Using Zebra for green screen

[Jeremy Garchow]

[Rick Pearl] “1. What should I be doing besides just turning on the Zebra option? “

Do you have a waveform?

What camera are you using?

Answers from 1 will dictate 2.

[Rick Pearl]

Using the HVX 200.

No waveform or light meter.

Thanks!

[Matthew Romanis]

Remember that zebra is only going to show a lower level on luminance, not the colour saturation which is almost more important to know. Too much saturation and the key will have a frequency range too wide.
I use this method sometimes for quick keys on professional cameras when there is no scope available.
First set the F stop to your desired stop, then on a camera with two stage zebra set the lower value to 47% and the upper value to 51%. Light the screen to see it covered with zebra 1 but no Zebra 2. Once you have done this, the lighting on the screen will be very flat and even with at most a 3% fluctuation.
Don’t adjust the F stop, adjust the 2nd zebra to 70% or your personal setting for average reflective skin tone, now light your foreground talent to your desired levels (you may have to turn zebra 1 off), then drop the 2nd zebra level back down to 51% to make sure that none of the talent light is spilling onto the screen and make any alterations such as shielding or flagging lights.
If you are using a camera with only one zebra that cannot be dropped below 70%, drop the F stop by 1 full stop, light until the the zebra is just filling the screen, then set the zebra 5% higher and make the lighting adjustment to the point where all the zebra is just not displaying, go back to 70% to make sure you haven’t gone too far, then open the iris back to the original setting and take care lighting your talent not to spill any extra light on the screen.
It’s always a good idea to have as much separation between the talent and screen as you can achieve.
This is very much a bush method, but yields satisfactory results.
There is another bush method for keeping an eye on saturation levels of the green screen, but it is too difficult to describe without a hands on demonstration.
Matthew.

[Noah Kadner]

Zebra is semi helpful for getting you a more even illumination. By cranking up the iris until you start to see zebra you’ll also reveal areas that are not in the zebra area and are therefore under illuminated. A good green screen is as evenly lit as possible. But a good monitor with a waveform and vectorscope along with as suggested above a light meter to check areas around the screen will help a lot more.

Noah

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[Rick Pearl]

Hi guys,

Unfortunately, a good monitor with a waveform and vectorscope along with a light meter is out of my budget now.

1.) Why did you suggest to set the lower value to 47% and the upper value to 51%. What are these based on?

2.) “Once you have done this, the lighting on the screen will be very flat and even with at most a 3% fluctuation.” Why 3%? How did you determine that?

Thanks!

[Matthew Romanis]

Hi Rick,
I normally use a good grading monitor and scope when I do Key work, however there are times when the old chroma flexi has to come out on light fly away gigs, so I decided a several years ago to sit down in my test area and set a standard that worked with the above “bush” method.
The values you seem to be questioning are the result of playing around with the cameras zebra settings to meet what I normally try to achieve in studio on a wave form monitor. Setting the upper limit 1st at 51% and adjusting the lighting so you see no zebra means it is less that 51% more likely 50% so the mathematical difference is 3%, that’s what I set as a benchmark to achieve when keying with a scope. 3% differential is very flat.
The numbers are not arbitrary, they were achieved with a lot of testing to match the scope, then in being keyed. Numerous successful jobs with this method have been shot in the past.

Another method you may consider is attaching the HVX 200 to your lap top via firewire and using the scopes in your NLE’s capture window. There are cheap programs such as Monitor Magic that can also do this.
Matthew.

[Jeremy Garchow]

[Rick Pearl] “1.) Why did you suggest to set the lower value to 47% and the upper value to 51%. What are these based on?”

Have you ever looked at where the color green falls on a waveform when you send it color bars?

Jeremy

[Rick Pearl]

Hi everyone,

Using the HVX, under display setup, I am unable configure both zebras below 50%

Am I doing something wrong?

Thanks everyone for your assistance!!!!

[Jeremy Garchow]

Nope, HVX only goes to 50 IRE. Do you have a laptop with FCP on it?

[Rick Pearl]

I do have a laptop with FCP on it. Can I connect the HVX200 to my Macbook Pro to check the green screen this way? If so, how do I do this. I know nothing about this so I very much appreciate all the info you can provide 🙂