Rodenstock TV Heligon 42mm f/0.75
This lens was intended for fluoroscopy. The image from a fluoroscope appears on a very dim fluorescent screen (it could be much brighter if you didn't mind exposing your patient to MUCH more radiation...heh...) and to capture images they had a camera with a fixed-focus lens pointed at the screen. To get images that were usable without having to use extremely fast film (which would introduce unwanted grain) they had very fast lenses made.

So this lens was intended to shoot a screen from about six inches away. It was never intended to photograph 3D objects at varying distances. Using this as a general-purpose photographic lens is an extreme perversion of a once extraordinarily expensive industrial lens. But it's fun, anyway.
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