Green screen Issue shooting with dslr (background green changes)

Question

I don't have a real green screen, just a piece of canvas painted green and not even the "best" color of screen. I've taken some short videos with a dslr (Nikon D7100) of me using a mobile phone and some work (I can key out the green) and other don't. I'm trying to understand the reason the key operations aren't working.

I think it is because the light from the mobile is affecting the way the camera sees the green background. When the phone's screen is a light color the green darkens; when the phone screen is black the green lightens. How do I prevent this, as I can't key out such a wide range of greens?

Camera is on auto, but have tried Manual also. Lighting is sub par but I think it is good enough.

Answer 1

It doesnt really matter what type of lighting instruments you use, tungsten, hmi, led, flo, so long as you use the same color temperature lights to light the screen as you do to gauge the lighting of your scene and subjects.

I do a lot of greenscreen work, and my best "short" advice:

1.) Get as big a screen as you can afford. And get your subjects as far from it as possible. This will reduce back and side spill.

2.) Use 1/4 magenta CTM rosco gel for hair/cut lights, behind your subjects. The subtle magenta is complementary to green, and will give you a better key and better edges in post.

3.) Avoid stray hairs.

4.) Use best codec possible. 444, 442, 422 being the worst. I shoot RED 3:1 and still have issues sometimes.

5.) Test shoot, and do your key in post, before you shoot. If you have fast moving action, you will want to shoot at a higher shutter speed. Motion Blur can be a problem.

6.) It's easier to shoot at a higher f-stop, and get a sharper tack, and add back in blur/dof effects in post, than it is to shoot wide open with a long piece of glass. Blur causes masking issues.

7.) Get Keylight for AE.

8.) Have a dedicated waveform monitor if possible. You want your luma to be a straight line at 75 IRE. You will not be able to light your screen evenly by eye, you really need a waveform to get a spot on straight line, with no dips or falloff on the edges.

9.) Dont let your subjects wear green.

Cheers.

Answer 2

1. DO NOT USE "AUTO". It is working against you. A nice, even video image of the green is needed. Setting your camera to "auto" just means that there is no single "formula" for which green to key out.
2. USE BETTER LIGHT. it is not clear why you say "I think it is good enough", and then tell us that it is not good enough? We typically light the green screen separately so that the light is nice and even regardless of what is in the foreground (in front of the green screen).

A popular solution is to use cheap "shop" fluorescent lights. Typically we don't like to shoot with fluorescent light because it has an intense green spike in the spectrum. But that is exactly what you WANT for lighting a green screen! Be sure to use a modern solid-state (high-frequency) ballast, not the old "buzz-box" kind that flicker.