John: very unusual bokeh effect. can you explain these lines?
Frank, I can not explain exactly why these are rendered when they are from an optic engineering viewpoint because my understanding of optical engineering is not up to that level.
But what I do know is that a good example of a lens that reliably renders these "double hard lines" in OoF areas is the 85mm f1,8 AF-D Nikkor.
When at medium distances from the main subject a different subject displays a hard transition between light and dark, then the edge shows up like one of the lines seen in Johns image.
The 50mm f1,2 is more prone to this then the 55mm f1,2 is, but the latter also displays this behaviour with subjects close to, or about 1m away and behind the main subject.
I try to avoid elements in this zone to work around the issue with most fast lenses.
This behaviour is also part of the reason why I like the 5cm f2 Nikkor-S and the 135mm f3,5 Nikkor-Q.C as much. Both are relatively free of these double lines.
Guessing at optics I would say the problem is caused by internal reflections, which also explains why faster lenses, or lenses with bright frontallens groups tend to suffer morepronounced from this behaviour
