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Filed under: Cameras •Post Production •Production •Training •

The RED Camera and Green Screen

Meet Adam Loften. He was our assistant editor/data wrangler on the shoot, and he was kind enough to stand in for our little test. The light is all tungsten and read around 2900k on my Minolta Color Meter 2.

You may notice some dimples and such in the green background. That’s because we saved a bit of money and went with a 30’x40’ fabric green screen over a cove in order to save money on paint.

Here’s the green channel:

It’s very clean. My previous tests on RED Build 15 (this test was shot on Build 16) showed that green was consistently the most solid color channel under both daylight and tungsten light. On 18% gray card tests shot under daylight the RED’s histogram showed green in the center with the red and blue channels almost perfectly aligned under it. Under tungsten tests green stayed in the center but red advanced to the right of the histogram (overexposed compared to green) and blue receded to the left (underexposed compared to green) with little overlap of color.

Knowing this, it seemed pretty safe to expect a good key based on green lit with tungsten light. I’ve also noticed, as have others, that the RED is overly sensitive to green and magenta color shifts in light. (In pure light terms this means the RED is sensitive to too much green and too little green. There is no magenta color in the visual color spectrum: magenta is simply an absence of green in light.) When not shooting full body green screen we often super-saturate the green with Super-Green Kino Flo tubes or by using green gel (“plus green” or Lee Fern Green). Modern software keyers look not just at the brightness level of the screen but also at how far the screen’s hue is from the next nearest color. The farther away it is from the next nearest color the better the key. Full body green screens don’t allow for the use of colored light, as it will contaminate the subject, so we’re forced to use only white light. In this case, having a camera that’s a bit oversensitive to green turned out to be a good thing.

Here’s the red channel:

As you’d expect, there’s not much red in green screen. (When looking at color separations, light tones show where there’s more of the separated color, and dark tones show where that color is absent. In this case, Adam’s skin and the slate have a lot of red, but his shirt and the screen don’t.) Zooming in shows a little bit of a rough edge between his shirt and the screen but it’s not too bad. We’re not really keying based on red but noise in an underexposed channel can result in a noisy image and, possibly, a compromised key.

Here’s the blue channel:

Gulp.

There’s a fair bit of noise in there. The lightness of the screen tells me there’s blue in the screen, which isn’t unexpected: if you look at any green screen on a vectorscope you’ll see that it’s not pure green but is a little shifted towards the blue vector. What’s surprising is how noisy it is. Even though the screen is light, telling me there’s blue exposure in there, it’s very noisy, as is Adam’s shirt. How can a green screen that obviously has some blue in it look noisier than the relatively clean red channel? There’s a lot less red in a green screen than there is blue!

Here’s my theory:

Tungsten light has a lot of red in it, so the red channel has a healthy exposure even though there’s not much red in the green screen, because what little red is there is healthily exposed. The 3200k viewing preset in the camera is probably holding back a lot of red that is actually present in the raw image. So while the screen looks dark in the red channel it’s actually getting enough red exposure that it’s not very noisy.

The blue channel probably looks bright because it is being boosted due to RedAlert trying to mix three color channels to create a white balance of 3200k. Red is reduced and blue is increased to match the level of the green channel. But the blue channel boosting in RedAlert is only increasing the brightness in this image. It may increase the visibility of some of the noise but it’s not increasing noise overall, because noise is a sensor artifact and we’re way beyond the sensor’s influence now.

Silicon sensors tend to be least sensitive to blue, which is why the blue channel tends to be the noisiest. Many cameras, such as the F900 or the Varicam, are balanced for tungsten light by boosting the blue gain in relation to the others. Most of the time this isn’t noticeable until the camera’s overall gain is boosted, and then the blue noise quickly becomes apparent. When shooting outdoors, an 85 filter simply balances the light for the tungsten-jiggered sensor.

The RED has no such filtering and gain manipulation. It records a “RAW” signal (at least in theory—let’s not get into what really constitutes RAW these days!) supposedly with no manipulation of the red, green and blue gains. (There is probably some, particularly in the highlights.) The result is a camera that does very well in daylight because there is a LOT of blue in daylight, and that increase in blue makes up for the lack of blue sensitivity in the sensor. But under tungsten light the red channel gets more exposure because of the increased warmth of the light and the blue channel is a bit starved.

Let’s see what difference a filter makes…

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Well done article.  Your articles are always informative and make potentially difficult topics, easy to understand.

Posted by .(JavaScript must be enabled to view this email address)  on  11/04  at  08:18 AM

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Did you place the 80D filter on the camera or on the light source?

I’ve often seen the green screen lit with one set of instruments and the talent lit with another. If the green screen is lit so it’s instruments do not spill onto the talent, would you then light with green on the screen and white light on the talent?

Peace,

Rob:-]

Posted by Rob  on  11/05  at  01:46 PM

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You said, “... the 80D was half the correction of an 80D filter ...”. Is there a typo in here?

Peace,

Rob:-]

Posted by Rob  on  11/05  at  01:47 PM

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Please clarify a couple of points:

When you say “full body shots” are you referring to the seeing of skin tones on your subject ? Is that your concern ?

If that is the case then why not just make sure that there is no contamination of the subject by the light illuminating the
Green screen ?  Would using Super Green Kinos or a Lee green on your tungsten lights significantly improve the key ability of the screen ?

I have lit hundreds of green screens for films,  Keeping green
contamination off the foreground subjects is simply a matter of light control and having a large enough space/greenscreen to
keep the subject far enough away from the screen to make sure
that there is no bounce spill.
I would light the green screen to its best keyablity and then light the subject to the proper ratio of screen to subject in terms
of over/under stop wise.
I would much prefer to gel my lights than to filter my lens because when I filter my lens I am affecting not just the green screen but also my subject.

Perhaps I am misunderstanding , again my experience is primarily with film. We are starting to be questioned about whether the RED is a viable option for some of our work.
thanks-J

Posted by .(JavaScript must be enabled to view this email address)  on  11/15  at  11:47 AM

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Great article Art!

Posted by .(JavaScript must be enabled to view this email address)  on  11/22  at  05:40 PM

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