Originally Posted by dougri
Mark... I understand why mounting an ND filter normal to the projection path could introduce reflections and reduce contrast, so why do high-end projection companies include threads on the lenses? Do you think multicoated B&W and the higher end Hoyas are poor, or are you just talking the mounting? I'm much pickier about optical imperfections on my photography than I am with movies, and the filters are definitely not the weakest link
No. The better ones are OK but with big buck projectors I didn't want to interject a filter below the quality of the lens. With projectors in the under $10K class I wouldn't worry too much. My filter is a Schneider, it cost between $100 and $200, was 3"x3" sq and was very very difficult to find involving multiple phone calls to Schneider. I gave up trying to get a 4"x4".
With hardly any exceptions, projector manufacturers buy rather than make their lenses.. Many more expensive projectors brands use the same lenses as their competitors. Cheaper projectors seem to use different lens each. Lens manufacturers use stock parts and since camera lenses all have groves for installing UV filters etc, its pretty standard to provide groves on projector lens. The groves won't affect performance at all and having the groves might result in a few more sales of the projector. However, I suspect very few projectors end up with a filter installed on the lens. And the vast majority are used to cameras and screw on filters normal to the light path but I think you can see the difference here. Its one thing to reflect light back to what you are shooting, most often a fairly large place vs reflecting back to the primary lens on the projector. We tend to be lazy, but if you are going the ND filter route, why not minimize adverse impacts by tilting the filter. It isn't hard to figure out something to hold the filter at an angle. Manufacturers ofter provide for extreme amounts of lens shift, it means more sales, but it doesn't mean you should use extreme amounts just because the manufacturer provided it.