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Lighting for Green Screen


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#1 Alexandre de Tolan

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Posted 05 February 2016 - 06:05 PM

I have more than 20 years operating a camera and now and then someone turns to me to DP some project or as a second unit DP.

 

Today was one of those days when some director came to me asking to DP a commercial. I said yes even though it's going to be a first time DPing on a green screen, so I'm a bit nervous.

 

The set has 43 meters (roughly 150 feet) by 18 (59 feet). The lighting grid is 7 meters (23 feet), off ground and they have all lighting requirements I can ask for. 

 

The set will be dress by the Art Direction with some walls where a couple of guys will be Parkouring. 

 

The action will be shot 60p and 150p, so about 1 1/2 stop difference.

 

My initial thoughts are to put a bunch of space lights in the grid to light the green screen evenly. They will be on dimmers to dim them down for 1/120 and up for 1/300 shutter speeds.

 

Then backlight/Kick the action with HMIs through frame diffusion (I was thinking LEE 250 to give it some edge), and bouncing that light into the actors with Ultrabounce. 

 

Questions for the experienced in these situations out there:

 

Hang the space lights only behind the action or also above it to raise exposure levels?

 

How can I avoid green spill on the action? Push it as must as possible from the green screen? I believe that the Art Direction will also be greening the ground where the fake walls will sit. Use Minus green on the HMIs to separate the action?

 

How many stops difference have the green screen to read comparing to the bounced light on the actors or even the edge/back light on them?

 

I'm under the impression that this will be shot on a Sony F55, S-Log3, so ISO 1250 base exposure.

 

Any suggestions are greatly welcome, even on different lighting approaches.

 

Thank you in advance.


Edited by Alexandre de Tolan, 05 February 2016 - 06:06 PM.

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#2 Satsuki Murashige

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Posted 05 February 2016 - 06:46 PM

I would not have the space lights on dimmers as the color temp will change when you dim them down. If they are 6k space lights, there should be 3x 2k globes in each so you can simply turn off the globes to reduce intensity without altering color temp. You could also have flicker issues with high speed, though that would unlikely with tungsten if you're not over cranking beyond 120fps.

Whether or not the space lights are above the action depends on if you want a silhouette look or not. Keep in mind that the darker the foreground, the more the green spill will show up. It's not a bad idea to put minus green on your backlights if you're going to shoot silhouette.

With regard to spill, yes keep the actors as far away from the screen as possible. If the floor will be green as well, have white showcard available to put behind the actors and at their feet to eliminate as much spill as possible when not in the shot. Basically, any green that is not actively being used to key the actors should be covered in white or black so as not to create more spill. How much spill you can get away with depends on how much time you have budgeted to clean up this stuff in post. Top quality VFX artists are pretty amazing but they are expensive.

I'd talk to the post production sup and see how they like their green screens exposed. Some like around 50IRE for more saturation, others around 65IRE for less noise.

I don't think the Sony XAVC codec is really the best choice for greenscreen since it is 422, I would push for raw or a 444 codec. You can easily add an R5 recorder to the back of the F55, or shoot Red or Alexa Prores 4444, though you'll have to make some compromises to get 120fps overcranking whichever way you choose to go.
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#3 timHealy

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Posted 05 February 2016 - 10:04 PM

I wouldn't use space lights to light the green screen. The light will go everywhere unless you use a few different ways to cut light from where you don't need it. Also if the space lights are too close to the green screen, the top of your green screen will be hotter than the lower part. See your fx advisor on how to proceed. I would use DMX image 80's or DMX Wall of lIghts from above the whole length of your green screen(but you can adjust or light from the floor depending on you set) You can get super green bulbs to really make things pop. or tungsten with some sort of green gel. Or you could get away with tungsten bulbs lighting a green screen. I'm not sure what your sets look like but you could have a row of Kinos on the floor, but to be honest with the height 23 feet, you only need kinos from above.

 

Then use a light board to turn kinos bulbs on and off depending on your exposure. You can't dim Kino's but you can turn bulbs on and off with the light board. 

 

And your main actors should be pulled away from the green screen as much as you can and lit independently.

 

Best

 

Tim


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#4 Alexandre de Tolan

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Posted 08 February 2016 - 05:11 AM

One more thing. Do panning shots need tracking marks or only tracking shots need them?


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#5 David Mullen ASC

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Posted 08 February 2016 - 11:35 AM

If the camera is not locked off, then the compositor might want tracking markers.


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#6 aapo lettinen

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Posted 08 February 2016 - 12:27 PM

if possible, mask all the green which is not used as Satsuki suggested.

I believe your vfx sup is going to be on set when shooting, if not then you should be extra careful about tracking markers and pre plan everything more carefully with the vfx department. 3d match moving is especially tough with green screen because the background is so even that you can't necessarily pull enough tracks to track the camera move correctly and the marks also have to be at different depths so one may have to add extra markers on stands to different depths and sometimes even fill half of the frame with them if it's difficult shot. if it's that kind of difficult shot you should reserve some small light stands for the parallax markers.

 

with tracking markers you have to know the camera move and what the background will be, even a simple tripod panning shot may sometimes need 3d track if it has 3d objects added close enough so that the parallax can clearly be seen. so you will need from 1 to >500 markers depending on the shot  ;)

 

blurry markers are more difficult to track but you can usually get away with them most of the time. 


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#7 Satsuki Murashige

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Posted 08 February 2016 - 04:05 PM

One of my close friends is a VFX artist, she tells me that while all that info is very important, she will often end up eyeballing elements to get it looking right. Be sure to record all the lens info per shot and to shoot lens grids.
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