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Currently, I have my G11 ceiling mounted at the back of my Theatre, lens about 12.5 feet from the screen. This turns out to be (just barely) enough to fill my 100" diagonal 4:3 screen (80" x 60"). In fact, when I was first building my Theatre, I thought I might need to mount it from behind the wall just to fill my screen. The room behind the Theatre was going to be finished also, and my wife wasn't wild about having the back-end of the projector sticking out. After a rather nervous circus act with portable DVD player, a board, a ladder, and a $9000 device balancing upside down, we happily confirmed that our original idea would suffice.


It has been that way for about 9 months, now. Although I have yet to hear any guests complain about the noise and the heat (from the projector), I have noticed that I often have to bump up the volume a couple of notches during especially quiet scenes.


As it turns out, my wife would much rather have the projector protrude from the backside than have any kind of hushbox. I tend to agree, since the projector is currently mounted under a soffit and directly above the rear row of seating. So I'm thinking, maybe I should give the idea of mounting the projector through the wall another chance.


After a little poking around in the false ceiling of the room behind Theatre, I was suddenly reminded of why I abandoned this idea so quickly before. It seems the perfect placement of the projector to be centered with the screen would be directly behind a 6x6 post which was now neatly finished inside the rear wall of the Theatre.


So, (finally) to the subject of my Thread. How much keystone will the image actually have if I move the projector off-center by, say 8 inches:confused: Assuming my new projector throw is going to be 15' from the screen, it looks like I should see a keystone of about 1/2" from left to right. Here's the math I used.


With the projector dead center, the distance from lens to screen is 180", while the left and right edges of the screen have a distance of 184.4", no keystone:)


Now, we move the projector off center to the right by 8 inches. The distance to the screen is still 180", but the point of perpendicular to the screen is shifted to the right by 8" also. That would put the left edge of the screen at 186" from the lens, while the right would only be 183" from the lens. Based on what I can tell from the throw calculation, that means the image on the right will be about 1/2" smaller than the image on the left. Alas, keystone:(


How noticeable is this really going to be? Is everyone really THAT accurate in their installation?
 

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One thing you can do is to take all your linear dimensions - screen width, throw distance, and

the offset and multiply them by some number

somewhat less than 1.0; say 0.9


Now setup that same geometry in your present theater - and because you've reduced all the

dimensions by the same factor - you can set up the scaled geometry, and the projector will still be

in the room instead of protruding into the next.


This scaled down version will have the same degree of keystone as the full sized setup - i.e. the same

angles - and you can judge for yourself as to whether the distortion is acceptable.


Greg
 

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I think your 8" is going to cause other problems beyond the keystone issue. Focus is one that immediately comes to mind. You'll never get edge to edge focus.
 

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Discussion Starter #4
Yeah, I never thought about the focus.


So, I take it everyone has their projector mounted dead center?
 

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jhill32


I have a similar problem except instead of a verticle post I have a horizontal beam.


Initally I thought I would be satisified with the upward angle of my g15 projector required to fit the image where I wanted it, this was made worse by the addition of the panamorph.


The additional distortion of the panamorph seemed to just push the level of distortion beyond my level of aceptance. My sig other doesn't notice a thing of course.


I've decided to project thru the beam! Some of the beam is made up of cripple wall (short studs that are spaced 16" on center) and the rest is 12"x4" lumber. I'll cut an opening between the 2"x4" studs and also notch the 12"x4" beam about 3".


If you're anything like the rest of us fanatics you will be bugged by the compromise. If you must accept the compromise you must but what about moving the post or placing 2 6"x"6 posts 8" apart so the image can shoot between them, this might also provide a mount for your hushbox. Call a carpenter and get some advice.


Good Luck
 
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