Originally Posted by sxr71
One thing about lens throw is that from what I know a short throw allows in the case of some projectors 40% more light output than the long(est) throw zoom.
Originally Posted by techman707
I don't believe there's any truth to that. It's the screen size not the distance or focal length that affects light output. In drive-in theatres the only other thing, besides screen size, that affects fL measured off the screen is the atmosphere between the projection booth and the screen (and then only sometimes). As for the lens itself, the actual light output is determined by the f-speed of the lens. However, short focal length lenses generally have a higher f-speed than long focal length lenses, since short focal length lenses with low f speeds are more expensive when compared to longer focal lengths. Zoom lenses present the worst of all worlds, but, like anything else, there are cheap and expensive ones. Finally, getting an even light across the screen is much more critical on short focal length wide angle lenses. SMPTE screen brightness spec. calls for the sides to be between 75%-90% of the center (which on a film projector is tough to achieve using a short focal length lens below 60mm - "standard" 35mm film flat lens size is generally considered to be 100mm).
Actually, sxr71 is correct. Given a constant screen size, the shortest throw of a zoom lens will typically yield the highest light output and the lowest contrast while the longest throw position of a zoom lens will typically yield the highest contrast but lowest light output. There are some zoom lenses that are constant aperture and the light output and contrast stay constant through the zoom range but they tend to be expensive and rare in projectors.
Also, this argument about pixel density is a bit confusing. You make it sound like the pixel density on the screen is different depending on the focal length, with vertical compression having a shorter focal length than an HE lens. However, the pixel density on the screen is going to be the same no matter what type of anamorphic lens is used. The pixel density on the projector lens area will be different by using a different zoom factor and I think you are arguing for an HE lens because you will be using a longer throw with it than to get the same size screen with a VC lens. Your benefit in using the longer throw would come from using as much of the center of the lens as possible and minimizing use of the edge of the lens where the quality isn't as good.