Is it not just the size of the screen? The smae lumens shared between more square inches?
It's this as well, but it's quite possible to have two setups with the same size screen and one could be 40% brighter than the other (imaginery identical projectors
). The one close to the screen but with more zoom applied will make the same size picture, but be brighter and have less contrast purely because of the way that most projector lenses work in changing the aperture when zooming. The one far from the screen, but with no zoom applied, but same sized picture will be darker and have more contrast.
Being aware of this effect can help you to prioritise your setup accordingly: I want to maximise contrast, I like having the projector behind me (like a real cinema) and long throw also helps with an A lens. I chose a screen with enough gain that I would still be able to hit the typically recommended 12-14fL even after lamp dimming, so for me it suits well. In a setup with a shorter room, there may not be the possiblity to do what I've done, or it may even be a temporary setup with the PJ in front on a table. Each has it's pros and cons, so more just a matter of being aware of the effect and how it might effect your setup.
For example, something like an AE3000 really struggles to light up a bigger screen especially in the more accurate modes like Cinema and Colour. Therefore it might be one of these occasions where every last lumen matters so a short throw high zoom setup will maximise the brightness.
