Homemade center filter

Goofing around with old glop from carbon printing
dilute, nearly clear coating that covers the filter thinly
then dropping single drops of increasing density into the center from decreasing heights to form an even dispersion

How well could that crap work?
When dry the gelatin layer must be "incredibly" thin but it would have a slightly thicker central area
or would it have a slightly thicker outer area? Both?
I can't see it by eye but
Any chemical that might aid in the evenness?
I figure taking the glass out of the filter ring would help but please


someone on here said smoking a piece of glass to produce the effect might work
reverse printing the fall off onto mylar etc


just wondering which would produce the best quality easiest




I tried this as a grad ND on picture frame glass and it works well enough



not trying to reinvent the wheel
just playing
but I'm also never buying a center filter till I can take a photo that will pay for center filter

Try smoking a filter over a candle.

I included you, man

but I'm not supposed to play with fire

I once made an experimental centre-dark filter from two 75mm diameter spectacle lenses, a +2dioptre and a -2 dioptre. The +2 was dyed dark brown for use in sunglasses. Held together the powers cancel but the centre-dark effect was perfect. Total cost was $50 and the optical quality was ok. Colour neutral? No way!

You could try dyeing a CR39 resin filter.
Its not that hard to gradient tint an eyeglass lens, iirc 50% took about a half hour and we had to move it every minute or so to keep it even enough. IIRC (its been years) there was also a bleach that would remove most but not all of the dye. The stuff is used hot, about 180f if I remember right. I believe Cokin uses CR39 as the resin for their filters. Don't expect true color neutrality, there's a reason these things cost...

Not sure where you'd source the dye but any one-hour eyeglass place would have the stuff and you might talk one into giving you their old stuff when they change it.