Quote:@Wim - Regarding the "graduated aperture" - is that even possible ?
Diffraction does only occur at the edge of materials that have a different refraction index, doesn't it ?
I have a hard time to believe that the APD element has a variable refraction index.
THAT BEING SAID - when looking at the bokeh highlights - the effect is only extreme (and I mean really extreme) at f/2.8. It is already massively reduced 1/3 stops down. That could indicate that the APD element does indeed consist of two different segments - with the outer ring having a very different refraction index.
Who knows ....
I need to look at the formulas for a while, as it does sound in a way like a graduated aperture indeed.
In principle diffraction is based on slit-like aperture transitions, IOW, a sharp edge going straight from light to dark. I do not know whether there is an element involved that may affect diffraction when there is not a sharp transition, but a graduated one. It could well be; I've never looked into this type of behaviour in any detail.
In addition, I do not know how the APD element(s) are implemented, whether they still work at F/11, or whether it is a single fixed element of which more and more of the APD effect gets excluded by the actual diaphragm when stopping down.
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....