Question about so-called "Special Effect" filters

[kwesi]

Is anyone familiar with filters like these, and what they're used for?

Screen Shot 2021-12-09 at 11.23.23 AM.png (133.83 KiB) Viewed 1530 times

I found them on eBay and I'm curious about them. The one on the bottom right looks like a COL filter without the central patch stop, the others, especially the top two, I'm having trouble wrapping my head around what they'd do to the illumination of the sample. Are these just a gimmick, or would they have actual applications?

[josmann]

Similar ones came with the Journey to the Microcosmos scope kit. I tried the ones like the top two and never got anything except a crummier image. From an optical analysis point of view they don't appear to make much sense.

[zzffnn]

I have tried a few but not all of them. The only one that kind of make sense to me is the bottom left one. Even with that one, I would think the center should be solid instead of hollow.

Sometimes filter makers just copy each other's designs, without actually testing out the effects.