24 Phat Tips for Chromakey Lighting
Robbert-Jan van der Does has done an extensive amount of chromakeying. As we’ve all noticed every time we attempt chromakey magic, Robbert-Jan knows that lighting is the most important factor in determining whether your key will look wonderful or terrible. In his 20 years of trial-and-error, Robbert-Jan has discovered numerous techniques that can assure you of chromakey success. Here are two dozen of his tips for lighting your next chromakey project.
Place your subject at least 9 feet (3 meters) in front of your blue or green screen, 12 – 20 feet (4 or 5 meters) is better but needs a bigger key area.
Place your camera as far away as possible. This will result in a narrow lens angle. Advantages: green or blue screen is out of focus, irregularities will be less visible; a smaller portion of the green or blue screen is visible, which is far easier to light evenly; your foreground subject doesn't cast shadows on your background.
Don’t use too much light. Light for a foreground f-stop of, let’s say, 2.8 (following the steps described below you will understand that in some cases, depending on your type of screen, a 2.8 f-stop might need the same amount of light necessary for f4-f5.6 on your background, because you have to overexpose). This will reduce the depth of field, which also helps to eliminate irregularities (of course it is best to avoid wrinkles or blots on your background, but if there are any, they will be more out of focus compared to f8 or f11, your set-up will also produce less heat when you use less powerful lights).
If you do blue screen, you might want to use an orange/yellow toned backlight. Not too colored, half CTO or Straw will do. This will reduce possible blue fringing in hair or clothes, because the color of this backlight is complementary to the blue. The result after you have keyed will look like a normal white backlight.
If you use green screen you can use a pale pink or magenta correction filter (1/4 or 1/2 minus green).
You can purchase these gels at professional light rental houses, where film and TV crews rent their lighting gear. Or maybe you know someone working in the film or TV industry whom you can ask to collect some used gels for you.
If you own an exposure meter, or if your camera has a zebra function, you can use this to tweak your green or bluescreen even further.
Here’s a step-by-step guide for setting up the ideal chromakey:
1- Set up a blue or green background of the type you are going to use in the shoot.
2- Light it evenly
3- Put your camera on manual iris
4- Place a white card directly on your background (you can also write the f-stop that you use on this card for reference). Make a series of recordings, about 10 sec each, at different f-stops, starting at the underexposed side. So f11, f8, f5.6, f4, f2.8, f2.
5- Capture the footage and put it in the timeline
6- Apply a color correction filter to the clip
7- In the effects palette click setup
8- Turn on the scopes and choose vectorscope and Y (luminance)
9- Locate that part of the clip where the white card just reaches 100% on the waveform monitor. It might be just below 100% at a certain f-stop and clipped to 100% in the next one. In that case it will be somewhere in between.
10- Note the f-stop indicated at this point in the timeline.
11- Now advance in your clip while looking at the vectorscope (waveform monitor is irrelevant at this point, because the white card gets clipped to 100%). You will see the blue or green vector moving towards the outer circle of the vectorscope until it reaches its maximum saturation. You can recognize that point because when you open the iris further the blue or green vector starts moving back towards the middle of the circle.
12- Note the f-stop at maximum saturation.
13- Note the difference in f-stop between step 10 and step 12. On a particular type of blue screen I use very often, this difference is 1 3/4 f-stop
14- When you are ready to shoot your chromakey set-up, just light your background evenly, use an exposure meter (incandescent metering) calibrated to your video camera, measure the light on the background, note the f-stop, add the amount of overexposure found in step 13, set your camera’s iris manually to this value and DO NOT TOUCH THE IRIS AFTER IT IS SET.
15- Light your foreground for this f-stop. If your camera has a zebra function which is set to indicate a level of 100%, you can use this instead of an exposure meter. Then go from step 13 to…
14a- When you are ready to shoot your chromakey set-up, just light your background evenly, stick a white card onto your background, set the iris at a level that just shows the zebra pattern on the white card (100%), note the f-stop, add the amount of overexposure found in step 13, set your camer’s iris manually to this value and DO NOT TOUCH THE IRIS AFTER IT IS SET.
15a- Light your foreground for this f-stop (use the same white card in your foreground to check the level, you might want to have 100% white (= zebra pattern) at your highlights.
Take care not to adjust your foreground level with your iris. Only adjust the amount of light on your foreground by adding or removing lights, or closing your lights in or moving them further away.
Following these steps will give you very nice, clean keys.
Robbert-Jan van der Does is a professional freelance cameraman/editor/director based in the Netherlands who's been working in the TV industry for almost 20 years.