Neutral Density Filter on CS-Mount Fisheyes

Getting a neutral density filter on a fisheye is a horrible mess. Some fisheye lenses solve this in a sane manner by having an integrated filter mount somewhere in the lens barrel (usually in between some lens elements close to the camera, not the front). That’s about the worst possible solution for the problem. Some people use tape, glue or magnets to fit ND filter foils to the rear element of their lens. For all the other lenses, there are … curved, globe filters. Since I plan to use the lens on a fixed camera, smoked acrylic domes used for surveillance cameras may be an option as well.

Acrylic Dome

But … maybe there is an alternative: the camera I use is a Raspberry Pi HQ Camera Module with a screw-thread-type CS-mount. Maybe there are screw-type filters for the mount itself?

CS-Mount

Yes! Indeed, there is one company which is selling exactly what I need: Midopt. The issue: really hard to get if you are not a business customer and do not have an address in the United States.

So, time for a homebrew solution:
I ordered a plain 20mm glass piece coated with an ND filter from Aliexpress and 3d-printed a springy friction-fit holder. Designing the holder is a bit tricky since it needs to be easily removable while keeping the glass piece from moving around (causing abrasion and chipping).

Fusion360

The holder is printed with a 0.25mm nozzle on a Prusa Mini: Slicer

Does it work? Indeed.

Filter in mount Filter in mount