Interesting idea. Maybe something that holds the mirror in the raised position?
I've been thinking about behind the lens filtration and the commercial versions I looked at physically hold the mirror in the raised position.
There are several benefits to using a filter behind the lens instead of in front of the lens, like being able to filter wide angle lenses that don't have filter threads and the light bouncing off of glass filters is less of a problem. Back in the days when view cameras were a staple of professional photographers placing the filter behind the lens was very common.
The
VAF filters that reduces aliasing on DSLR cameras when shooting video are placed behind the lens as are
some light pollution filters for astrophotography. There is also a
behind the lens filter holder for Arri cameras. In any case putting a filter behind the lens is still a thing.
By the way, what I'm thinking about experimenting with is behind the lens net filters--basically using women's hosiery to soften the image. This trick goes way back and it is still being used today though
the implementation is usually very low tech. What I've been pondering is a 3D printable filter holder, that would be cool.
