I am pretty sure Otus maxes somewhere around f/4, see figure below from Lensrentals. DxO-mark shows similar data.
Erik, the LR curve shows on-axis performance of the Otus at f/4 slightly better than f/5.6. My testing shows it slightly worse. The difference is not large, and could be due to:
statistical variation (there are no confidence lines or bars on the LR curves)
difference contrast targets (I used a contrast of 4, in Imatest terms)
difference lighting spectra (I used a studio strobe)
sample variations in the lens
not using the same shutter speed for all tests (I varied the light level. If someone didn't do that, but compensated for exposure as the f-stop is opened up by increasing shutter speed, that could reduce vibration on the widers aperture test images
a different sensor (I'm assuming that the LR test images were made with a D800E, although you didn't say so. The D800E has a strange "non-AA" filter. I believe the a7R has none)
many other things
I don't consider the differences in the two test results to be particularly important.
My understanding is that the classic definition is that on a diffraction limited lens the first Airy ring [you mean the zero, I believe] can be clearly seen.
I don't have an optical bench. If I did, that's the test I would have performed; it's the gold standard. A 36 MP sensor can't clearly resolve the Airy pattern even af f/16. SO I used a more indirect way to get the answer. I thought it was pretty clever, but I've always been a big fan of my own ideas.
Note that Lensrentals uses LP/IH and not the more usual LW/IH, which gives half the figure and also that they don't use sharpening.
We both use Imatest. I didn't use sharpening either. I took the readouts in cycles/pixel, and converted them to cycles/picture height for the full frame. As has been discussed in this thread, a line pair is a cycle, and line pairs/PH should be close to (exactly?) cycles/PH.
But the LR numbers are way too low for a 36 MP sensor, which is 4912 pixels high and can theoretically resolve half that many line pairs. I think I know what happened. Imatest
reports results in LW/PH based on the height of the sample image it sees. I always feed it a cropped image, and thus any readout that is per picture height is looking at the wrong height. It's possible that the LR folks didn't correct for that, figuring, quite properly, that the absolute numbers weren't important, just the relative ones.
Eric, have you seen any claims that the Otus is the sharpest at f/2?