Get the Pentax K system at B&H Photo.
This apertures series studies focus placement, depth of field, focus shift and field curvature with respect to aperture choice. It is essential reading for getting the best out of the Pentax DA HD Limited 15mm f/4 ED.
Indeed, it is a valuable study for anyone looking to understand the general issue of focus placement and focus shift, regardless of which camera system is used.
Ice on Lundy Canyon Beaver Pond (in DAP)