Sharpest f stop?

six-five-two:
Sorry I don't know enough about Pentax lenses to recommend one. One thing you probably shouldn't do is get too caught up on the numbers. Yes sharper is better, and so is low vignetting and low CA. But ultimately the softness of the image will not be apparent until you start using your sensor to its fullest. A 10x8 print will look tac sharp with even a crap kit lens. It's when you zoom in 100% on the monitor, or start printing A3+ sizes that you need to worry about sharpness of lenses.

xfloggingkylex:
Diffraction is like a noise floor that gets worse as the aperture closes. It exists in every f-stop. Diffraction sets in at apertures where the sharpness peaks. So if the lens is sharpest at f/8 then at f/11 diffraction is starting to cause the problems. If it's sharpest at f/5.6 then at f/8 it's an issue. This is usually how it goes, but I am sure an optics expert can give a counter opinion. Also mounting one of the lenses on a full frame camera is likely to give you different resolution figures because the diffraction element changes based on sensor size.
 
I've read and been told that one stop from wide open is going to be your sharpest. I also have heard from people that f8 is going to be sharpest, but the one stop from wide open I learned in school so I'll stick with that.
 

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