Since digital cameras became mainstream with pros, there's been an increasing interest in Chromakey photography. Chromakey is the process of using a greenscreen or bluescreen as a background and then removing that background (and replacing it with something else) in software. Usually this is either done in stand-alone applications, like TriPrism or ExpressDigital, or as plug-ins for Photoshop, like Primatte Chromakey and Cinematte.
However, I'm not going to discuss the software. The well-kept "secret" about Chromakey is that the whole process has more to do with the photography side of it than the software. Understanding what the software is looking for means the images you shoot fit the bill. If everything is shot right, then it's pretty easy once you get your images into the software. If not, then you potentially have some very green, and probably very angry, clients. A lot of the info out there that talks about avoiding green, angry clients is oriented toward video. So our goal here is to provide you with tips that apply specifically to photography.
There are a few components that go into a good Chromakey photo shoot: lighting, subject positioning, and the background. There are many different ways to combine these successfully, as they all interact with each other. What "works" will depend on your equipment, what the subject is, and the environment you're shooting in.
Like many things in photography, shooting Chromakey is as much art as science. There's a learning curve, and you definitely want to try your set-up before throwing clients in front of it.
"Spill" is the term that refers to: color that reflects off the background or walls of the room to cast a noticeable tint on areas of the subject or color that's showing through semi-transparent areas of the subject, like hair or a wedding veil. The goal of your set-up is to reduce spill as much as possible so that you don't have to deal with it in post. If it's not dealt with when shooting, dealing with spill can easily take longer than the photo shoot itself. It's the main reason that people give up on Chromakey photography. However, if the shot is set up correctly, it's mostly avoidable-you just need to be aware of it and make adjustments as necessary before you shoot.
Usually you see spill on the back of the shoulders, side of the arms or legs, and through the hair.
You want to get the subject as far away from the background as possible. This reduces the possibility that light reflecting off of the background will spill onto the subject. Eight to 10 feet is ideal, and four feet is the minimum. The more light you're using, the further away from the background you want to have the subject. It is possible to have a successful shoot with the subject closer than four feet, but other problems start being introduced, like shadows on the background, and you'll be setting yourself up for a lot of Photoshop work.
If the walls or ceiling are close to the subject, it's helpful to paint them black or hang black fabric on them. Light bounces all over the place, and that light can turn your background into a big green or blue light. The black material helps soak up that light and avoids spill on the front of the subject, which can be especially problematic.
There is no set rule for lighting. It really depends on how much room you have and what you're shooting. For things like Little League shots, two umbrellas in front of the subject work just fine but produce very flat lighting; and you need to be concerned about shadows falling on the background. For senior portraits, you might want to use a three-light set-up to create more moody or artistic lighting.
You have more flexibility if the subject is 10-feet away from the screen. Light from the background falls off after five to six feet, so you naturally end up with less spill. This also makes the foreground a bit darker than the subject, which is helpful. You want the background to be about a stop below the foreground. You can light the background with separate lights if the background is too dark. You don't want the background blending into the shadow areas on the subject. However, you need to be careful about lighting it too much and increasing the amount of spill.
A hairlight or backlight can be helpful as well. This can make it easier to separate the hair and reduces spill on the shoulders. If it's too bright, it can add a glow to the hair, which looks fake when you remove the background.
The background needs to be a flat, consistent color, but you want to achieve this with the least amount of light possible. You want to avoid turning it into a big blue/green light, but any significant variation in the color of the background will cause you problems. Sometimes you'll get a flat color naturally with the same lights you're using for the subject, but you can also light it separately. Again, especially with green, you want to keep the background at about a stop below the foreground.
The color also matters. You want a blue or green that is a pure medium-tone blue or green and very saturated. Most software, like our Primatte plug-in, works with saturation, so having a screen that's too light or dark will make things difficult.
In addition to proper lighting, the material the background is made of can have a huge effect. You don't want to use something like paper or polyester, which can be very shiny. This causes a lot of the light to bounce off of the background, creating spill. Felt-like materials work best, and in particular we recommend a fabric made by Velcro. It's inexpensive, the right color, wrinkle resistant (wrinkles make lighting tougher), and works beautifully. It's a very soft, foam-backed fabric that diffuses the light and prevents it from bouncing off. It's available from many places including www.chroma-key.com and www.filmtools.com.
Jim Tierney is president of Digital Anarchy. He's been developing software tools for film/video pros, photographers and graphic designers since 1995. His experience working with film and video chromakey as well as photography, gives him a great perspective on the challenges and differences of using chromakey for each discipline.