Chroma Key tips

Chroma king or green screen as it is also known is a technique where a person or an object is composted onto a background and with the correct lighting techniques and post production the shot is believable by the viewer. The most common use of Chroma key is on the evening news when the weather segment comes on. When you saw Superman flying through the sky this was also a Blue/ green screen shot. In both cases the Christopher Reeve and the weather presenter were in front of a coloured screen Blue or Green.

WHY Blue or green? There are many colour’s in the spectrum that can be used to do a key. Though blue and green are the furthers away from Human flesh tone that is why . Blue is favoured more for Video and green is favoured more for Film work. Personally I prefer green as my key background. If somebody has blue eyes you could be in for a bit of trouble. Sure you can fix it in post though you don’t want to be responsible for a budget overrun. When the blue eyes will have to be matted back in that is going to cost more from the post budget.

Lighting the background is very important you need an even spread of light on the screen. Your light should be flooded and soft. Once you have placed your lights. This is where a big room is useful if you are not in a studio. Use a light metre to check the exposures and that they are not overlapping. It is also possible to see hotspots on the key in with the zebra pattern, or light metre.

It is now time to begin lighting the foreground. This will entail lighting the subject relative to the background. If you have a copy of your background or plate, it is easier create a believable foreground. If you have shot the back ground plate then you will have made note of all the lighting parameters, Sun position ect….

That is the essence here of a successful key. Allowing the viewer to believe that they are watching a real shot not a composite. There are times when you get a verbal creative of what your key will look like and then it is guess work. Good luck!