Chroma-key backgrounds and depth of field.

Laurence wrote on 3/25/2010, 9:16 AM
I know that chroma-key stuff has been beat into the ground on these forums, but I am working on a project and have tried something new to me at least that I kind of like:

I have a guy talking in front of a greenscreen. It's just the one guy talking so I thought I would vary it up a bit and shoot some phrases waist up, some head and shoulders, and some close up on the face. I was thinking how when you do this with a real background, as you zoom in you often get a shallower depth of field as the f-stop increases during the zooms.

I thought about simulating this effect with my static background and so in addition to zooming in on the closer shots, I defocused them a bit with the Sony defocus filter.

It seems to look very natural. Is this a common technique, and if it is where can I read more about it?

Comments

Laurence wrote on 3/25/2010, 4:50 PM
Well after a little experimentation I think I am going to give up on blurring my chroma-key backgrounds. The problem is that the keyed edge really stands out against a blurred background. With the background in focus it is hardly noticeable if I am careful, but with the blurred background the edge kind of stands out like a sore thumb no matter how I treat the edges. I guess depth of field is easier to shoot than it is to generate.
Cheno wrote on 3/25/2010, 4:54 PM
Laurence,

there are ways to soften your keyed edge by either the chroma blur or even creating an alpha channel mask with a blurred edge that you can put on a track above your clip and shrink just a tad to soften that edge. Lots of DOF done in post and can truly sell an effect if done correctly.

Also - are you using Vegas' keyer or another?

cheno
richard-amirault wrote on 3/26/2010, 6:49 AM
Assuming you don't zoom while shooting ... try de-focusing the background pic in Photoshop instead of Vegas. Don't know if will make a difference, but it might.
Laurence wrote on 3/26/2010, 7:09 AM
Well this particular project is going to show tomorrow without any DOF effect. I used a combination of the Vegas Keyer and the New Blue Video Essentials one and had to do it in two passes because of some unevenness in the green screen lighting.. I tried various amounts of blur at the edge of the key but they all became obvious as soon as there was any blur in the background. I could probably make this work if I had more time. Without the blurred background it looks great though so I'm happy for now.
farss wrote on 3/26/2010, 7:55 AM
What I think you need to achieve is the natural blur at the edge between the subject and the background. The more advanced keyers such as the one from Boris do this for you. AE itself has the primitives to do it by creating a mask that is just the edge, from that I can see how you blur just an edge which in your scenario would give your the blur / bleed needed to make the subject look like it belonged in your out of focus background.

I can mentally see a way to maybe do this in Vegas but that'd be a fair amount of work. You can expand / shrink a mask using blur. Then you can create the difference between the two masks which gives you just an edge mask.

Don't give up, go back to your thoughts when you have time to play around. You're certainly onto something.

Bob.
Laurence wrote on 3/26/2010, 4:56 PM
I have Boris FX and that has something called "light wrap" which may well be what I am looking for. Does anyone here use Boris FX's "light wrap" feature, and how does it help?

By the way, here is the video I've been working on (without the simulated DOF on the closeups):

http://vimeo.com/10459011
Myerz wrote on 3/26/2010, 5:06 PM
Watched it... pretty cool.

Hey, somebody told me Boris FX didn't work in Vegas Pro 9.
Is this true?
Laurence wrote on 3/26/2010, 8:02 PM
Boris FX works ok in Vegas albeit with some limitations because of Vegas only being able to pass one frame at a time. Still useful though. I use it mostly for the chroma-key options although not so much lately as I like the simplicity of the New Blue chroma-keyer.
DGates wrote on 3/26/2010, 11:13 PM
The trick is to keep the blurring subtle. It doesn't need to be a full blown blur to get the slight DOF look.