From my recent testing of my lenses with Reikan Focal I have yet to find one that has a sweet spot higher than f5.6. Although I have yet to actually test all of my lenses. So I am wondering if the old thought that lenses were best at f8 is still valid?
Here are two of the graphs I have available. The first is the Tamron 24-70mm f2.8 VC lens. I can't remember what focal length I used for this testing now. It's on the report.... Was tested at 70mm.
This one is the Tamron 70-200mm f2.8 G2 at 200mm.
All lenses will of course be slightly different. But it is interesting to see the actual sharpness of a lens and the point where diffraction kicks in. Not sure if it changes anything, but these were tested on a D810.