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You're seeing bleed because the matte is not strong enough. You need to do more tweaking than a simple choke matte. You should always look at the combined matte and the status when using Keylight. Adjust the white and black clipping to clean up the matte. A Google search for keylight tutorials will give you a handful of great keying tutorials.
Your lighting may have a little to do with the quality of the key, but I couldn't tell that unless I saw a raw frame. Keylight is capable of pulling satisfactory keys from some pretty terrible footage.
Generally speaking, there's almost always spill on your subject. (Especially with very luminous green like Rosco DigiComp). You can minimize this by draping black curtains around areas that are not visible by the camera. It's less of an issue with flat wall screens versus soundstages you walk on.
But as usual, Rick is absolutely correct. Your core matte simply isn't strong enough. To get a really great basic key, there's only a couple things you need to mess with on Keylight: Screen Matte> clip back and clip white.
Switch the view mode to Screen Matte. Adjust both so that your talent is completely white inside and the green background is completely black. You can also use View>Status for an exaggerated view of this (Black is 100% transparent, white is 100% opaque, and gray areas are translucent. Green areas are also receiving despill correction.)
And that's pretty much it to get a really good basic key.
You might want to check out Primatte 5 for After Effects. It's a more intuitive keyer that has a surprisingly good one-click "auto compute" mode. You can also draw on the screen to tell it what parts to keep and what parts to key out.
Thank you Rick and Navarro for some very helpful tips.
Please accept my apology for not posting my Keylight settings. Here they are (were):
Looking at the key in Screen Matte view, it was clear the settings were woefully inadequate. I had to adjust both according to your simple guidelines, Navarro, and presto, no more bleed. My settings were based upon one tutorial I watched 1-1/2 years ago, so I guess I just got lucky with the background images I used until this one.
Rick, I will indeed watch a bunch of tutorials now on Keylight.
For what it's worth, here is the raw frame if you want to comment on the lighting. Now that I look at it again, maybe it isn't so evenly lit.
Thank you, gentlemen. You have made me a much better "keyer". It certainly cannot be a much better matte[r].