How to Tilt Screen to Compensate for Projector Image Offset? - AVS Forum
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post #1 of 13 Old 08-20-2007, 11:54 AM - Thread Starter
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I'm looking for advice on tilting my ceiling mounted projector (HD80) and screen to compensate for image offset with my lower-than-desirable ceiling.

For those who have done it or are knowledgeable in this area, is this simply a matter of trial and error to get the best picture (using some sort of test pattern, I assume), or is there a more scientific method? I searched, and found very little usable info. Anyone know of any helpful articles or sources?

Thanks!

- Nuke
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post #2 of 13 Old 08-20-2007, 12:38 PM
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For a ceiling mounted projector tilting up, you need to tilt the top of the screen out away from the wall until the image is square. I've only heard of trial and error for the amount of tilt.
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post #3 of 13 Old 08-20-2007, 01:13 PM
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I'm pretty sure the answer is that you tilt them both by the same angle. For example, if you tilt the PJ up by 10 degrees, you would also tilt the screen out (at the top) by 10 degrees. My reasoning here is that the PJ bottom is normally supposed to be horizontal and the screen vertical for zero keystoning. In other words, the angle between the PJ bottom and the screen is normally 90 degrees. I think you want to keep that 90 degree angular relationship constant.
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post #4 of 13 Old 08-20-2007, 02:24 PM
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Matching angles is the key, the problem is, how do you accurately measure the angle of tilt? So you end up doing trial and error. In my case, the top of my screen was fixed 10" out from the back wall and I moved the bottom in until I got a square image (zoomed in to make it easier) and then secured that.
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post #5 of 13 Old 08-20-2007, 04:24 PM
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Quote:
Originally Posted by jvos View Post

Matching angles is the key, the problem is, how do you accurately measure the angle of tilt? So you end up doing trial and error. In my case, the top of my screen was fixed 10" out from the back wall and I moved the bottom in until I got a square image (zoomed in to make it easier) and then secured that.

Yeah, from a practical standpoint I would agree. I was just trying to answer the question asked in the OP. I think he wanted to get a rough idea of how far he would have to tilt his screen before actually installing it.

P.S.- For a few bucks you can buy a tool that looks like a protractor with a couple of additional movable parts for measuring/marking any angle. Stores like Home Depot and Lowes sell them.
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post #6 of 13 Old 08-20-2007, 08:59 PM
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Before tilt, my keystone was -5%. After tilt (arrived at by trial & error) and zero KS, the tilt pushed the bottom of the screen 2.5" away from the wall compared to the top.

Now this is purely anecdotal and possibly circumstantial/coincidental, but I wonder if the 5% KS was related to the aprx 2.5% tilt (relative to the diagonal of the screen, which is 104"). That would make it a 50% ratio. It would be an interesting theory to explore.

Is there a trig whiz in the house? My trig is way too old and wasn't all that good anyway. In any case, as others have said, the angle would have to be the same for the screen and PJ so as to maintain perpendicularity.

Edit: Upon thinking about it, the height of the screen is ~51", which makes the 2.5" of tilt ~ equal to 5% of the screen height. Makes more sense to me IMO.
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post #7 of 13 Old 08-21-2007, 12:13 PM
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I calculated but felt that using trial and error would probably work as well and faster. Start with tilting the projector until the picture is at the desire height - i.e. bottom of picture aligned with bottom of screen. Then tilt the top of the screen outward until you get a square (well - rectangular) picture. I used hooks and chains (with quarter inch eyes) for the adjustment. I am sure you can buy fancier gadget to adjust the tilt.
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post #8 of 13 Old 08-21-2007, 12:27 PM
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Search out the IN82 thread in the other forum - GetGray posted a very handy screen tilt calculator.
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post #9 of 13 Old 08-21-2007, 02:38 PM
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I went to the trouble of making an online calculator for projector tilt angle, given a desired amount to raise the image by:

http://stephenmason.com/misc/projectorkeystone.html

It gives the resulting size of the tilted image for various zoom options. Of course, as you might expect, I did all this and then just set the projector tilt by eyeballing it on my screen. I ended up not tilting the screen at all since the 1/2" or so of light spill onto the masking doesn't bother me.

-Stephen M
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post #10 of 13 Old 08-21-2007, 06:42 PM
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my screen is about 12" from the ceiling, and about 4" out from the wall, the bottom of the projector is flush against the wall. I have a low ceiling about 7.5' so my screen is pretty low, but not that low, eye level is still center to the screen, infact you still have to look up slightly which is great... I have my screen ( made from 1X2's ) sitting on "cleats" that extend about 5" from the wall giving a good surface to suspend the screen on. works very well...

My projector is tilted slightly up, and my screen tilted down towards the audience... It was trial and error but took very little time... I love my home theater, even if it's in an unfinished basement...

) and I love my HD80 as well...

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post #11 of 13 Old 08-22-2007, 05:37 AM
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When an image is projected "square" the top/bottom and left/right distances are equal, otherwise you do not have a perfect rectangle.

So with that in mind I approched this issue by titlting/rotating one or both items until I achieved a perfect rectangle.

The biggest problem was being able to just nudge the projector in some cases and make so it would say at that setting.

Paul
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post #12 of 13 Old 08-23-2007, 06:47 AM
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Quote:
Originally Posted by Paul Hives View Post


The biggest problem was being able to just nudge the projector in some cases and make so it would say at that setting.

I hear you. I left the adjustment screws on my mount a bit loose as I was aiming it, got it lined up, then found that tightening the screws actually shifted it a bit more. Very frustrating. I'd love a mount with some sort of screw-driven vernier adjustment for small changes.

My last two projectors were DLP, and I'm happy with them, but lens shift is starting to look like a must-have feature.

-Stephen M
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post #13 of 13 Old 12-08-2012, 04:05 PM
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I found this thread from another here and I might have to do this I did not think you could tilt the screen and not have to just rely on the keystone settings.
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