Pentax HD DA 40mm f2.8 Limited lens review: Compelling choiceBy Kevin Carter - Thursday January 02 2014 Lens Review
An overall DxOMark score of 16 points is slightly disappointing but the 40mm f2.8 Limited has very low chromatic aberration, and good control of both vignetting and distortion.
Pancake lenses like this rarely have consistent sharpness across the frame, and this model only has good sharpness in the center when stopped down. Wide-open, sharpness is only moderate, but fortunately that improves in the center significantly at just one stop down from the initial aperture. Peak sharpness is measured at just 8-Mpix at f/5.6, which is a little low on the K-50.