[Brightcolours]

I do not think (as opposed to know) that PF is a reflection effect caused by the lens. It feels highly unlikely that reflections create a halo effect. (Re-)Reflections would be distributed across the image field - essentially re-reflections are a freak effect.

 

If I had to GUESS, this may be caused by the micro-lenses that sit in front of the photodiodes but not by the lens. These micro-lenses are certainly NOT coated. It feels viable that sensor-surfaces reflections are distributed to adjacent micro-lenses. This would also explain the halo effect.

 

Just guessing.

But is that were the case, one would see similar with every lens with a similar f-ratio. And we know that is not the case... Even the OP notes a difference between his 75mm f1.8 and his 50mm f1.8.

 

Do you have any ideas about how to do a quick and easy PF test? Then I could conduct some experiments.