I bought a canon 200-400 f4 that comes with a geletine filter holder. However Canon does not sell this filter, nor can I find one elsewhere. I bought an IRIX ND geletine filter set, but these do not fit the shape and size of the holder. Someone shed me some light on where to find such filter?
Lee Filters makes the gels. They have both a "photographic" line which are where you find the slide-in filters that they're well-known for.... but they also have a theater line ... to gel lights (photographers use these to color the studio lights).
Lee lists their filters based on "optical density" (OD). In that system, each ".1" is 1/3rd of a photographic stop. So a "0.3" filter is 1 full stop. A "0.6" is 2 stops. "0.9" is 3 stops, etc.
The filters you probably want to look at are:
These are sold in sheets. You have to cut them out and fit them into your gel-holder.