Canon RF 400mm f/2.8 L IS USM Instructions Manual page 26

Drop-In Filters
Because the lens optics are designed to include
O O
a filter, you must always attach either a Canon
PROTECT filter or a commercially available filter
before attaching a Drop-In Screw-Type Filter
Holder.
Note that filter frame thickness may prevent
O O
some commercially available lens filters from
being used.
The Drop-In Gelatin Filter Holder 52(WIII) and
O O
Drop-In Circular Polarizing Filter PL-C52(WIII)
are also available (sold separately).
The Drop-In Filter 52/52(WII) series can also be
O O
used. However, their exterior color is different
than the lens body.
ENG-25