Is a filter required in telephoto lenses having a built-in filter mount? For example, the nikkor 400mm AI-s f/3.5 has a 122mm front thread and a built-in slip-in filter holder for 39mm filters. I have heard that the inner filter is mandatory for this lens to function normally but I have also read otherwise from a photographer 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?