Oh, and the Cirkut doesn't have a curved film plane. But the slit where the exposure is happening describes a circle in 3D space as it rotates, so it *emulates* a curved film plane.

I'm thankful that my day job is mechanical design, or a buncha the concepts on Cirkut and swing-lens stuff would just dumbfound me. Instead I can think my way through, after a bit of reading and thought.