Originally posted by Cipher You can still use f4 - f5.6, it really doesn't become too bad until f8 and you gain depth of field and better edge to edge sharpness.
I've got the M 100/2.8, it is OK on the Q but is prone to heavy fringing. The M50 1.7 and 1.4 are better but you should really try all of your lenses, some may surprise you. For an example, here is
a test of the Pentax 110 lenses adapted to M4/3, you'll notice that the "partway" results (which would be the edge on the Q) improve dramatically going from f2.8 to f5.6 while the center results stay the same.
Q images generally need some careful PP sharpening to get optimum results.
I did some back yard testing three years ago with a paper target, so the results were based on a consistent subject. I tested at 300mm the five lenses I had that actually go to 300mm
(1) Sigma 70-300mm APO
(2) Pentax-DA 55-300mm
(3) AdaptAll 60-300mm
(4) Ricoh 70-300mm
(5) Takumar 300mm
I started with the widest each lens would go and tried each aperture to f/11; based on the common "received" knowledge I expected the best for each lens to be f/5.6 or less, so this should be me the best for each lens. In actual fact, however, the Sigma lens's best images were at F/8 or f/11. I know this goes against everything that everyone "knows",
except people forget one factor - for many of these lenses, decreasing detail from diffraction is offset by increasing detail from using a "better" part of the lens {the innermost cylinder}.
By the way, the numbers above show how I ranked the lenses; (1) and (2) were essentially died at f/5.6, but the Sigma lens continued to get sharper beyond that point while the Pentax-DA did level off as expected.