@MarkJerling
Yes, RAW (PEF) files exhibit the same problem. Stronger sharpening set in DarkTable or RawTherapee can mitigate the problem (in case of slightly out-of-focus images). The problem isn't caused by JPEG compression.
No filter was attached, only the original Pentax lens hood (slot type) and the lens isn't dirty (checked that).
Imagine taking 10 pictures (landscape/architecture) focused 50m (160ft) away taken with the same aperture setting with the results:
01 very slightly out of focus
02 slightly out of focus
03 perfect!!!
04 slightly out of focus
05 totally out of focus
06 very slightly out of focus
07 out of focus
08 slightly out of focus
09 very slightly out of focus
The results while focusing on closer subjects are usually better but not always (see those leaves through the grating).
@photogem
I've added 3 sample pictures with
SMC Pentax-DA 17-70mm F4 AL [IF] SDM Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database, focused to the distant building or ship. They're f/4 but believe me - if focused correctly they could have been tack sharp.
Maybe I exaggerated a bit with 9 out of 10 spoiled images. The score could have been as above - actually only 1/10 totally out-of-focus, the rest from almost acceptable to acceptable or perfect.
Isn't this why that lens has Autofocus rated only 6.2 out of 10?
The K-S1 may be at fault. But why it focuses very reliably with the cheap and noisy
SMC Pentax-DA L 50-200mm F4-5.6 ED WR Reviews - DA L Zoom Lenses - Pentax Lens Reviews & Lens Database (user rated AutoFocus: 8.4 out of 10)?
I'm definitely spoiled by my previous K-m body and the kit 18-55 lens I used for almost 10 years. While browsing through old albums (the same type of scenes), I can't almost find misfocused shots at all.
Thanks again for all the tips. If there isn't a hidden setting I haven't tried yet, I should indeed try some of the suggested lenses and hope that the problem disappears.