This is nothing new but I have tried it myself and it works. Using a flip over glass of one circular polarizer filter (CPL) stack/screw over another CPL filter, you have a variable or fader ND filter.
At near maximum fade out, you will experience hue change depending on the filter brand, so you have to try it and figure out how to compensate the RGB yourself.
Here is the quick video I have recorded. (if it gets jerky, just replay after buffer)
I removed the glass secure ring (extreme caution and patience required), reverse the glass and put it back into place. * Note that the once reverse, the filter looses it's polarizing ability.
The stacking orientation must be lens, CPL then reverse glass CPL filter. See below:
- by turn either one of the CPL (not both) will increase darkening intensity.
- If you put the reverse glass CPL (top) first, then the normal CPL (bottom), you get a change in color instead.