Zoom lens question

[Alan Paterson]

The best way to be test this would be to use a lux meter at the screen and measure the difference in readings with the projector at different distances from the screen. Good luck in your research.

[Brian Fretwell]

Thinking, possibly too much, about this - there must be an ideal distance betrween the lens and the film so that the maximum part of the cone of light from the lamp/condenser system to get the maximum ammount of light through the rear element and not hit the surround. The change in focusing when zooming the lens may mean that in full telephoto it does not catch as much light as at wide angle (or vice versa). It's getting too complicated for ma!!!