Pentax K-5 II S Operating Manual page 371

Circular polarizing filters and vignetting
Circular polarizing filters are thicker than regular filters. Depending on the lens that is
mounted and the shooting conditions, vignetting may occur in the four corners of the image.
Refer to the "Compatibility table for PENT AX circular polarizing filters" on the following page
for information on compatibility of PENTAX circular polarizing filters (currently not in
production) with various lenses. There are two types of PENTAX circular polarizing filters.
The A type (on the left in the diagram below) and the B type (on the right in the diagram
below). The B type filter is thinner than the A type filter.
A type B type