Some telephoto lenses such as the nikkor 400mm AI-s f/3.5 (and many others) have a built-in filter holder. Is a filter required in that built-in holder for the lens to perform according to the manufacturer's specifications? I have heard that the inner filter is mandatory for this lens to function normally but I have also read otherwise from a seller saying he uses this lens without the inner filter and without any problem.
Trying to reformulate my question in optical terms, is the inner filter positioned along the optical path where its presence modifies the light rays (i.e. change the focus distance or/and has an effect on the aberrations) or are the light rays parallel (not convergent, not divergent, i.e. like the rays between an infinite conjugate microscope lens and an infinity corrected tube lens) and the presence or absence of an inner filter has no effect on the aberrations?
edit: I have found the optical design of said lens, and the filter appears to be the last element, I'd conclude the filter is required, but I would appreciate a confirmation and some comments.