Is a filter required in the built-in holder of telephoto lens having such holder?

Question

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.

Answer 1

I think I have the answer from these 2 user manuals extracts, though from an optical point of view I don't know for sure how the image suffers from the absence of the inner filter.

From the user manual of the nikon AF-S_VR_200-400mm:

From the user manual of the nikkor 400mm f/2.8 IF ED:

While the above (nikkor 400mm) only says "always use a filter" without specifying whether it's a front or rear filter, the remaining of the recommendation points toward the inner (rear) filter, which is clearly mentioned for the 200-400mm. If someone wants to constructively elaborate by explaining what happens to the image without the filter, welcome!

Answer 2

It'll shift the focal position slightly (I think by around 1/3 the thickness of the filter, for typical materials). Depending on how the lens focuses, this may or may not have any impact on the quality since the AF should compensate - so if focusing moves all the lens elements in or out, you're probably OK. If focusing only moves some of the elements, you may or may not have issues. In the case of astrophotography, especially with focal reducer/flatteners, the spacing between the flattener and the sensor is critical for proper performance, and people routinely figure filter focus shift into their calculations. Not sure how big an impact leaving the filter out in a telelphoto would have, especially since the thichness of gelatine and glass filters is quite different, but presumably there's a reason the manufacturer says it should be there.

Answer 3

High quality optical filter are optical flats. In in other words, the front and rear surfaces are ground super flat and they are parallel to each other. That being the case: An optical flat can be mounted anywhere in the tube meaning, before the lens, after the lens, or in-between lens elements. As to an optical effect, a filter will shorten the focal length by a miniscule amount. This change is so small it will be difficult to measure.

A wicked effect is induced by the addition of two added polished surfaces. Both sides reflect away some light and this becomes stray light that comingles with the image forming rays. This can be as high a 4% or more but lens coating mitigates to 1 thru 2%. This comingling produces flare. Flare is devastating as it robs the image of its contrast.

Optical filters have their place; the polarizing filter is the most valuable of all as it mitigates reelections from non-conducting surfaces and adds boldness to sunlit scenes without inducing a color shift.
The bottom line is: Filters have their place but the best advice is: Never add a filter unless the benefit outweighs the harm.

Answer 4

I have no direct knowledge of that particular lens, but based on what I have read regarding similar lenses in the past, the absence of the flat could affect image quality slightly, particularly at either infinity focus or the MFD.

Beyond that, though, what does it say about the way an owner of a lens (or a previous owner) cares for their gear when they've lost a piece that the manufacturer says is integral to the functioning of the lens?