PMW-200: Settings for green screen

[Oliver Buckie]

I need to setup my PMW-200 for a green screen shoot of a spokesperson (head to toe). The talent will have a dark suit on. I am a little concerned about too much noise on the subject and obtaining a good key. My thoughts are:

- Format: HD422, 25P
- Shutter: 1/50
- Iris: f4 to f2.8 (check using zebras)
- Gain: 0dB
- White balance: Manual using a grey card (or WarmCards 3.0?)
- Focus: Manual (check using peaking)

Should I be using TLCS? What about a particular picture profile, i.e. using HG4?

Any tips would be appreciated. Thanks, Oliver

[Gary Huff]

None of those settings matter. What matters most is that the green screen is properly illuminated, and exposed in as much of a straight line across your waveform monitor, ideally at about 30-40 IRE, and that your talent is properly exposed as well.

If you underexpose the suit, you'll get noise regardless, which is why lighting is key.

[Oliver Buckie]

Gary, thanks for your feedback.

I have done a few tests. I used WarmCards 3.0 to do my white balancing and played around with different white balancing values, iris, gain & shutter values. Please check out http://www.edv-089.de/downloads/dvin...reen-tests.zip for my results.

I feel that the pictures with -3dB gain have less 'noise'. My favorite is "Warm1, 5500k, F2.4, -3dB, shutter 50, vortex-n.jpg", although with the shutter off the results are similar. I think the green screen is OK and should provide a nice key.

What are your thoughts? Thanks, Oliver

[Alister Chapman]

30-40 IRE is a little too low for green screen in my opinion. You want your green nice and bright, the darker it is the noisier it will be so the key will be less clean. I would light and expose the green at between 50 and 60 IRE where possible. It is very difficult to pull a decent key if the background is not sufficiently exposed.

Lighting is the most important part of getting a good key, that and separation of the foreground from the background, often best achieved simply through good physical separation.

The one thing I would consider is to turn off the detail correction in the picture profiles. Don't use a hyper gamma, you have control of your lighting so light the scene within the standard gamma dynamic range and you'll get a better key. Progressive is best for keying. One last thing for a head and shoulders shot consider get a bracket so you can use the camera on it's side to shoot portrait for a higher effective resolution as you fill the frame better.

[Alister Chapman]

Just saw your grabs. They look pretty good, exposure is about right, maybe a fraction low, it will help if you can even out the shadows a bit.

[Dave Sperling]

Adding my 2 cents...
Definitely record 422!
I usually end up with green at ~50-55 IRE.
I try not to have too much out of focus, since green creeps into soft edges.
If the shot isn't full height then I may also add some green gel on the lights illuminating the background to optimize the green chroma. (It also minimizes any non-green kicks on the background if there is any reflectivity coming back off the back wall.)
If the shot is full height, think about using a polarizer on the lens to minimize floor glare from the backlights (it's amazing how much this can clean up the key around the feet.)
When there is a lot of movement I may go with a higher shutter speed to keep the edge blurs cleaner.
Speaking of the edges - I often put 1/4 or 1/8 minus-green on the backlight(s) and back-edge lights to minimize green spill on edges.
In terms of hypergammas, for me that really depends on the look I want for the foreground. If someone has shock-white hair then the gammas might be useful. For a subject who doesn't need hypergammas to keep within standard video levels they shouldn't be necessary. My feeling is that the hypergammas will have very little effect on the green since it is in the middle of the scale, so it really depends whether there's anything in your foreground that needs them... and it then becomes a matter of personal preference. If I know I've shot my background plates with a specific hypergamma, I may use the same settings for the foreground green shoot.

[Oliver Buckie]

Wow, thanks for all your valuable feedback!

What's the best way of measuring the green-screen's IRE? I have no special waveform monitors... can I do this via my PMW-200 or in After Effects CS5.5?

@ Alister: I used the "vortex-n" picture profile from Doug Jensen. You mention I should turn off detail correction... to do this should I turn turn Level & Crispening from +10 back to 0 under HD Detail? Gamma is also set to HG4 - should I change this?

@ Dave: What do you mean about having "not too much out of focus"? Never heard of green gel - is this something you can 'smear' on the lights? Never thought about using a polarizer indoors... will this not make my overall image darker, adding more noise - that means I need to add more gain / open up the iris more? Finally, not sure what you mean by "I often put 1/4 or 1/8 minus-green on the backlight(s) and back-edge lights to minimize green spill on edges".

Thanks again.

[Dave Sperling]

To check your IRE, turn on Brightness Display and the Center Marker to use the camera as a 'spot meter' and be able to see a number for the brightness level of the area inside the center marker.

Gels are colored filters that can be clipped over the lights. They can be bought in sheets or rolls. Minus Green is a magenta-colored filter that counteracts the green spill. I find that using full or half density minusgreen ends up being a bit too strong for my tastes, so I use a less dense (lighter) version - 1/4 or 1/8. There are also various shades of 'plus-green' as well as other green gels. I've done some experimenting, and chosen specific colored gels to use depending on the light sources which are illuminating the background.

In terms of 'not too out of focus' -- Most of the time a green screen will be one person at a fixed distance -- no problem with focus there. But what if you have two people interacting in the shot and one is close to camera and the other is far away? If selective focus is very soft on one of them the out of focus edges will be a soft combination of the person and the background - which makes pulling a decent key far more difficult.

Adding a pola filter means that you may need more light for exposure - or may need to open the iris. You don't want to increase the gain in the camera since noise will tend to degrade the key.

[Oliver Buckie]

Thanks Dave for your feedback.

I turned off all lights apart from those lighting the green screen. I used my camera's Brightness Display and it showed between 45% and 58% depending on exposure. Is this equivalent to IRE 45 - 58? Are these good values?

I also tried a polarizer - however, the results were far too dark, even with the iris at f/1.9. To get anything exceptable, I needed to increase the gain to 6dB - wasn't too happy to do this in order to prevent noise.

[Alister Chapman]

The spot meter measures IRE. I would say that your shadow areas at 45 are too low, I would want at least 50 IRE.

In the picture profile just turn Detail to OFF, this turns off all the detail corrections. The picture will look a little softer but it eliminates the double edge or fine halo that you will most likely get between the foreground and background. This should result in a cleaner and less busy edge to the key. Once you have composited the foreground/background you can add in a little sharpening if you feel the image needs it.

If exposing Hypergammas correctly (HG4 middle grey is designed to be at 33 IRE) you will end up with a darker than necessary image. Most people over expose the Hypergammas which pushes skin highlights into the compressed part of the curve. While very often this looks fine, if you do a comparison with standard gamma and correctly managed lighting you'll find your loosing texture in the skin highlights giving faces a smoothed low contrast look. Skin tone with Hypergammas should be around 55-65 IRE, not the more normal standard gamma levels of 65-75 IRE.

I sometimes employ a slightly different lighting strategy to Dave (but Dave's should work very well) in that I will light the foreground using daylight balanced lights and the background with tungsten balanced lights. I find the extra colour contrast between foreground and background helps get a cleaner key. A back light on your subject will help reduce spill from the green, but you need to consider how this will look in you final composite image.

Don't add gain. Adjust your lighting. Remember when you do the key you can mask out any background areas that are outside of the box that contains your subject. So don't worry about lighting the extreme edges of your foreground frame, concentrate on getting the center area behind your subject as even as you can. Your green doesn't need to extend beyond the area behind the subject, less green means less spill to deal with.

[James Kuhn]

Alister...thank you for letting us know the 'Brightness Display(spot meter)' is measured and calibrated to IRE on the PMW-200. I didn't know what 'scale' was being measured when I enabled the 'Brightness Display' and for that reason I didn't feel comfortable using it. I'll test it with my 'Vortex Media' Gray Card. I'm not sure I'd use the 'spot meter' function for critical exposure, but if you work with it, you get a 'feel' for what the proper exposure should be.

Thanks again,

J.

[Alister Chapman]

The spot meter is very accurate. Great for setting exposure with a grey card.

[James Kuhn]

Thanks, Alister! Now, that I have 'independent confirmation' regarding the accuracy of the 'Brightness Display', I'll perform some additional testing and see how my camera responds.

Best regards,

J.

BTW, did you receive my email regarding the '2014 Storm Chasing season?

[Alister Chapman]

Yes I did get it James, sorry been travelling. I'll get back to you by email.

[James Kuhn]

Thanks, Alister!

Regards,

J.