gelatin filter

Then either a round threaded filter is attached, or one can use a gelatin filter.

Rear gelatin filter (no front filter thread).

The lens also incorporates a built-in lens hood and a gelatin filter holder behind the rear lens element.

Rear gelatin filter holders are used by cutting out a sheet of gelatin, to the size shown on the back of the lens and then sliding it into the holder.

Wide angle L-lenses typically have a gelatin filter holder on the mounting point of the lens, which allows the photographer to cut a small, square piece of gelatin out of a larger filter sheet and place it on the lens.