Today, I tested three Hasselblad HC lenses for this witchcraft term "diffraction" that I've heard thrown around on this forum. I thought it was hogwash.
It is not. Maybe it's more critical with digital than film. I don't know. I'm not a scientist. But I know sharp from soft.
I tested the 35mm HC, the 50mm HC, and the 80mm HC, on an H2 body with a P45+. I tested it tethered, inside, using strobe to eliminate camera shake. Shutter at 800th. Profoto head adjusted each time, dialed down one stop so the light would not have to move.
The range was, in general f8 to f22, in full stops. The files are too large to post here; no use in posting an artifacted JPG, when we're discussing subtleties like this.
But in general, my advice is to NEVER shoot an HC lens any smaller than f11. Even f16 shows noticeable softness, and 22 is a joke, and let's not even talk about 32 or 45.
So boys, instead of just "stopping down and carrying focus", you'd better be learning to shoot focus brackets, and build the layers. No, not good news at all.
If Mr. Grover reads this, meet me on the corner of Main and Sorrow, at high noon, and I'll turn over the bad news files.
My hunch is that this factor affects every brand, although I'm sure there are some differences between brands. My advice: TEST YOUR OWN GEAR. Do not rely on my information, or any other hearsay. Just take this as a warning. Just because it says Zeiss or Planar or whatever on the front, it only means that some of the fstops are sharp. Test your own stuff so that you know your own gear.