Opening a Fujinon T 400mm f4.5

[fdion]

This lens is supposed to have a drop in filter at all time. Mine doesn't. I was expecting something easy to access, like on my Tamrons.

Question 1:
How do you open the rear of the lens to gain access?

Question 2:
What kind of filter is it supposed to be, is it a plain clear glass? thickness?

Thanks

[eggplant]

For reference, the optical design doesn't show a filter present (X-Mount):



However

In this photo of the instructions from an eBay listing, you can clearly see the rear can come off (X-Mount):



I am guessing if there is ever a place for the filter, it will be there.

[fdion]

[Ultrapix]

I bet it's stuck. I often solved it this way: take a roll of paper tape, and wrap it around the ring you marked with the blue arrow; put a lot of it on, until it creates a significantly greater thickness than the original one. At this point it usually unscrews effortlessly.