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Chroma Keying Primer: Part One

Once you’ve got your hands on some appropriate material, you will need to hang it in the background of where you intend to shoot. This is an art in itself. In particular, creases and folds will still be visible after the keying process. So make sure your background is not crumpled. If it is, consider ironing it before use! The way you hang the background also needs to leave it smooth. Special stands are available for mounting backgrounds, but they’re not cheap. For the more budget-conscious amateur, careful wall pinning or attachment to an existing curtain rail will suffice. But if you do use an existing curtain, beware that light through the window could cause the background to be unevenly illuminated.
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Probably the most important factor of all in effective chroma keying, however, is lighting - we described the standard lighting methods for shooting video in a previous article. But keying adds another level of complication. First, the background needs to be more brightly lit than the subject matter (the rule of thumb is two F-stops, but you probably won’t have a light meter handy to be this precise!).

Even more importantly, the background needs to be as evenly lit as possible. This is why the background material needs to be smooth and lacking in folds or wrinkles. Shadows will reduce the effectiveness of the key. These can be caused by an uneven background, but also by how you have lit your subjects. Shadows from your actors should be prevented from falling on the background as much as possible, so place your main front lights as high up as you can.
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The best way to achieve an evenly lit background is to light it separately to your subject, from as many directions as possible. In a dedicated TV studio, multiple lights are used to illuminate the background from different directions. But you probably won’t have this luxury. If you followed our suggestions in our feature on lighting and invested in some halogen work lights, you can use a couple of these on the background, and make do with a single key light near the camcorder, pointing at the subject. If you don’t have any video lights, even a few home lamps would be better than nothing.

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