When shopping for your home theater projector, of course brightness - lumens, are one of the most obvious specs you see. The interesting thing, is that depending on how/where you set up your projector, brightness can vary by almost double!
That's right - a projector placed in one position in your room, might do 350 lumens, but placed more optimally, it could produce as much as 700 lumens. Sound easy? Not! There are always trade-offs. Before I get into all of this, let's be clear up front:
The purpose of this piece is to improve your understanding of the impact of positioning a projector, the lens it has, and your need for achieving a level of brightness. While the information here, may cause you to change some of your installation assumptions, it is unlikely that you will make totally different choices in projector, screen type, or whether you ceiling mount or shelf mount a projector. These decisions will all have some, but minor impact on your overall theater, and its performance.
OK, back to brightness, first, this two to one range of brightness isn't true for all projectors. The huge shift in brightness occurs with projectors that are exceptionally flexible in placement, by virtue of wide range zoom lenses.
When a projector's lens is in full wide angle mode (largest image from a given distance), more lumens make it out the lens. In full telephoto, less lumens.
Many projectors for the home, (mostly 3LCD and LCoS types) now offer 2:1 zoom lenses. That is a placment range from closest to furthest, of 2 to 1. Perhaps, for a 100" diagonal 16:9 screen, that might be from 10.5 feet to 21 feet away.
If you mount it 10.5 feet from the screen, you will get almost double the lumens as mounting it 21 feet back.
There is no straight simple formula, because different lens designs will have some impact on the actual amount of change, but, let's say that with a 2:1 lens, it will be close to a doubling.
If, on the other hand, the projector has a far more limited lens, say with a zoom ratio of 1.2:1, then, the change in brightness from one "extreme" to the other, becomes minimal, and not a serious consideration.
Now, most of the DLP projectors have limited zoom lens (1.1:1 to 1.3:1), so there isn't much to concern ourselves there, but with the 3LCD, and LCoS projectors, where all of them seem to have at least 1.5:1, and most are around 2:1, where you place the projector can matter a lot.
Consider, the reason you get a 2:1 zoom on most of these projectors, is to allow the flexibility not just to ceiling mount the projector, but to give you the option of mounting the projector on the rear wall of your room. The projectors with wide range zoom lenses means that probably 90+% of people can rear shelf mount.
And, rear shelf mounting can save money, in terms of length of cables as well as installation costs - as it's harder to get power to most ceilings, and if you have fairly high ceilings, working up there means ladders or scaffolding. For most, shelf mounting is just plain simpler and less expensive.