Green Screen with 4K RAW, suggestions

G

Gene Sung

Veteran
I'm not the most experienced at Green Screen so I want to see what you guys suggest. Going to be using 7Q 4K RAW and filming 1 to 2 people.

1) What F-Stop is a good one? For green screen I usually try to keep it between f5.6 to f8 for a deeper DOF. Is that OK?

2) Focal length. For close ups on talent, I usually prefer long focal length (85mm at least). Should I avoid these longer focal lengths because of the shallow DOF they cause? I could stop down the aperture, but once you start hitting f11, the IQ starts to suffer IMO.

3) Is the goal to always have the talent as sharp as possible (with deep DOF)? Should any type of shallow DOF be avoided since the key might be tough?

4) What if I'm doing an OTS of two people talking and one person is in the foreground and the other person back a little from the camera. One person would most likely be out of focus on Super 35mm format even stopped down a lot. Should this be avoided? Should I shoot the two people separate and add the depth blur in post?

Thanks for any help.
 
A lot of this will depend on your particular green screen setup and how much action your shooting, particularly if your subjects are moving from and away from the camera.

The only DOF that matters is keeping your entire subject in focus. The focus on the green screen theoretically shouldn't matter, because if lit correctly, it should be only be a single even color value. Focus and out of focus of the green screen will look the same to the camera. However, if your green screen material/lighting are less than ideal (or your subject is static) then you'd want just enough DOF to keep your talent in focus, but allow the green screen to be out of focus so that it blends the lighting/wrinkles/etc. As long as your subject is in focus, you'll have clean edges to key. Motion blur can also be a problem, so balancing your aperture with high shutter and available light can also come into play.

It's not impossible to key out of focus subjects, though more difficult and will definitely lengthen your time in post. On the flip side, blurring in post doesn't always capture some of the same richness in bokeh, proper DOF fall-off or lovely lens characteristics that you shot on. If you want to do it properly in post, you'd have to estimate and animated depth maps and apply those to a lens blur. I've done both before and I'd prefer to key out of focus subjects, as building depth animated depth mattes takes quite some time for me. Then again, if your client doesn't know that you've slapped on a Fast Blur in AE, then the point is moot :). But definitely try it out beforehand to see what you can stomach.

Your background plates might also make a difference for out of focus areas. Sometimes it blends really well or the lighting conditions can mask the blur. Other times it stands out like a sore thumb. So test as much as possible and if you can't test beforehand for location/availability reasons, I highly recommend bringing a laptop with you on set for a quick key test before you commit.

Personally, I prefer shooting as much of the performance in camera, to allow the actors a more natural performance and also to let the lens characteristics come through.

A major upside is that the 7Q 4K RAW is amazing for green screen work and also incredibly forgiving.
 
Hey Gene,

Depends who's doing the keying and what they prefer. As james mentioned above focus for your talent, keep it looking natural. Hair, edges and focus totally depends who's keying really. If it's a simple 'slap a plug in on in After Effects' then stick to hard edges if you can. If it's going to a place that does this for a living then you can afford to be more natural.

A big question is what is the background going to be, because that would motivate the entire foreground lighting scheme.

Look for decent exposure on your green screen, if you're hand held then you may need tracking markers but watch for markers crossing talent, the less of that the easier the work. Remember that the only thing a compositor really cares about are the edges. Other stuff can be roto'd and garbaged out pretty easily. Don't wrap people in green or wrap booms in green, to me that's a face palm moment.

The only issue that i've encountered that is quite tough to solve is the situation where light from the green screen diffracts around your subject giving very different RGB edges, it makes edge work really tough. In my experience this is with a darker foreground than the green screen when the edges are in focus. I mention it just as a heads up because it's difficult to know when it's happened (it seemed to be with a particular lens in my case)

You know you can mail me if you like!

cheers
Paul
 
Thanks for the response guys. Much appreciated.

Hey Paul, when you say: "The only issue that i've encountered that is quite tough to solve is the situation where light from the green screen diffracts around your subject giving very different RGB edges, it makes edge work really tough."

Are you talking about spill from the green screen? Kind of like when the person is too close to the back green wall and the color kind of wraps itself around the person? We are actually shooting on a legit, large green screen cyclorama with a curved elbow and all, so I'm hoping if we keep the subject at least about 6 or 7 feet from the back wall we can avoid this (If this is what you are talking about).

Thanks again,

Gene


 
gene_can_sing said:
Thanks for the response guys. Much appreciated.

Hey Paul, when you say: "The only issue that i've encountered that is quite tough to solve is the situation where light from the green screen diffracts around your subject giving very different RGB edges, it makes edge work really tough."

Are you talking about spill from the green screen? Kind of like when the person is too close to the back green wall and the color kind of wraps itself around the person? We are actually shooting on a legit, large green screen cyclorama with a curved elbow and all, so I'm hoping if we keep the subject at least about 6 or 7 feet from the back wall we can avoid this (If this is what you are talking about).

Thanks again,

Gene
Click to expand...

No, spill is fine. I've had a situation with a hard edges where the red green and blue are not aligned, one of the channels shrinks relative to the others so you're dealing with spill, and a colour edge. Keyers have a lot of issues with this and you have to jump through hoops to get the channels matching. I've seen it on red footage so it's not related to the 700. I think it might even be lens dependent. IMHO it's the light diffracting differently for each channel. You know if you wave your fingers really close to the lens and they're out of focus. The background will distort around them, the light diffracting around the edges? Something similar to that but more subtle.

Distance between the screen and the subject will alway help, not least you can light the screen separately.

Really depends on what you're comping into.

But back to your original question, focus, spill and DOF are day to day tasks. What colour hair are you shooting? Some people believe blond is easier to key off blue than green for example?

cheers
Paul
 
gene_can_sing said:
Thanks for the response guys. Much appreciated.

Hey Paul, when you say: "The only issue that i've encountered that is quite tough to solve is the situation where light from the green screen diffracts around your subject giving very different RGB edges, it makes edge work really tough."

Are you talking about spill from the green screen? Kind of like when the person is too close to the back green wall and the color kind of wraps itself around the person? We are actually shooting on a legit, large green screen cyclorama with a curved elbow and all, so I'm hoping if we keep the subject at least about 6 or 7 feet from the back wall we can avoid this (If this is what you are talking about).

Thanks again,

Gene
Click to expand...

I don't think spill is too big an issue as Paul said - but you should try and avoid it if you can (you get an easier, cleaner key if you do). To be honest, having a big 'proper' green screen can be more of a hassle to deal with than a smaller simple one: ideally you want just enough green to surround the subject and no more. More means more spill because the bigger surface reflects more green onto the subject. In practice this means having black poly board and black bolton cloth to hand to add some negative fill around the subject, out of shot. And - as you say, keep the subject as far from the screen as you can place them.

Bigger studios also need more lighting and can be hard to light evenly. The usual approach is to use space lights or a mix of them and cyc lights. The issue with space lights being that they produce a hell of a lot of fill which you might not want.

Also, as a general rule, you want the chroma key to be a stop under key - again, this might mean a lot of light when dealing with a big studio (or having to open up the stop a lot on the lens to compensate)

The up shot of this is that you might need bigger, more powerful lights to light your subject to get the balance right.

Paul's problem with diffraction was an interesting one. It sounds like maybe they were using older cine or stills lenses (i.e. ones designed to work on film rather than digital) as these may be designed to focus red green and blue in slightly different planes. They may also have stopped down a bit (more diffraction effects) as they under expose their subject to "make it look right" but not really thinking it through properly: it's a chroma key. You can grade it down after you key.

But yeah. Green screen shouldn't be too tricky. Try to light the screen evenly (a stop under key, as I said) - you can use the false colour/zebra or waveform on the O7Q to make sure it is even. Also, depending upon the background plate, it is good practice to add a backlight to the subject - feel free to throw a little 1/8 minus green (magenta) onto the back light if you want to have a cleaner key. You can also add plus green to the lights lighting the chroma key as well if you want to make it greener.
 
I would think that a major problem would be strands of hair that have green screen behind them. These are tough enough when sharp, and become impossible when blurred. Unless they are blurred enough that they effectively disappear.

Jim
 
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