New Q

Agreed, diffraction is entirely optics specific, but pixel size determines when you can see impacts from diffraction.

I think you might be seeing that those lenses perform best at those apertures because of properties of those lenses.

From my blog erphotoreview.com for the Pyxis/Spacecom/etc 16mm f/1.4 5MP lens.

This doesn't look high resolution, but is significantly higher than 5-15mm kit at 15mm.  Notice, there is no huge drop from f/4 to f/5.6, but there is a noticeable decline, and even bigger from f/5.6-8.  I.e. I wouldn't use this lens much above f/5.6 unless I needed depth of field.

What you will likely see for the 55-300 is you start in that diffraction limited f/5.6 area, but you are actually limited by the optics in the lens, so you see a slight improvement stepping to f/8, although I am guessing the optics at f/5.6 should be better than diffraction at f/8.  Maybe it is some other reason, like depth of field or something?

I can't test the 55-300mm, maximum I can do is about 150mm in full frame equivalence with my chart without going to extreme measures (taking the chart outside).

Eric

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