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Discussion Starter · #1 ·
HI all ,

I have a question

As i am not an expert in any way , i was hoping you all could help me out with the placement of my ceiling mounted ht1000 (when i buy it)


Is it true to say that the brightness will be identical on a closer placed projector and with the zoom all the way in as a farther away projector with the zoom all the way out , both achieving a 92" diag screen


the reason i want to know this is because i want to know to maximize brightness , does it make a difference if the Projector is closer or further once you have used the zoom to get the 92" screensize



Projector central says to achieve the size i need between 9.9 ft. - 12.5 ft.

where should i place my PJ , at 9.9 or 12.5 ft or does it not matter


any comments will be greatly appreciated
 

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Interesting question. I think the brightness measured at the screen would be different at different zoom settings. Having said that, I do not have experience on the HT1000 to tell you if this difference will be meaningful in viewing video.


Also, as f/stop of the lens at wide and at tele, is not published, we won't be able to tell what is optimum. By f/stop, I don't mean a mechanical aperture (or iris) which can be opened up or shut down, but by the optical and mechanical characteristics of the lens, the f/stop may not be constant throughtout the range. If they are the same, that is if the lens is constant f/stop throught the zoom range, then the answer to your question would have to be that the light output at either end of the zoom is the same. Otherwise, it would be dim or bright according the to the f/stoo.


Another consideration would be that most lenses are optically optimum at a particular focal length, rather than at all focal lengths throughout the zoom range. I would suspect that the variation to be fairly small, but there would be a golden distance for the PJ.


I would suspect, though that the lens is not constant aperture, but hopefully the variation is small. Anybody with the specs of the f/stop of the HT1000 lens at wide open or at zoomed in?


Any thoughts on this?
 

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Discussion Starter · #3 ·
wouldnt the distance that the light from the PJ have to travel affect the brightness , as in a closer projector with more zoom would be brighter than a further away PJ with less zoom
 

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I guess so, but that would only be minimal - only light that is being absorbed by the air. I think what you are thinking of is the inverse square law of intensity. This doesn't come to play because the total area you are illuminating is the same at either end of the zoom scale, and you are manipulating the light source with the zoom lens, and not allowing the energy of the light to be dispersed naturally.


Just looked up the spec...from the data sheet, it appears that the F/stop seems to be from 2.5 to 3.7, zooming from 22.4 to 27mm. It doesn't say if the F/stop is linear across the zoom range, but I guess, its reasonable to assume that.


So it would appear that from both the F/stop of the lens (nearer = brighter) placing the PJ as close as it would allow would yield a brighter picture. Now is this noticable in viewing video?


I too am almost at the verge of buying the HT1000 (will decide after I see the Optoma H76), and in my installation, I would have the PJ at 12.5 feet away from my 100" 4x3 screen...so it would be the dimmest.


Would Guitarman or other comment on this and on the which focal length the lens would be most geometrically optimum?
 

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Quote:
Originally posted by crumpet
wouldnt the distance that the light from the PJ have to travel affect the brightness , as in a closer projector with more zoom would be brighter than a further away PJ with less zoom
All that matters is the area that is to be illuminated.


Take the light output of the projector, measured in "lumens" and divide by the number of square feet in the image - and

you have the luminance in "foot-lamberts"


It doesn't matter whether the PJ is close and zoomed in, or far away and zoomed out - as long as the image size is

the same.
 

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For a generic light source (lamp, candle) the distance would matter. However, with a focused source the information on calculating foot-lamberts is correct and distance doesn't come into play.
 

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For a given image size, the brightness is essentially the same.I've seen tests where differences are measurable between min and max zoom, but they are not detectable by the eye.
 

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Not accounting for room conditions. The PJ would be brighter the further away you mount it. It also gives it more of a conical shape, which makes it better for reflection pruposes. PChong is correct that this is a factor of the Fstop that the more you zoom the higher your fstop becomes. Geometrically less zoom would be better as well.


However, unlike cameras these zooms are relatively small and the differences probably aren't noticeable to the eye. If these has 3x, 4x or greater zooms like cameras do then the differences would be more noticeable.


Bob
 

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Quote:
The PJ would be brighter the further away you mount it
Nope, as stated above, the only variable is the screen size.


Blatently stealing from a previous post:


Take the light output of the projector, measured in "lumens" and divide by the number of square feet in the image - and

you have the luminance in "foot-lamberts"


The above, of course assumes a 1.0 gain screen.



Vern
 
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