Originally Posted by Ellebob
I've done this several times, real world it is about 20%. Calculated it is about 25% screen size difference and about 20% pixel difference minus A-lens light lost and zoom difference. You usually don't make up the difference with the zoom and light loss through the A-lens. I'm not where I can check these for a few days but I can try it with one of several projectors we have here now, an Epson 3500, 4030, 6030 and 10000 and I know we have a Cinevista A-lens kicking around which is the lower end of A-lens scale. Plus the lower end projectors should have a bigger difference when using the zoom. I'll post the measurements.
A lot may have to do with the projector model (different light loss curves as you zoom) and if you are on the telephoto or wide angle end of the spectrum. According to Projector reviews, the first half of the 600ES zoom is relatively flat, with little light loss from wide angle to mid zoom (~4%). However, they show just over 19% drop in light from mid zoom to full telephoto.
So, in my room, and therefore my calculations, I'm basing it on A-lens (and 1.78 viewing) at full telephoto, and zoom method being not quite mid zoom. Therefore, the projector is picking up nearly all of that 19% by being near the mid zoom point, even though the light is being spread out over more square feet.
If on the other hand, you were dealing with the other end of the zoom spectrum, with 1.78/A-lens at mid zoom, and zoom at nearly full wide, then based on Projectorreview numbers you would be looking at 34FL for A lens and 27FL for Zoom, because they show picking up less than 4% on that half of the zoom curve.
FWIW, Projector Central claims that in their testing the light output from full wide to full telephoto is linear and drops 30% in total, with roughly a 15% drop at mid zoom. Using PC numbers weather you are on the long side of the throw curve or the short side, there is only a 1-2 FL difference between Zoom and lens (a lot depends on real world A lens light loss -- 3%, 5%, 8%??)