Projector Lumens in Cinema Scope - AVS Forum
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Old 11-06-2012, 10:48 PM - Thread Starter
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When you are projecting cinema scope movies in 2.4:1 using 4K projector, you are NOT using 30.5% of 17:9 chip. Does that mean you are also loosing 30.5% of brightness? eg) 2000 lm projector -> 1400 lm uncalibrated.

4096 x 2160 => 17:9
3840 x 2160 => 16:9
3840 x 1600 => 2.4:1
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Old 11-06-2012, 11:30 PM
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This depends on how you're doing it. If you are using the zoom method then what you said holds true. Not only are you only using part of the chip but you are also making the image larger therefore your image will be less bright. If you are using an anamorphic lens you will be using more of the chip when you enable the vertical stretch mode and some lenses will actually increase the lumens due to optical changes in the light path. Obviously there are pros and cons to either method but if lumen output seems like an issue the latter option is the way to go.

By the way, there is a dedicated forum for scope discussions. You may get more responses if you ask your question there.
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Old 11-07-2012, 05:45 AM
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That's not totally correct. With the zoom method you are not losing brightness, but with an anamorphic lens you are gaining brightness...

More detailled:

The lumen number gives you the luminous flux that the projector radiates. This is independent of a specific area.

The brightness you perceive on a specific area that is illuminated by the projector is called luminance and typically given in foot-lambert (fL).

Luminance = luminous flux / illuminated area.


With or without an anamorphic lens the projector always radiate the same luminous flux (lumens) (simplified because the A-lens absorbs and reflects some light)
But the illuminated area will be different.

Lets say your projector has 2000 lumen and your screen is 10' wide and the native panel AR is 16:9.
If the screen is 16:9 it would be 5.625' high, the (illuminated) area would be 56.25 ft² and the luminance then would be 2000/56.25 = 35.6 ftL (supposed the screen gain is 1.0)
If you have a cinemascope-screen with the same width of 10', height would be about 4.17', the area of the screen would be 41.7 ft².
When using the zoom-method, the illuminated area would be the same as with the 10' wide 16:9-screen! With the cinemascope-screen, the black bars are projected outside of the screen and with the zoom method they will still be there and contribute to the illuminated area, so the luminance will be the same as with the 10' wide 16:9 screen.
So you don't lose any lumens here.

With an anamorphic lens, the actually illuminated area will be changed! Now the illuminated area would be the same as the visible area of the cinemascope-screen, about 41.7 ft², the projector (roughly) radiates the same luminous flux of 2000 lumens, but the luminance now would be 2000/41.7 = 48 fL!

So you don't lose any lumens, and you don't lose luminance (brightness) with the zoom method, but you gain luminance with an anamorphic lens!
Theoretically about 33% but in reality the A-lens "swallows" some light as it absorbs it or reflects it back to the projector etc.
So the real gain of luminance with an A-lens wouldn't be that high. It depends on the quality of the lens (or prism) how much gain there will be.

Christian
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Old 11-07-2012, 10:36 AM
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While your explanation is sound, the only light that counts is the part of the image that has active content. With a scope screen, the total screen is filled with active image both with the zoom method and anamorphic stretch. The active image fills the entire screen area. Because the whole chip is used to light up that area with vertical stretch and an anamorphic, more lumens will hit the same active area. To your eyes, the anamorphic will be slightly brighter. Without being totally accurate, the image will be about 6% brighter to your eyes. In terms of measured brightness per sq ft of the screen area, the increase would be about 1/3 more, or going the other eway, the zoom would be about 1/4 less.

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Old 11-07-2012, 12:16 PM
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Ignorance is bliss...that is, in the absence of side-by-side comparison, zooming can produce a satisfying result. If you compare a zoomed image to that produced with a decent anamorphic lens, you will no longer be satisfied with a zoomed image. So if the expense of an anamorphic lens is not something you are eager to incur right now, don't expose yourself to one.
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Old 11-07-2012, 01:24 PM
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Ignorance is bliss...that is, in the absence of side-by-side comparison, zooming can produce a satisfying result. If you compare a zoomed image to that produced with a decent anamorphic lens, you will no longer be satisfied with a zoomed image. So if the expense of an anamorphic lens is not something you are eager to incur right now, don't expose yourself to one.

I don't find that true. I had a lens, but didn't find the negative effects on the picture offset the gains ( maybe I needed to try an ISCO III lens ). As far as brightness, I use an electric 2.35:1 screen that has a higher gain than my 16:9 screen - so there is no loss in brighness.

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Old 11-07-2012, 02:23 PM
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While I'm a lens owner myself (bought as a test and kept it ever since) there is something to take into account that seems to be glossed over on many of these threads: Changing the zoom of a projector will very often change the brightness of the image as the aperture changes on the projector's lens. When using the lens I have to reduce my zoom to minimum which gives me a reduction in light output compared to if I zoom. In my particular setup there is little measurable difference in the lux measured at the screen (Tecpel 531 lux meter) when zooming or when using my (Isco II) lens, so having more light output with a lens is not a given.

However, I have a particularly long throw setup which tends to exacerbate this issue, though it also means I get minimum pincushion, maximum contrast (due to that same projector lens aperture change) and a nice sharp image due to the smallest area of the anamorphic and projector lenses being used. Simple maths using the 33% figure doesn't always result in a real world increase of this magnitude as I found. However, I still prefer the image using the lens rather than zooming possibly due to a mixture of lack of overspill ( I can't surround my electric screen with black velvet), higher contrast due to the lens aperture effect and higher pixel density (even though I can't see the pixels when zooming, the image still seems more 'solid' for want of a better description). As said I bought my lens to try, knowing I could sell it on for little or no loss, but I chose to keep it. I could have used the money to improve other areas of my setup, so it's not a decision taken lightly or just to 'prove' the lens vs zooming argument in my favour as it's just what works for me, in my particular setup.

In a short throw setup the negatives of a lens could impact much more on the result (more pincushion and possibly a softer image as more lens area is being used in the A lens and the projector too), plus these days we have the option of E-shift or 4K projectors and lens memory options which might reduce the A lens improvements (bar the overspill which would still be an issue for my setup).

Zooming: Been there, done that, bought the lens...
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Old 11-07-2012, 11:50 PM
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In a short throw setup the negatives of a lens could impact much more on the result (more pincushion and possibly a softer image as more lens area is being used in the A lens and the projector too),

Ya, that's what I have - a very short throw. In my case the issues out weight the benefits.

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Old 11-10-2012, 03:18 PM
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A good spherical anamorphic lens used in conjunction with a medium to long-throw projector lens will always look better than zooming. I have never seen it to be otherwise. A prismatic lens over a short-throw lens is not recommended.
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