I spent a month in hesitation between 31mm Pentax and Sigma. In the end I decided for the Sigma (400 cheaper than the Pentax ... anyway!) And frankly I do not regret my choice. The only drawback to this goal is to be a bit heavy. For the rest, nothing but happiness. I use it on a Pentax K5. It is very well to the more open, and absolutely perfect from f / 2. Piqué in the center as at the edges. Depth of field blur (bokeh) remarkably smoothly. Speed of installation Excellent focusing. Prime.