Evening,
UWA lenses - so let me launch in to my standard overview - somewhat...
Landscapes taken by professional photographers usually are with mid 20 to mid 30 focal lengths. The reason for this is that at these focal lengths there is little to no distortion. So when the lens get wider than say 24 to 28mm you start to see distortion. The reason for this is as the Angle of View gets wider, the lens is pulling in more of the scene around the edges of the frame - sides along with the top and bottom. In order to accommodate this the center of the image tends to get pushed back into the distance. The distortion along the edges tend to manifest itself as some pulling and stretching. So the wider you get, the more of this distortion. The complexity of the lens' optical design also increases in terms of the engineer trying to mitigate this distortion as much as possible. What this results in is more glass, higher quality glass, greater and more precise lens grinding, which drives up cost.
Also, lets say that you camera body has a 2K x 3K pixel sensor. As the focal length decreases (gets wider) and the angle of view increases, each pixel on the sensor records a larger and larger area within the scene being captured. The end result is that at the individual pixel - since its recording a larger area, is recording a less sharp image.
The popular and available UWA lenses come in several types:
- Rectilinear - This is the normal type of lens, where the lens tries to keep straight lines straight
- Zoom - Is a lens that has a variable focal length from one limit to another limit - i.e., a 10-20mm lens
- Prime - Is a lens that has a single focal lens, i.e., 31mm This enables the lens to have a simpler design, thus is able to produce higher quality images.
- Non-Rectilinear - This is the type of lens where the distortion is uncorrected. Straight lines are bent, and that is what you get.
- Fisheye - Again uncorrected and can come in either a prime or a zoom. Pentax is unique (along with Tokina who they co-designed the lens with) by offering the DA 10-17 Fisheye
Bottom line - the wider you go the more distortion, and usually the higher the cost. Also, it tends to be very difficult to understand what you will see by just going off the focal length - as its pretty meaningless for most folks. Look at the angle of view for each lens (for the specific sensor type - Pentax is the APS-C).
Got to go watch Burn Notice - there is more I want to include here - but a bit later. Been in a meeting all day 6am to 6.30pm. I want to go vegetate for a while.....