A simple way to use filters in conjunction with select Canon super-telephoto lenses, the Drop-In Gelatin Filter Holder 52 (WIII) can be used to hold cut gel filters in the rear of the lens. Gels are held in place by a clamping mechanism.
Compatible Lenses
EF 400mm f/2.8L IS III USM
EF 600mm f/4L IS III USM