I will also admit to distrusting the M lenses categorically due to their smaller size, even with plenty of examples that bigger does not necessarily imply better optical performance.
It seems to me that what Pentax and some other manufacturers have tried to do in the 70s (following Olympus) was to make smaller and lighter lenses that are AT LEAST ALMOST AS GOOD as their bigger couterparts.
I compared the big Takumar 200/3.5 with the M 200/4, taking the same photo and checking details in 2700 dpi scans and they showed the same resolution and practically the same contrast (maybe a dash better in the M lens). In that case, Pentax has not significantly improved the optical quality of these 200mm lenses between the 1960 and 1980 designs, they have improved their portability.
(The missing link here is the 200/4. I only know that Modern Photography in feb 77 gave the K lens very low corner contrast at all apertures.)
I found size an important factor for normal and moderate WA lenses when you need ease of use, as in stage photography. The smaller M focussing rings are a pain to use when you have to work fast. But for landscape photography, for example in the mountain, when you have all your time after putting that heavy pack-sack besides you, it's another matter : smaller lenses are good for your knees!
Andre
