Yes Notice Cap has 2 parts... Remove both
If it's necessary to do that, it means you can't use filters (as COKIN, or others) in front of tle lense?
And for the nikor 10,5 it's the same?
Sigma use GEL filters that fit to the rear of the lens.
While gels are applied at the lens back side.. it still is necessary to remove both the cylinder and lens cap in order to obtain the 180 degree view from the FRONT side of the lens,