Pentax HD DA 21mm f3.2 AL Limited lens review: Enhanced image qualityBy Kevin Carter - Monday January 13 2014 Lens Review
Pentax HD DA 21mm F3.2 AL Limited versus Pentax smc DA 21mm F3.2 AL Limited mounted on Pentax K-3: Enhanced performance
Compared with its smc predecessor, the HD version performs very similarly as you might expect, but there are some improvements. From the initial aperture to f5.6, the new HD coated lens has slightly better uniformity, although from f8 through to f22 the two remain alike. Although peak sharpness and transmission remains unchanged, the new HD coating technology, which, according to the company shuns the conventional vacuum evaporation process, appears to improve peripheral illumination while also lowering lateral chromatic aberration.