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Join Date: Dec 2006
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You did not specify which Benq, but assuming you are speaking of the Benq w1070:
1) I believe you misread their calc or their Flash app sometimes gets stuck in certain browsers and shows the wrong zoom (maybe), because your Zoom should be about 1.11 at 12 feet back from 120 inch screen.
Zoom can affect the image on some projectors, but to a lesser degree generally on these tighter throw ranges. Most Benq w1070 owners have said they cannot tell any difference depending on the Zoom, though I have played around with the w1070 myself, I did not really have enough time to compare the sharpness at different zooms, but it didn't appear to have much effect. As far as effects to contrast, if you mount it farther back then you will get a tiny tiny boost to contrast (doubt you will be able to tell). On some projectors, the contrast loss is more extreme due to the larger throws and different lens confi.
2) For 120" 16:9 screen, you can mount the projector with the center of the projector's lens 3" to 8.5" above the top of the screen's viewable area. Generally speaking this means the physical bottom of the projector can be pretty close to the top of the screen, or slightly above.
3) Middle of screen can be eye-level, but take into account a few things. Do you watch movies reclined or sitting more upward. If you recline, make sure to keep the screen high enough to not get into the way of your feet. Generally depending on your chairs, this can be anywhere from 2 feet to 3 feet above the floor for the screen. Also where you place the screen is really just personal preference, but I wouldn't put it too high above the ground. I keep my screen just about 24" above the ground if I recall, though I could lower it more to make it so that my eye level is matched to the vertical center (optimal), then it would get in the way of my feet when I recline. Compromises...
Hope this helps...