Originally posted by fgpinarli Hi. My K20 D underexpose with M 100mm macro and K 135/2.5 with wider exposure, f:4 and f:2.5 respectively. When I stop down the lenses, the exposure become correct without compensation. I use them im M mode metering with the green button. Related the use of the central part of the lenses? Any explanation? Thank you
Your observations are correct.
Unfortunately, the only explanation you will get from Pentax is in the manual. To quote the manual regarding M-42, Pentax-K, and Pentax-M lenses..."an exposure error may occur".
The causes for this issue are poorly understood, but it appears to be related to the type of focus screen installed, camera body used, and corrections applied to the meter output when "A"-mount lenses are used.
Those of us that use manual lenses have gotten in the habit of checking the histograms and adjusting exposure appropriately.
Steve
(A little off-topic, but still worth answering...)