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Interesting, I did not know they were very different [optically]. But yes, I've read opinions about 3D advantage CY/Rollei vs ZE/ZF, better sharpness ZE/ZF vs the older, color talk and so on but I honestly cant tell the difference. Lack of fidelity perhaps.
The only one with characteristics that sticks out is the Rollei with the triangular bokeh which you may or may not like. Other than that I'd be very easy too fool with any of the versions. Got a CY (MMJ) myself.
I haven't tried the C/Y or Rollei but I own the ZF, and from what I've seen, there is not much in common between them. The older lens gives much smoother bokeh in the middle at medium short distances, but is also very "nervous" towards the corners. The new one is more uniform across the frame and maintains a relatively good bokeh at large distances (like the Planars), with a low contrast background.
So, I'm a bit annoyed with people posting images without telling what lens they used. I mean, this thread is about pure gear freakery, so let's get there. Otherwise we could also have a thread named "The official Zeiss 50 mm thread". As you might know, there are like 10 lenses that fits, which are all different.