Hello, I'm new here. I just got a new lens - 600mm Pentax 6x7 - and a filter was included. It doesn't seem to fit on the back inside. I have to look again but it didn't straight away.
Does anyone know about this beast? Thanks
Michael Carter

Michael did a review of this lens some time ago. You can find it here: http://www.luminous-landscape.com/reviews/lenses/600mm.shtml

In it he talks about mounting filters by screwing them on the rear element.

That article is why I bought it, but, I confess that I didn't see the bit about the filter. The Pentax camera instruction book has a little about filters. Looking again at the small end of the lens, the flat glass there is held in by a roughened ring. It was a little loose and unscrewed all the way.
Surprise, it was a filter retaining ring.
I never saw the like but it works great.
Now I have 2 filters as one was already in there.