selections from the " In Depth Review " of the D FA 100mm 2.8 WR
The lens covers the full 24x36mm format, not just APS-C. The aperture blades are rounded, delivering a smoother bokeh which is important for a macro lens. However, unlike its predecessor, it has no aperture ring
The optical design is the tried-and-proven formula of the previous generation D FA 100mm F2.8 Macro lens, which in turn had inherited the optical design from the well-respected FA 100mm F2.8 and F 100mm F2.8 macro lenses introduced in 1991 and 1987, respectively. None of these earlier lenses had rounded aperture blades. . .
Macro Capabilities
This is a true macro lens with a flat field of focus and which focuses all the way down to life-size (1:1) magnification. Next to the distance scale it has a scale showing the magnification. Close-up lenses which screw into the filter ring are the best option for achieving magnification beyond 1:1, since bellows and extension tubes cannot be used: there is no way to set the aperture with bellows and extension tubes since the lens has no aperture ring.
Pentax-D FA 100mm F2.8 WR Macro Review - Introduction | PentaxForums.com Reviews
I like my D FA 100mm F2.8 ( non WR ) it is very good even when not used as a macro lens
however I don't use a flash with mine