Just received a 300 f2.8 lens. It had a large 122mm UV filter on the front, but no filter installed in the 39mm holder in the lens. I had thought that the 39mm filter was necessary for proper focus, but in researching, I get conflicting opinions. Of course, the holder must be inserted, but no consensus regarding the filter. Also, the holder can go in both ways so not sure which way is correct. I assume that it goes in with the filter screwed in from the front like a regular filter on the outside of the lens.
All help is appreciated,