Projector placment...? - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 06-17-13, 04:27 PM
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Re: Projector placment...?

Quote:
My ceiling is 8 ft tall and I'm projecting about 15 ft away from the wall. How low does the projector need to be from the ceiling to prevent the need of using a keystone adjustment?
According to ProjectorCentral.com's Projection Calculator for the Optoma HD65, the smallest 16:9 image you can throw from 15' is 122", and to get that centered onto a screen the mid-point of the lens has to be 11" above the top of the actual screen (and not above any border that might surround the screen).
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Old 06-18-13, 12:07 AM
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Bob

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Re: Projector placment...?

Quote:
eljay wrote: View Post
According to ProjectorCentral.com's Projection Calculator for the Optoma HD65, the smallest 16:9 image you can throw from 15' is 122", and to get that centered onto a screen the mid-point of the lens has to be 11" above the top of the actual screen (and not above any border that might surround the screen).
If you have a full 122 inch diagonal, that gives your screen a 60 inch height so if the very top is flush with the ceiling (which it probably isn't), the projector would have to be 30 inches (60/2) below that to have a keystone-free image. If the top of the screen is 6 inches below the ceiling, add 6 inches to come up with 36 inches below ceiling level for the projector. Bottom line is that the lens of the projector must be centered on a line extended from the center of the screen (perpendicular to the screen surface) to obtain a keystone-free image. There's no relevance to the top or bottom of the screen other than the projector must be located half-way between them.
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Old 06-18-13, 06:37 AM
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Re: Projector placment...?

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Bottom line is that the lens of the projector must be centered on a line extended from the center of the screen (perpendicular to the screen surface) to obtain a keystone-free image.
That is incorrect.

Projectors such as the OP's Optoma - and my previous InFocus SP7210 - have a fixed, angled throw, and you have to place the projector at a set distance above the top or below the bottom of the screen in order for the image to be thrown perfectly onto the screen.

Keystone correction only comes into play if your PJ is at a point other than the recommended distance above or below the screen and, in order to compensate, you have the projector physically tilted upward or downward to center the image onto the screen.

OP, here's how it works:
- Using 122" (diagonal) as the size of the image you're throwing, you have an image height of 60".
- If the center point of your PJ's lens is 8" down from the ceiling, the top of the screen should be 19" from the ceiling.
- If your screen happens to be, say, 26" from the ceiling, you'll need to drop your PJ by another 7" inches (so that the center point of the lens is now 15" from the ceiling).

(NOTE: The screen refers to the screen material itself. If you have a fixed-frame screen with a 3" black border, ignore the border - that is, don't measure from the center point of the lens to the top of the black border.)

Last edited by eljay; 06-18-13 at 07:21 AM.
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Old 06-18-13, 07:13 AM Thread Starter
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Thanks for all of the information everyone. I really appreciate the help. It's all starting to make more sense now. It looks like I will just need to drop the shelf a few inches. I don't have a screen yet. I'm going to build one in the next couple of weeks. I can't wait to fix this issue. With the amount of keystone I'm currently using, I'm loosing about 10 to 12 inches of screen size. I measured the distance last night again and I'm actually projecting about 13 feet back.
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Old 06-18-13, 07:22 AM
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Re: Projector placment...?

Be sure to use the calculator to determine the exact offset (center point of lens to top of screen) that you need, based on i) your throw distance and ii) the diagonal size of the image you're trying to project.

Last edited by eljay; 06-18-13 at 07:41 AM.
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Old 06-18-13, 09:38 AM
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Bob

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Re: Projector placment...?

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eljay wrote: View Post
That is incorrect.

Projectors such as the OP's Optoma - and my previous InFocus SP7210 - have a fixed, angled throw, and you have to place the projector at a set distance above the top or below the bottom of the screen in order for the image to be thrown perfectly onto the screen.....................
So what you're saying is that the OP's Optoma and the InFocus have built-in fixed lens shift. I was referring to projectors without lens shift. I do however stand corrected in the cases you're mentioning in that they wouldn't need to be aligned on-axis, and that certainly helps in the OP's case.

Now, what a previous poster mentioned about mounting the projector upside down makes sense. With a fixed lens shift, it would definitely make a difference between shooting from the ceiling level or shooting from a table below screen level.

Sorry, I wasn't aware this was a feature in the OP's projector.
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Old 06-18-13, 09:56 AM
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Re: Projector placment...?

Quote:
So what you're saying is that the OP's Optoma and the InFocus have built-in fixed lens shift.
Not so much a fixed shift as a fixed - but angled/offset - throw.

Quote:
I was referring to projectors without lens shift.
If a PJ doesn't have lens shift, I think it normally has an angled/offset throw. But where it would have neither, then you are correct that the lens would have to align with the center point of the screen.

Quote:
Sorry, I wasn't aware this was a feature in the OP's projector.
No worries.
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Old 06-24-13, 09:38 AM Thread Starter
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Well I lowered the shelf this week and am very pleased. I gained about another 5 inches in screen size so I'm finally at 100" screen size. I still have to some keystone adjustment but the setting went from 13 to 3. I also started building my own screen this weekend. I'll probably have it finished this next weekend.
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Old 06-29-13, 05:55 PM Thread Starter
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Well my final projection size is now 102". Built a frame today and now it looks insanely good! I love it!
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Old 06-29-13, 07:50 PM
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Re: Projector placment...?

Quote:
jstanley214 wrote: View Post
Well my final projection size is now 102". Built a frame today and now it looks insanely good! I love it!
what material are you building the screen out of ?
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