Projector Placement - AVS Forum
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post #1 of 11 Old 03-30-2014, 12:35 PM - Thread Starter
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Hi,

Are there advantages/disadvantages of placing the projector all the way back in the room.

Thanks
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post #2 of 11 Old 03-30-2014, 01:40 PM
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Yes. smile.gif Do you have a follow up question?

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post #3 of 11 Old 03-30-2014, 01:44 PM
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I think it will be easier to answer your question if you ask something more specific. What are specifically concerned with about placing the projector all the way in the back of your room?

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post #4 of 11 Old 03-30-2014, 05:13 PM
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OK. I have more time and I would be glad to elucidate on the issue of projector placement and picture quality. In a nut shell every projector with a zoom lens has a throw range with respect to the size of the screen. Basically your can place the lens measured horizontal to the plane of the screen any distance within the throw ratio range times the screen width. For example, a JVC projector uses the same lens on all its projectors and they have a throw range of 1.4 through 2.8. that is a huge range. let's suppose you have a 1.777777 screen and it is 8 ft wide. you could place the projector as close as 11.2 ft away to as far as 22.4 ft away. The screen size will always be a constant. If you move the projector closer than 11.2 ft, the screen will not be filled completely with image and if you place it further than 22.4 ft away the image will be too big for the screen. If you examine the size of the image on the front of the lens glass where it exists, it will be the very largest at 11.2 ft and it will be smallest at 22.4 ft. And its this size difference and therefore how much width of the lens glass within the lens that affects the brightness and contrast parameters of the image. At close throw, its as if you were flowing the water through a wide pipe and lots of light can be sent down the pipe. At long throw, the pipe diameter is much less and not as much light can exit the lens. So at close throw your image will be the brightest it can be. At long throw, you will get less light out. maybe 30 to 40% less depending on the lens and its zoom ratio (the long throw ratio number divided by the short throw number). the higher the ratio, the more light will be lost at the extremes. While you do give up light going to long throw, your on off contrast ratio goes up and your black reference level gets smaller ( a good thing, meaning blacker blacks). Now where you should place it having the freedom, if so, to do depends. On the size of your screen, the screen gain, and how much light the projector will put out after some hours on the bulb and with calibration. Before you glibly conclude that you will place it half way between the extremes, as many glibly conclude, let me tell you the fall off from the extremes is a log function. what this means is that the fall off is a steep curve that levels off rather quickly. In the middle you will lose most of the light advantage of close throw placement and most of the contrast ratio improvement from long throw placement. Generally lenses perform better optically at long throw because the image is usually more concentrated towards the center rather than being bigger and using more glass away from center. I won't even go into lens shift and how that moves the image from its ideal place within the lens glass.Hope this helps.
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post #5 of 11 Old 03-30-2014, 11:58 PM - Thread Starter
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Brilliant!!!

Amazing reply.

When projector review sites report lumens do they measure them at short throw or long throw.

I am going to purchase Sony hw55es which has calibrated produces 950 lumens(as per projectorreviews.com) and I shall use elune vision AT screen with .95 (reported) 135" screen.

A short calculation suggests that I can get 17 ft-lambert from the projector/screen if it produces 950 lumens.

Would it be advisable to place the projector all the way back.
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post #6 of 11 Old 03-31-2014, 07:20 AM
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Is the 135" the width or the diagonal? If the width is 135, that's a big low gain screen and I would go with close throw. Generally, a manufacturer gets it lumens spec at closest throw with most everything set to maximize the brightness. Once you set the parameters to maximize the PQ rather than the brightness the number could go down by 1/3 or even more. After the bulb wears a bit, the light output will be even less. Some screen calculators take the highest number and then adjust it with the throw ratio. They do this by getting the effective F stop at the throw extremes from the manufacture, The F stop will be lower at close throw and higher at long throw. The lower the F number, the more glass the light path is using. Once again F stops are not linear. Starting at say an f stop of 2.8, each of the following F stops represents a 50% in size or area reduction 4.0. 5.6, 8, 11, 14. most calculators are linear in their calcs which is dead wrong of course. I think you will be marginal at long throw if the 135 is the diagonal but of course this depends on light control etc in the room and that you might need to replace the bulb more frequently as it dims. But bulbs for the sony's are cheap. You won't have a really bright image but it depends on you. Your eyes can live with as little as 5 ft lamberts. Theater standards are 12 to 14 ft lamberts. 17 ft lamberts will be short lived.

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post #7 of 11 Old 03-31-2014, 07:59 AM
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Also depends on, if that screen actually measures .95 gain. The highest measured gain I have seen for a woven AT screen is .98 and the lowest is .85. I would mount close to max the light output. This is based on guessing the gain is closer to .9 and the dimming of the lamp. I estimate that you will get 16 FL in high lamp if mounted at short throw and 14.3 FL using high lamp.

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post #8 of 11 Old 03-31-2014, 08:52 AM
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Quote:
Originally Posted by AV Science Sales 5 View Post

Also depends on, if that screen actually measures .95 gain. The highest measured gain I have seen for a woven AT screen is .98 and the lowest is .85. I would mount close to max the light output. This is based on guessing the gain is closer to .9 and the dimming of the lamp. I estimate that you will get 16 FL in high lamp if mounted at short throw and 14.3 FL using high lamp.

Maybe you have the numbers reversed? smile.gif or left something out of the sentence?

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post #9 of 11 Old 03-31-2014, 09:08 AM - Thread Starter
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Quote:
Originally Posted by mark haflich View Post

Maybe you have the numbers reversed? smile.gif or left something out of the sentence?
Screen is elune vision 135 diagonally AT which spec say is 1.15 but people say it is .95.
The lumens number is for Sony projector hw55es which I got from projector reviews post calibration at close throw, the same site reports that at maximum distance it is 844. Based on those number I will get around 15 fl at max distance, not bad?
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post #10 of 11 Old 03-31-2014, 09:13 AM - Thread Starter
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Quote:
Originally Posted by mark haflich View Post

Is the 135" the width or the diagonal? If the width is 135, that's a big low gain screen and I would go with close throw. Generally, a manufacturer gets it lumens spec at closest throw with most everything set to maximize the brightness. Once you set the parameters to maximize the PQ rather than the brightness the number could go down by 1/3 or even more. After the bulb wears a bit, the light output will be even less. Some screen calculators take the highest number and then adjust it with the throw ratio. They do this by getting the effective F stop at the throw extremes from the manufacture, The F stop will be lower at close throw and higher at long throw. The lower the F number, the more glass the light path is using. Once again F stops are not linear. Starting at say an f stop of 2.8, each of the following F stops represents a 50% in size or area reduction 4.0. 5.6, 8, 11, 14. most calculators are linear in their calcs which is dead wrong of course. I think you will be marginal at long throw if the 135 is the diagonal but of course this depends on light control etc in the room and that you might need to replace the bulb more frequently as it dims. But bulbs for the sony's are cheap. You won't have a really bright image but it depends on you. Your eyes can live with as little as 5 ft lamberts. Theater standards are 12 to 14 ft lamberts. 17 ft lamberts will be short lived.

Please see the above post
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post #11 of 11 Old 03-31-2014, 09:48 AM
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Quote:
Originally Posted by AV Science Sales 5 View Post

Also depends on, if that screen actually measures .95 gain. The highest measured gain I have seen for a woven AT screen is .98 and the lowest is .85. I would mount close to max the light output. This is based on guessing the gain is closer to .9 and the dimming of the lamp. I estimate that you will get 16 FL in high lamp if mounted at short throw and 14.3 FL using high lamp.

Quote:
Originally Posted by mark haflich View Post

Maybe you have the numbers reversed? smile.gif or left something out of the sentence?

and 14.3 FL (high lamp) long throw.

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