Pentax HD DA 15mm f4 Limited lens review: Compelling choiceBy Kevin Carter - Friday January 03 2014 Lens Review
An overall DxOMark score of 15 points is a little on the low side but the lens is well corrected for distortion and vignetting.
Peak sharpness is also on the low side at 7P-Mpix, which is a little discouraging, at least on the 16-Mpix K-50. Sharpness wide-open is a little low in the centers while edge sharpness is lower still.
Stopped down one stop to f5.6 though and sharpness improves dramatically, though the edges and periphery remain less impressive. However, image sharpness doesn’t really improve on stopping down further, there’s nothing to be gained except depth of field, while some chromatic aberration may still be noticeable in the outer 50% of the field.