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Screen Size / Zoom / Throw Distance and Brightness

post #1 of 9
Thread Starter 
So this seems very simple, but I'm not really understanding this so maybe someone here can explain this to me.

I would think that once I determine the screen size and the projector, the brightness is completely fixed and independent of zoom / throw distance, yes? To say it another way, if I move the projector twice as close and zoom in twice as much, my image size is the same -- so isn't the screen brightness exactly the same??

The reason I ask is that it doesn't seem to work that way on this calculator. Namely, if I keep the screen size constant, then move the zoom slider, the brightness also changes. This doesn't seem right to me.

http://www.projectorcentral.com/Sony-VPL-VW1000ES-projection-calculator-pro.htm

Obviously if I *don't* keep the screen size constant, but instead keep the zoom constant and move the projector back, then I lose brightness on a per-unit-area basis, because the projected image will be bigger.

Can anyone explain?
post #2 of 9
There is an element that you've overlooked: When you change the zoom on a projector then the projector's lens aperture changes which impacts on the brightness and also contrast. The effect various from one projector to another due to the lens design and the zoom range available. Even if you have the same sized screen you can have these two scenarios:

1. Projector closer to the screen, more zoom has a brighter image but less contrast.

2. Projector further away from the the screen, less zoom has a less bright image but more contrast.

A further choice is that for scenario 2 you are likely to get a sharper image as this is using less lens area of the projector.

However, you'll find that on here it seems to be the default choice to go for scenario 1 due to most people wanting maximum brightness over all else. Personally I have a screen size and gain such that I still have plenty of brightness for scenario 2 even with my iris closed down, so I benefit from maximum contrast. Of course if you have large and/or low gain screen then you may have to go for scenario 1, though the truth would be that this is only fine tuning and the projector/screen combination should be chosen to give enough light output to allow for lamp dimming.
post #3 of 9
Quote:
Originally Posted by Kelvin1965S View Post

There is an element that you've overlooked: When you change the zoom on a projector then the projector's lens aperture changes which impacts on the brightness and also contrast. The effect various from one projector to another due to the lens design and the zoom range available. Even if you have the same sized screen you can have these two scenarios:
1. Projector closer to the screen, more zoom has a brighter image but less contrast.
2. Projector further away from the the screen, less zoom has a less bright image but more contrast.
A further choice is that for scenario 2 you are likely to get a sharper image as this is using less lens area of the projector.
However, you'll find that on here it seems to be the default choice to go for scenario 1 due to most people wanting maximum brightness over all else. Personally I have a screen size and gain such that I still have plenty of brightness for scenario 2 even with my iris closed down, so I benefit from maximum contrast. Of course if you have large and/or low gain screen then you may have to go for scenario 1, though the truth would be that this is only fine tuning and the projector/screen combination should be chosen to give enough light output to allow for lamp dimming.

Hello

Firstly sorry for my english. I wanna ask a question. What do you mean with a big screen? 150 inches? 120 inches, 130, 110? I am planning hw50 es with 120-130 inch screen 1.1 gain. will i face a problem about brightness, can you please give me some information
post #4 of 9
No problem, though you're now asking a different question. In my example I was talking about two set ups that use the same size screen (whatever size that happens to be) and how one could be brighter than the other depending on the zoom used.

I'm using a JVC X35 on a similar size screen to the one you suggest though I have a little more gain (1.5). With the Sony being a brighter projector than the X35, I don't think you'll hve any problems lighting up that screen size. You should have some brightness in hand which allows for lamp dimming or if you want to view with some ambient light for some reason.
post #5 of 9
Thread Starter 
Kelvin -- So first of all, it clearly sounds like you know more than I do about this stuff, so to the extent that I'm "arguing" with you it's more of me just trying to ask questions...

It sounds to me like the effect your describing should be a subtle one at best. Here's another related question that I think strikes at the heart of what I'm getting at:

A lumen is basically a unit of light energy. Foot-lamberts is (basically) lumens / area. So if I don't mess with the projector at all, but move it forward and backward, my brightness (measured in foot-lamberts) will change because the projected image area changes.

In my described scenario though, I'm adjusting the zoom as I move the projector back and forth, so that the image area doesn't change. If I'm also not changing the lumen output of the lamp, why should the foot-lambert brightness change much if at all?

To say it another way: Given that foot-lamberts is essentially a measure of the light energy (once the screen area is held constant), if it's true that the brightness is lower if I move the projector back and adjust the zoom, where is the extra energy going that used to be hitting the screen?
post #6 of 9
It's just because the aperture of the projector lens changes when zooming, which in a way makes the projector more or less efficient at passing light through. For some projectors it is a smaller difference, but I think with JVC and Panasonic projectors it's something like 40% difference in light output (and contrast) between the two extremes of zooming. Other models such as the Sony you mentioned is a smaller difference I think maybe as little as 10% (partly due to the smaller zoom range as well) so less benefit in either option of short throw or long throw.

Cine4home (a German website) always measure the light output and contrast at minimum, middle and maximum zoom so it's possible to confirm the data for the models they have tested, though it's in German and it's difficult to prove links to their reviews for some reason. If you use a translator program it's possible to read their reviews (the figures are easier to work out anyway).
post #7 of 9
Every lens has an effextive f stop. Its the way lenses are speced. a fixed lens has a maximum effective f stop. . a zoom lens can have a constant F stop at both ends of its zoom, but to do so the lens has to be much wider than the zoom lenses used on projectors So as just a relative example, at close zoom, say the F stop is 2.8 (just making a number up) and a long zoom its is 4.0. Those numbers mean the lens will transmit 50% less light at long zoom than at short zoom. The bigger nger the zoom ratio, the long throw multiplier divided into the short throw multiplier, in the case of the JVCs, 2.8/1.4 equals 2, the more light one will lose from going to from short to long. Think of it has at short throw, the image coming out of the lens is as big as it can get. As you zoom towards long, increasing the throw dfistance, the size of the image must get smaller in order not to overfill the screen area.. this means less glass in the lens is being used. sort of like sending water doiwn a 12 inch pipe and then switching to a 6 inch pipe (diameter). Holding the pressure constant, the pipe can only transmit 1/4 as much water. pi r squared to determine the area. Effective F stops are a log funnction and the sequence for each 50% cut is 2, 2.8, 4, 5.6, 8, 11, 14. some of the calculators use the effective f stops at each end and then use a log calculators, others are much more crude.
post #8 of 9
I am looking at a Runco LS-Hb and my throw distance is 192 inches. I plan on using with anamorphic movable lens. What is maximum screen size I can have.
post #9 of 9
This is the wrong thread to ask this question. But to help you out please refer to this calculator:

http://www.projectorcentral.com/Runco-LightStyle_LS-HB-projection-calculator-pro.htm

The screen size will depend on many things. One of the biggest is going to be how far away the projector can be placed from the screen. Another big thing is the gain of the screen.

According to your 192" throw the largest screen size you could obtain with the stock lens is 113" diagonal on a 2.35:1 screen. But that would yield over 60 FtL on a 1.0 gain screen. WAAAAAAY too bright for a screen that small. This projector is way too bright for your room. If I were you I'd look at something more modest like the Runco LS-5. It will perform much better in your room and will have dramatically better black levels and contrast.
Edited by Seegs108 - 1/21/13 at 4:39pm
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