Hi GibbyTheMole,
As in just about everything, I think it's a good idea to experiment for yourself to establish workable tolerances. It's been said by many (who don't own a Q, BTW) that diffraction will start to effect a lens' performance at f2.5 or even faster, and the Photozone tests show that the 01 and 02 Q system lenses show highest resolution between at f4, then degrade at f5.6. A lot of people came to the conclusion that the cut off point was at f4. Neither of these conclusions are based on sound reasoning. The calculation of where diffraction will start to effect a lens' performance is based on the "perfect lens" and doesn't take into account how much resolution can be gained by stopping down, and how that balances against how much degradation is caused by diffraction at those apertures. The Photozone tests, like all lens tests are based on the lenses tested, and only show that those particular lenses topped out at f4.
These assumptions have not been reflected by actual user results, as those who have done tests with Qs and adapted lenses that they own have seen that peak sharpness for a given lens can come at f5.6 or, in fewer cases f8. This is really a new frontier, and we each have to test and make our own rules with our own lenses and shooting techniques.
As far as the handholding "rule of thumb" -- personally, I've found that with SR, I can usually handhold and get acceptable results for a good percentage of shots at about 2 stops slower than 1/ crop factor eq FL when I use good long lens handholding technique. Everyone is different, and some can do better than others. I have the benefit of literally tens of thousands of shots handheld with long lenses and digital cameras that give me almost instant feedback through chimping, so I can see when I need to calm down and take it to another level in technique or get some help from whatever might present itself at the time. I also practice a lot indoors so I don't have to relearn everything at the start of each spring. I probably take as many throw away practice shots of my bookcase during the winter than many people take in total for a year.
Hope you have a good holiday season -- and a happy Q Year!
Scott
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