Wide Angle Lens

Just to add to the above comment - the Pentax 10-17 is a fish eye and has a whopping 180 degree angle of view Can't use filters.

The Sigma 10-20 will be less than that, but near enough straight Can use filters

Both can be straightened out near enough completely in software.
There have been other threads here on the differences in angle of view... plus some example shots too!
Matt