Here’s an pretty cool tip for those young filmmakers out there experimenting with Green Screen / Chroma Key.
In the process of recording a subject in front of a Green Screen, first and foremost make sure you have the desired framing of the person: a medium shot, a close-up or a wide shot. Then ask politely to the actor to step aside, and now your entire frame should be filled with green, or at least part of it.
Now, activate the Waveform window on your camera settings. Some cameras have it, others don’t, and some you can hack with a software that rhymes with Tragic Panther.
Normally, a waveform looks like this, if it’s analyzing an image with lots of colors and have objects that disrupt the luma levels: