very different types of lenses - rectilinear (normal) and fisheye (distorted). I have both and each lens treats the subject matter differently, and in many respects these lenses are complementary, as they support each other. First, the wider you go, the more contrast and richness in color you tend to get - however the greater the distortion that there is present. Depending on your subject and how you compose and frame, this may work for you, or can go horrible wrong.
- Fisheye lenses tend to treat things with natural curves better than straight lines and squares - unless you intend to emphasis these items. They are also 180 degrees wide - from corner to corner (diagonal).
- Rectilinear treats lines and squares much better - i.e., the eye sees what it expects to see with the rectilinear or normal lenses. There are some scenes, where a fisheye would appear to be the way to go and it just does not work.
Another aspect is stitching. You can stitch with both types, and there are times when its really beneficial to do so. Also, having a fisheye has given me the opportunity to catch some of my best images. Things in motion, where stitching will not work - where its one shot or nothing, and you need wide - the fisheye.
Just a suggestion - if you really intend to go with a fisheye, I would go with a zoom rather than a prime, since it does offer more versatility and flexibility. The only available fisheye zoom is the DA 10-17. I have it and it is wonderful. Just a note, based on how you frame the subject you can put the bend anywhere you wish, by slightly tipping up or down. Also, watch out where your shoes are. Guaranteed within the first few shots you will have your shoes in the picture. Also, the focal length is not a really good indicator for fisheye lenses since they all catch 180 degrees on the diagonal (corner to corner). The main difference among the fisheye lenses is the type of spherical projection that they use (think of how flat maps are used to represent the earth - a sphere).
You can get prime fisheyes from 8mm to 16mm from a number of companies. Bower, Sigma, Rokinon, Zenitar, Samyang, Vivitar, Pelangi are some of the names (Bower, Rokinon, Samyang and Vivitar are the same lens). Also, the prime fisheye lenses tend to be faster at f2.8.
- The DA 12-24 is very well corrected for distortion, and complements the 10-17 in terms of Angle of View where the 10-17 (180 to 100 degrees) leaves off, the 12-24 (99 to 60 degrees) takes over. You would not expect this from looking at the focal lengths. Sigma and Tamron have similar offerings, inn 10-20, 10-24, etc.
- The Sigma 8-16 is wonderful, and at times a bit too wide (117 degrees at 8mm), but the contrast and color richness is excellent. I picked it up for one specific reason (since I like the DA 12-24). I like to shoot square rigged sailing ships (think the USS Constitution, or the tall ships), and its the only lens that was sufficiently wide to grab from the waterline to the top of the mast in a single frame, pierside. Stitching didn't work too well. I wanted straight lines and thus the fisheye would not provide what I wanted.
A number of these lenses may appear to be relatively slow. However with such a wide field/angle of view, they are really pulling in a lot of light, and thus appear to be faster than what their aperture shows.
Shooting with a wide angle lens is a bit different. Landscape wise, they tend to push the main object in focus into the background, so you really need something in the foreground to anchor the shot for the viewer's eye. They are also very good for very close objects - example, the Rolls Royce in the posting above. The DA 10-17 is a fisheye and is always a fisheye - especially at 10mm. At the 17mm focal length, a great deal of the "fisheness" is dampened out, however it is still there.
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