I've started using ND and polarizer filters in my shots.
Since I'm learning to use them, I'd like to manually add this info to exif/iptc/xmp metadata of my shots, so later I can find images taken using a specific filter. I plan to do it manually at the end of each session, when I import images from my DSLR.
I already have the software needed to edit tags, but I'm unsure which tag should I use to add such information.
Is there any "standard" (even a "de facto standard") tag used to store filters info?
I looked at exiftool documentation (http://www.sno.phy.queensu.ca/~phil/exiftool/), bud didn't found anything.