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Green screen exposure


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#1 Steven Prio

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Posted 22 April 2007 - 02:05 PM

Hi all!
I just quickly want to ask a very simple technical question. What should be the exposure of a green screen in relation to your f-stop to help the post people do their job faster?
We are using greenscreens in computer monitors and mobile phone screens in order to replace the images afterwards.
Usually greenscreen reflected reading should be a stop under your aperture. Is this actually true? T
Thank you for your time!
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#2 Stephen Williams

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Posted 22 April 2007 - 02:22 PM

Hi all!
I just quickly want to ask a very simple technical question. What should be the exposure of a green screen in relation to your f-stop to help the post people do their job faster?
We are using greenscreens in computer monitors and mobile phone screens in order to replace the images afterwards.
Usually greenscreen reflected reading should be a stop under your aperture. Is this actually true? T
Thank you for your time!


Hi,

1/2- 1 stop under is fine. For the mobile phone screens you may be making lots of extra work by using green screen. Even tracking marks can be a pain, the corners are easy to see in any case.

Stephen
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#3 Tony Brown

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Posted 22 April 2007 - 02:23 PM

Hi all!
I just quickly want to ask a very simple technical question. What should be the exposure of a green screen in relation to your f-stop to help the post people do their job faster?
We are using greenscreens in computer monitors and mobile phone screens in order to replace the images afterwards.
Usually greenscreen reflected reading should be a stop under your aperture. Is this actually true? T
Thank you for your time!


Post houses vary. The latitude in the stock means you could be +2 or -2 it will make little difference. I generally go for the shooting stop so that tonally its a mid tone

Cant remember the last time I actually shot green / blue scree. The post houses generally prefer of rough match to whatever will be comped in these days.....
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#4 Stephen Williams

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Posted 23 April 2007 - 12:32 AM

Hi,

My reason to slightly underexpose a green screen relative to key is to reduce green spill.

Stephen
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#5 Tony Brown

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Posted 23 April 2007 - 12:41 AM

Hi,

My reason to slightly underexpose a green screen relative to key is to reduce green spill.

Stephen


Depends on your distance from subject to screen....... but yes if you are so close you are experiencing spill then it would be prudent to reduce the level
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#6 Steven Prio

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Posted 23 April 2007 - 01:47 AM

I just want to thank you all for taking the time to answer my question
Thank you!
Steve
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#7 Stuart Brereton

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Posted 23 April 2007 - 04:01 PM

I usually keep green screen about 1/2 a stop to 1 stop under. It helps with spill and increases saturation.
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#8 Mike Lary

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Posted 03 May 2007 - 07:30 PM

For ease of keying, even lighting on the green screen is more important than its relation to the key. From a VisFX standpoint the screen could be lit a stop or two below or above the key, so you might as well go under to reduce spill.

Testing is imperative if you can afford it. If you hand the test footage over to the VisFX crew and have them key it, you'll know exactly what you need to do when principal photography begins.
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#9 Stuart Brereton

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Posted 05 May 2007 - 06:31 AM

the screen could be lit a stop or two below or above the key



Lighting the screen a stop or two above key would reduce its saturation, whuch you definitely don't want.
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#10 Andrew Brinkhaus

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Posted 06 May 2007 - 02:06 PM

So I'm not new to lighting, but I have in the past shot mostly video, and I am just now beginning to work with a meter. When you say "light the screen 1/2 - 1 stop under" Your referring to already having a predetermined stop, such as f/4, and then lighting the screen at a f/5.6 or something similar? Which means that the f/4 would be the foreground or talent's range? Just wanting to make sure i'm reading this right.
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#11 Andrew Brinkhaus

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Posted 06 May 2007 - 09:15 PM

...Or does it mean, 1/2 - 1 stop under middle grey?
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#12 Frank Barrera

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Posted 06 May 2007 - 09:49 PM

So I'm not new to lighting, but I have in the past shot mostly video, and I am just now beginning to work with a meter. When you say "light the screen 1/2 - 1 stop under" Your referring to already having a predetermined stop, such as f/4, and then lighting the screen at a f/5.6 or something similar? Which means that the f/4 would be the foreground or talent's range? Just wanting to make sure i'm reading this right.

if you want to expose your scene at an f/4 and you want to under expose your green screen by 1 stop then your reading off the screen should be an f/2.8. that would be 1 stop under.
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#13 Stuart Brereton

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Posted 07 May 2007 - 05:59 AM

if you want to expose your scene at an f/4 and you want to under expose your green screen by 1 stop then your reading off the screen should be an f/2.8. that would be 1 stop under.


It's worth noting that we're talking about reflected readings from the greenscreen, not incident.
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#14 Frank Barrera

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Posted 07 May 2007 - 08:24 PM

It's worth noting that we're talking about reflected readings from the greenscreen, not incident.

yes. on a film job i would use an incident meter for the subject to achieve the f/4 and then a spot meter to get the f/2.8. and for video with caucasian skin tone i would take some 70 IRE on the face and around 50 IRE on the green screen using a spot meter to check that the screen was lit evenly. of course the running joke is always that its doesn't matter how we light it because "with a mac nowadays you can key anything".
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