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Discussion Starter · #1 ·
All of the mounting questions I have seen deal with distance back from the screen, or drop-down distance from the ceiling. I haven't seen anything addressed about horizontal position, though. What I mean is this: I plan to have my screen positioned in the middle of the wall. How exact does the mounting have to be so that the lens is dead center to the screen? Do projectors typically have some kind of L-R shift control in addition to keystone control? Any other suggestions or tips for mounting a projector and screen would be appreciated. My likely combo will be a Sanyo PLV60HT and a 92x52 Grayhawk.


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jj
 

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It should be fairly exact... you can tilt the projector left or right slightly, but this will cause the image to keystone. I think some of the higher end projector have horizontal lens shift, but none of the lower cost units do. It should be fairly easy (with careful measurement) to find the center point for the lens.
 

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jeffthx,

I just dealt with this myself. I had my D-ILA G11 projector mounted in the back of the Theatre, directly above the rear seating. Although nobody complained, I knew it could be better if I mounted it from behind the wall. The problem is, there was a 6"x6" post inside the wall right where I needed the projector to be dead-center. I contemplated mounting the projector to one side about 8", but I was concerned about side-to-side keystone.


First, I just did some math. With the projector at a specific distance from the screen (a), you can draw two right triangles that go from the center of the screen to the edge (b) and whose hypotenuse (c) is the distance from the edge of the screen to the projector. Using the pythagorean theory, a2 + b2 = c2 you'll find the actual throw distance of the image to the edge of the screen (c).


If you then move the projector off-center to the right, and redraw your triangles, you'll notice that the (b) leg of the triangle on the right is smaller than the one on the left. In turn, the (c) measurement on the right is less than the one on the left also. The difference in the values you get for (c) is the difference in the throw distance of the image. Using a throw calculator for your projector you can then estimate how much larger the image will be on the left compared to the right. This is your side-to-side keystone.


For mine, I calculated about a throw difference of just a few inches, with a resulting keystone of maybe 1/2". In the end, I removed the post and didn't have to deal with it.


I have not heard of any projectors that have adjustment for side-to-side keystone. Are you concerned about accuracy in mounting dead center, or is there a logistical reason why it can't be mounted in the center? If it just a matter of finding the center, just use a string and stretch it from the projector to each edge of the screen. Keep moving the projector until they come out the same.
 

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I built laser jigs and that helps but plane geometry will do just as well. If the diagonals are equal, the top is esthetically level and the two verticals are equal and the two horizontals are equal, that is as good as can be done. My approach is to achieve a perfect rectangle and then mount the screen to match the illuminated rectangle. It is much easier, but for most, counterintuitive.


Vince
 

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Discussion Starter · #6 ·
Actually, I shouldn't have any trouble mounting the projector dead center. The ceiling is unfinished, so I should be able to put it anywhere I want. My question was driven more by the fact that I wasn't sure what order things should be mounted. i.e., Do you put up the projector first and then mount the screen so that it lines up with the image? Or do you put up the screen first and then find the spot for the projector that hits it correctly? What I'm worried about is putting everything up and then realizing that the image is off center by two inches (for example) because I failed to take something into account when calculating the mounting position. I figured that if there was some kind of horizontal shift, that would help compensate for any slight errors in L-R position. Since that doesn't seem to be the case, any insight into the proper way to mount a screen/projector would be appreciated.


Thanks,

jj
 

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Actually, I think it is very difficult to mount the projector dead center. The walls are not square or parallel. The optics in the lens have variances. The floor is not parallel to the ceiling. Just getting the projector lens boresight exactly at 90 degrees to the screen surface in the horizontal dimension is hard. Using the projected light is almost the only way. Forget strings. Forget measuring tapes (except for measuring the illuminated rectangle and use steel and two people). Of course, if you want it kinda close, then the problem is easier.


You can nail most variables with projecting triangles and a plumb bob. It is far too much work. But if you succeeded in getting the center of the projecting lens exactly aimed at the center point of the screen, you still have yaw and pitch to deal with. Use the light from the projector.


Vince
 

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So is the conclusion here to mount the projector first?


I'll be mounting a Sharp 9000 and a manual pull down Stewart Luxus Communicator. Should I mount the 9000 first then have 2 people hold the pulled down screen and move it around until the projected light rectangle lines up just right?
 

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Never tried it with a pulldown. I developed an easy way to do the mounting of the standard Stewart 1.3 with the aluminum frame. But using my approach when there is no wall upon which to create the rectangle of light [e.g., the pulldown be in front of the stone fireplace or windows] would make it very difficult. After taking such a strong stand on my approach, I think I would be forced to install the projector first in such a case. Preferably using a mount with adjustments for yaw, pitch and altitude. The equal diagonals, etc. rules remain valid, but it will be difficult to accurately measure the distances on a screen that can move a bit. Live and learn. In this case, it was my turn to learn.


Vince
 

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I suggest you mount the screen first. Then when you get the screen up you can get the projector lined up with the screen and make minor adjustments before installing the mount. I find this works for me. ( try to have someone else with you just in case one person needs to hold up PJ and mount while the other does the lens tweaks before the drill is applied).
 
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