Minus Green, ND Gel for Chromakey Ceiling Spill

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Ken1212

Well-known member
I have setup the game room as a chromakey studio at home and experiencing major spills off the white ceiling paint, I know I could paint it black, however my question if I staple Tough minus green to the ceiling would that take care of the spill from the ceiling? And will use black curtains on the side walls.
 
It might contain the spill some, but that seems like an imprecise solution. How about pushpinning a thin black sheet up there, to prevent any spill in the first place?
 
Barry_Green said:
It might contain the spill some, but that seems like an imprecise solution. How about pushpinning a thin black sheet up there, to prevent any spill in the first place?
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Thanks Barry, Will do just that, this is my first setup of a green screen and wasn't sure what's the most ideal solution for color correction. Capturing a clean full body shot with white walls in a limited space has been a challange with the spill.
 
If it's a small space, you're going to have challenges in all sorts of ways. Key (pun intended) to getting a good key, is to get the subject as far way from the greenscreen as possible. That minimizes the spill. If you can't get the subject a goodly distance away (10 feet at least) then you're going to have issues with spill. Strong backlighting can help, as well as lowering the light level on the greenscreen itself; the less light you blast at the screen, the less that can bounce back and spill all over your subject. Good luck!
 
And cover up any part of the greenscreen that's not crossing the subject, and especially anything not in frame. the screen is basically a giant bounce. Smaller bounce with same light = less spill.
 
. . . "as well as lowering the light level on the greenscreen itself; the less light you blast at the screen, the less that can bounce back and spill all over your subject."

WRONG.

Your spill from the greenscreen is directly proportionate to its illumination. If you reduce the light level on the greenscreen you reduce the spill level proportionately, however, you will have to increase your exposure to bring the greenscreen back up to its proper level. That will bring the spill level back up, too.

Suggestion: 1) Stem you greenscreen lights up as high as possible and tip them down to smooth them out.
2) Gobo or flag the ceiling spill off of your subject.
3) Find a bigger venue in the future.

Good luck.
 
kennedymax said:
. . . "as well as lowering the light level on the greenscreen itself; the less light you blast at the screen, the less that can bounce back and spill all over your subject."

WRONG.
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It's only WRONG if you presume that the person doing the shooting is already exposing the screen absolutley perfectly. It is an extremely common error for people to overlight the screen significantly, thinking that the brighter the blare of green, the more green the computer will see.

If you prefer, I'd reword what I said to "don't overlight the greenscreen, as blasting more light onto it will just magnify the amount of spill you get on your subject. As long as the screen is lit evenly and completely flat, you don't need a ton of light blaring onto it."
 
I agree with Barry.

I see many examples of key screens that are over lit for the purpose. ie the bg exposure level is too high in relation to the subject exposure level.

FWIW. I never use digital green or digital blue screens in the studio.
I own traditional chroma blue and chroma green screens which have a lower reflectance value.
 
Proper exposure also helps counteract spill in and of itself. If the screen is exposed properly at 50-55 IRE, it's going to be less bright than most of what you're shooting, particularly all but dark skin tones, and quite a bit less bright than light colors and whites. It won't be able to "spill" as much because it'll be partially outmatched by a properly-exposed subject.

Of course, the more that's in your image of 50 IRE or lower (shadows, etc.), the more likely it's going to be green-tinted, but a pre-key color curve adjustment can help with that.
 
I, too, agree with Barry. . . partially. Most people do over-light their keying backgrounds. We certainly presume that in this situation the OP has established a proper exposure for his greenscreen background and then recognized his spill problem. Also, I think that you will agree that most people mainly light their backgrounds from a low angle. Reflectance off of the background (most likely in the OP's instance) heavily lights up a low white ceiling. To minimize this spill I would prefer steming the background lights up high, pointing them down and adjust them for an even spread. This will allow the green reflected background spill to fall more onto the floor rather than on to the ceiling. Spread a black cloth on the floor if you need to. It's much easier than attaching it to the ceiling. Still one should flag off any objectionable spill onto the subject.

Any reflected or spill light is directly proportional to its source. If this spill is objectionable with a properly exposed background lowering the background level will not change the background to spill ratio. Correcting the exposure for the lower background illumination will only cause the spill illumination to be brought back up and will not have corrected the problem.

You have to play around with an awkward situation like this to come up with a workable solution.

Its only a suggestion. . . and I'm gone.

Good luck.
 
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Thank you guys, I am hopping that finding the right balance of exposure to achieve the most acceptable key will be achievable for my limited space. The screen is 10ft wide x 16 ft long, room is 16ft wide x 19 ft long with 9ft ceiling. My subject is about 7 feet from the screen. I have only used so far 40w/4100k single lamp fluorescent tube fixtures for the screen. One on each side. I seem to be getting sufficient light , not harsh but nicely lit. I think the problem emanates from the 9 ft ceiling and the 16 ft wide room. I also had setup a background light with tough minus green for my subject. Used two 500w tungsten Lowell DP lights for key and fill, the fill is in a silver dome and the key has diffusion gels. The spill is sharpest around edges. Surprisingly Boris Keyer is doing a great job when my subject wears dark colors, but when having on a white shirt you really see how awful the key is. Just went to Hobby Lobby and bought a bunch of matte dull black poster boards, I am going to try to cover the side wall portions and ceiling where my subject stands and see how much that helps.
 
40 IRE seems a bit dark and tempts noise, but if you can pull a good key with it, then the spill risk is even less.
 
Ken1212 said:
Used two 500w tungsten Lowell DP lights for key and fill, the fill is in a silver dome and the key has diffusion gels. The spill is sharpest around edges. Surprisingly Boris Keyer is doing a great job when my subject wears dark colors, but when having on a white shirt you really see how awful the key is.
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that doesn't seem surprising, really. the white is going to reflect any splashed green more than a darker color. controlling the walls and ceiling may help, but separation (as mentioned) between subject and screen is important. failing that, if there isn't more room, it's important to have edge lights (also mentioned) so that you're controlling the color of light in the area where the green spill would be on your subject. 4 foot kinos on each side (for a standing subject) are ideal for this purpose. but, this is also a problem in a small space, you need room to position these slightly behind and to the side of your subject. and then you probably need to put a flag between those sources and the lens.

and don't forget the hair. i don't like hair lights, but in this case you're using it to fight spill on one of the more difficult parts of your key.

full body chroma takes a lot of space.
 
David Jimerson said:
40 IRE seems a bit dark and tempts noise, but if you can pull a good key with it, then the spill risk is even less.
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Actually 40% is ideal / exact exposure' for a blue bg (for key purposes) so there is no noise issue.

This exposue provides exact saturation of pure blue channel) and it is also well separated level wise from skin tones @ 60 - 70%.

Over exposing it lowers the saturation (lightens the blue channel).

To clarify that 40% is exact...check where the perfect blue swatch on SMPTE Color Bars sits in a waveform
monitor. (Actually its more like 38% to be very precise).

The additional bonuses of course are less spill and less light required on BG and FG.
Note: the BG always requires X amount of light and units to cover it flat and evenly... so that determines the rest of the lighting balance.
 
OK, 40 IRE may be ideal for blue, but he's doing green. Full-sat green is about 59 IRE and SMPTE green is about 49.
 
Right now I turn the room's ceiling light off when taping. My subjcet stands straight under it, it's a two halogen bulbs fixture, would it be helpfull to put on couple CFL bulbs and use it, would that do much to the spill, how about adding minus green to it? The ceiling will be coverd with black poster boards.
 
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