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Bluescreen exposure


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#1 Daniel J. Ashley-Smith

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Posted 15 July 2005 - 07:47 PM

I was just wondering if there's some kind of a rule as to what exposure you should get on the blue screen in correspondence to the subject.

i.e. If you expose for the subject, say 1/50th, T5.6, what exposure should you expect on the blue screen? The same, a few stops higher or lower?

I just want to know so that if I ever do it I'll know how bright to make the blue screen.

Thanks for any suggestions,
Dan.
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#2 David Mullen ASC

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Posted 15 July 2005 - 08:00 PM

There's no consensus on this -- some favor more underexposure of the bluescreen than others. Some even want an overexposed bluescreen (despite all the spill problems that creates.)

An underexposed bluescreen tends to reduce spill and reflection problems.

General rule of thumb is that with an incident meter (assuming a front-lit screen) the exposure on the screen and the subject should be the same. This means that using a spot meter, the screen would generally be at least a stop underexposed from a caucasian face (basically screen in Zone 5 and the face in Zone 6.)

An evenly-lit bluescreen is very important, perhaps even more important than the exact exposure used.

Generally you want to talk to the person doing the composites, perhaps even give them some tests, to determine what works best for them.
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#3 Daniel J. Ashley-Smith

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Posted 15 July 2005 - 08:23 PM

There's no consensus on this -- some favor more underexposure of the bluescreen than others. Some even want an overexposed bluescreen (despite all the spill problems that creates.)

An underexposed bluescreen tends to reduce spill and reflection problems.

General rule of thumb is that with an incident meter (assuming a front-lit screen) the exposure on the screen and the subject should be the same.  This means that using a spot meter, the screen would generally be at least a stop underexposed from a caucasian face (basically screen in Zone 5 and the face in Zone 6.) 

An evenly-lit bluescreen is very important, perhaps even more important than the exact exposure used.

Generally you want to talk to the person doing the composites, perhaps even give them some tests, to determine what works best for them.

<{POST_SNAPBACK}>

Oh right ok thanks David. So, if anything, matching the exposure of the subject or slightly underexposing. I'd probably underexpose it slightly.

Just another quick question, avoiding setting up an entire new topic, how do you actually apply those gels to lights without melting the gel?

Tnx,
Dan.
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#4 Bob Hayes

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Posted 15 July 2005 - 08:44 PM

Underexposing green or blue screen is the way to go. I usually go one stop under but I?m thinking of heading toward 2 stops under. It seems like reflection is my biggest problem. I?m thinking of draping black over much of the green screen so as to reduce spill even more. Then just have the post effects guys garbage it out
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#5 Mark Allen

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Posted 15 July 2005 - 09:16 PM

Then just have the post effects guys garbage it out

<{POST_SNAPBACK}>


Anyone with a lot of keying experience is making about six mattes anyway. Compositors never mind garbage matting things out, it takes seconds. What drives them nuts is spill - and, you're right on target - reduce the amount of green as much as possible - that helps.

Also know that if you're going to have a lot of motion in the frame - if at all possible - try to shoot things moving at different speeds on different plates. That's not something I've heard anyone mention ever online - but it helps a lot. If it's unavailable? Okay - nothing you can do... BUT... if you have two people talking in the foreground and then some people walking behind them in the background - ask if they can be done on different plates. Reason is revealed in the motion blur - the way you have to handle the keying contamination of that is different than with the still people.

And.. I'm thriding the underexposure vs. over exposure. Distance from the screen helps a ton as well.
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#6 Gordon Highland

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Posted 15 July 2005 - 11:09 PM

I do a ton of greenscreen-- on video, anyway. I'll definitely concur with underexposure (= more saturation which is a good thing for "chroma"-keying). I've personally found that about 40 IRE works really well for the background (assuming 70-80 subject and 100 highlight). If you don't have a waveform or a meter handy, use the zebra on a video camera to determine the evenness. I personally use several banks of Kinos. Subject lights come last, and consider judicial use of flags to keep key/fill lights off the background.

As others have stated, distance from the background helps greatly, but if that's difficult, a straw backlight can help wash off some spill. Recently I've taken to applying a light amount of BedHead stick gel on wispy split-end hair, especially blondes.

On the lower end, if you've never tried Keylight in After Effects, it's quite awesome.
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#7 Bob Hayes

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Posted 15 July 2005 - 11:47 PM

The second biggest problem I have doing green screen is out of focus tracking spots. When I shoot closer shots my tracking spots are out of focus.
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#8 Stephen Williams

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Posted 16 July 2005 - 04:08 AM

Underexposing green or blue screen is the way to go.  I usually go one stop under but I?m thinking of heading toward 2 stops under.  It seems like reflection is my biggest problem. I?m thinking of draping black over much of the green screen so as to reduce spill even more.  Then just have the post effects guys garbage it out

<{POST_SNAPBACK}>


Hi,

For a simple green/blue screen underexposing is ok. However if you want to pull the real shaddows that fall on the background or are trying to a shot with smoke or water then I think the blue should be correctly exposed.

To mimimise spill, use the smallest area of blue/green required, and cover up any area not used, including the floor!

I remember doing blue screen work for photochemical finish and in effect one was re-lighting the scene with the blue screen. Any dark areas in the blue came out dark in the composite and light areas came out as a highlight!

Stephen Williams
Lighting Cameraman

www.stephenw.com
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Willys Widgets

Visual Products

Wooden Camera

Technodolly

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The Slider

rebotnix Technologies

Metropolis Post

Ritter Battery

Paralinx LLC

Rig Wheels Passport

CineLab

Tai Audio