All of this points to the down side of classic glass. SR does not set itself for them, the 40-year-old SMC coating isn't the best option for modern digital sensors, and relative to other options it's easy to make silly mistakes!
On the one hand a DA21 would cure the problem nicely - but it does leave a larger gap between 21 and 50mm (return of the DA35/2.4?). On the other hand the Sigma 18-50 covers the range with modern optics and coatings and is silent to focus unlike the DA21 (and DFA 50 for that matter). A bit more bulk for sure, but at 21mm it just straddles the f/2.8-3.2 maximim so the DA21 is no faster. And the zoom is already in hand - so no further expense. The age-old conundrum of carrying a few great primes vs. a convenient zoom reappears, especially when one of the few primes is not quite "great"! Must check the 100/2.8 for veiling flare in similar circumstances now . .
|DA15, K24, DFA50, M100, DAL55-300 on K-s1; Sigma 18-50 above.|
While updating the group photo I noticed another new thing: not only does the K-s1 not come in the Pentax 'stormtrooper/panda' White and Black trim (it has grey trim), its white is not in fact white! I know this because I put my broken white DAL 18-55 next to the camera and it's clearly not a good match - the lens is more intensely white than the body. That saves me some effort, as I have a black DAL with a good mount which I could use to repair the white lens' cracked mount. Now that I know the colors would look really wrong it saves me the effort. To spare the fashion-conscious I left the hideous, clashing lens out of the photo.