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Green Screen Shutter Angle


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I did a search but cannot find the answer, hoping someone can help me out.

 

I am shooting on a green screen for my first time and have been doing some testing. For this spot I am very limited in space, talent will be about 4 1/2 feet away from the screen. It's waist up, 2 camera angles, and not a lot of moving around. My cameras will both be Blackmagic 6K cameras with prime lenses on them.

My question is...what shutter angle should I use?  I have looked in numerous places and the only answer I can find, in one space,  is move my 180 degree shutter angle down to 90 degrees. This will get rid of the motion blur and I can put that back in during post, if needed.

What shutter angle should I be using and does it vary with any variables?

Thanks all!

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I can understand the logic but I've never done that simply because I'm shooting a greenscreen shot -- it sort of depends on how well your compositing software handles transparency from blur.  But if it's a waist-up shot on someone standing in front of a greenscreen, I doubt there is a lot of movement so either 180 or 90 degrees is probably fine.

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Yeah I've never seen that technique before but it sort of makes sense and I wouldn't be surprised if someone were doing that. The issue is for fast action, motion vector software gets a little weird and for slow action a normal 180º shutter works if you pull a decent key. I think I have choked in on a key before and put another key below it with Twixtor or something in the past. 

In a small room your worst enemy will be spill. I forget exactly but I think you want your green screen to be around 18% gray or maybe a half stop over through the spot meter and evenly lit. And the subject lit normally. If you light it too hot especially on a darker subject you'll get a lot of spill. Curious to hear from vfx sups and DPs what their green screens read through a spot meter. 

Don't use a diffusion filter, even if you're using one elsewhere. Diffuse the shot in post if you have to.

There's separate advice I'd have for your post team but if you're just shooting it I'd do 180º shutter, avoid too bright a green screen, usually I'd rate that camera at 400 ISO or 800 ISO but if you're not clipping maybe shoot 200 ISO or something – Arri recommends the same thing for keying Alexa footage I think. (I find both braw and ArriRAW fairly noisy at 800 ISO.)

Edited by M Joel W
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Thanks for the thoughts so far David and M Joel. Right now I am planning on keeping the screen close to 40 IRE and the talent close to 60 IRE. I have done some test shots and think I am managing the green screen spill. However they have not yet picked a background so I really cannot color correct everything perfectly yet.

Right now I am testing at 5.6 aputure with 400 ISO. However once I get this dialed in I'll need to redo it all because I will be placing a 70/30 teleprompter glass in front of the lens. I THINK that will take 1 to 1 1/2 stops away, so I'll have to adjust for that. I actually have no idea how to measure the amount of light stops the glass will take away. Does anyone know how that is done?

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By Twixtor I meant real smart motion blur btw. It sounds like you are off to a good start.

Never worked with a teleprompter before but if you have a spot meter that would be the easiest way to tell. If not, shoot a blank white wall at f5.6 and look at the IRE reading then put the teleprompter in front of it and then open up until you get the same IRE and note how much you had to open up.

If it's 70% transmission that is about 1/2 a stop I think. If it's 30% that's about 1.5 stops.

Edited by M Joel W
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  • 1 month later...

One stop over? That’s going to create more spill, especially with the subject so close. 
 

I metered my green 2 stops under the subject on a commercial last week and it keyed really cleanly. 
 

Would be interested to hear how anyone else is metering their green. Also, how to deaL with curly and frizzy hair and clothing. Always found this a bit of a nightmare afterwards. 

Edited by Scott Jones
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  • 2 weeks later...

One of the secrets of good keying is shooting cleanplates if possible - the greenscreen background without actors. The best way of dealing with frizzy hair, defocus and motion blur is Nuke IBK keyer that can precisely subtract the background using a cleanplate; almost always VFX artists have to recreate cleanplates digitally (through a tedious cleanup process),  but a real cleanplate is much more preferable. Learned this technique from from Andrey Savinsky, lead compositor artist at Framestore.

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9 hours ago, Alissa Alexina said:

One of the secrets of good keying is shooting cleanplates if possible - the greenscreen background without actors. The best way of dealing with frizzy hair, defocus and motion blur is Nuke IBK keyer that can precisely subtract the background using a cleanplate; almost always VFX artists have to recreate cleanplates digitally (through a tedious cleanup process),  but a real cleanplate is much more preferable. Learned this technique from from Andrey Savinsky, lead compositor artist at Framestore.

I've heard it said this is a good reason for motion control, even if it's easier to just track the shot to markers in the end (which obviates a lot, if not all, of the messing about with lens rectification and other maladies).

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