Zoom lens question

[Ed Gordon]

I think you will always get a better image without a zoom lens, but I prefer to have a zoom lens so that I can enlarge letter-boxed images in order to maintain "constant height". There is a discussion on this topic at: https://www.avsforum.com/threads/doe...ality.1226740/

If there is loss of brightness when using a zoom lens, a good light meter will give you an idea of the actual impact of zooming. You might go over to http://www.film-tech.com/vbb/ and search the subject there. I know the pros use special meters to measure screen brightness and determine how evenly the light is distributed. I believe most projected images tend to be the brightest in the center of the image.

[Paul Adsett]

I have been thinking that the brightness of the projected film depends not only on the size of the projected picture but also the distance (throw) of the projector from the screen. In other words for a fixed projected picture size of say 72 ins wide, you will get a much brighter picture if the projector is as close to the screen as possible, with the zoom lens zoomed out for max picture size, as opposed to projecting further back from the screen (larger throw) with the lens zoomed in to fit the same 72 ins size screen. This must be because the f number of a zoom lens (focal length/aperture) is not constant and varies with zoom setting. For example the Elmo f1.0 12.5mm to 30mm lens is only f1.0 at the 12,5mm setting and is f2.4 at the 30mm zoom setting. Do I have this right?