Overview

The Pentax smc DA 40mm F2.8 XS is an ultra-compact pancake lens (9.2mm thick, 52g) designed for the K-01 but compatible with K-mount cameras. Optically, it offers excellent center sharpness from f/2.8, with very good edges; peak performance is at f/8-f/11. Weaknesses include soft corners wide open and increasing chromatic aberration when stopped down beyond f/11. Vignetting is minimal. Autofocus is fast but noisy (screw-drive) and the front element rotates. It suits compact shooters prioritizing portability over ultimate image quality.