I remember getting my photo taken at a studio as a child. I would sit in the chair wondering, when will it all be over. The bright lights and the strange white or silvery umbrellas were what I remembered most. I also noticed the painstakingly painted backgrounds that were placed behind me. This was the way photographers could shoot in a studio, but have their subjects placed in all sorts of silly scenes. Sometimes the effect was complete, but mostly, it seemed artificial.
Photographers could get a more realistic effect by projecting slides of various scenic photographs on a screen behind the subject. Because of the labor involved in painting backgrounds, this was a cheaper alternative. Naturally, since the photographic slide was more realistic, the effect was more complete. Green and blue screen backgrounds, combined with advanced darkroom techniques, were also employed in the past. Fortunately, today we have digital cameras and image editing software.
Chroma key is a great technique to use for both amateur and professional photographers. You can expect quality results if you put effort into setting up the green or blue background with the fewest imperfections, adjust the lighting on the subject to properly match the intended background image, and take the time to use the image editing software correctly. Sloppy setup of this system will result in an image that looks like your used scissors to cut out the subject and glue it on a background, which would be much less time consuming.
To make digital chroma key images, you will need at a minimum; a camera to record the image, a solid colored background, and image editing software to composite the subject with the background image. While this is the absolute minimum, better tools will create better results. A digital camera with a zoom lens is recommended. A film camera can be used but there are the added steps of processing film and scanning. The solid colored background is the most important ingredient to creating perfect results. Green and blue are the best colors to use for this technique, especially for using people as subjects. Image editing software exists in all forms from free web-based applications to full-powered production suites that only business can afford.
Chroma key is often referred to as "green screen" because solid green is the best background to use for chroma key. Green is best because it's the color that is furthermost from the human skin color. Blue is equally as distant but is also a very popular color and often times, the subject is wearing something blue, like blue jeans for example. If you are not shooting a human subject, you may use different colored backgrounds. Just be sure that the color you use is opposite if your subject color for easier removal with your image editing software later. Green is the most popular color used, and will be referenced for now on.
The background material used, should be as least reflective as possible, and have a consistent color across the whole surface. Most photographers use a felt type cloth. Cloth is portable and convenient, can store away easily, has a very low reflectivity, but it can get wrinkles. Wrinkles cause shadows. Shadows in your background make the digital removal process much more difficult. Paper is very smooth and stores away easily but is somewhat more reflective. Paper has a slight sheen to it and therefore, is more difficult to adjust lighting to remove the sheen. You can paint a board or wall a nice flat matte green color. A painted wall is very permanent but can not get wrinkles. Care in lighting should be taken to avoid any reflective sheen. The background must be lit well. If the background is not lit, then it's a dark-green background and will not be easy to remove later. Lighting should be consistent across the whole surface. Use diffused lighting to get the most even coverage.
When lighting the subject, keep in mind that you will be inserting another image background using digital manipulation later on. Know what the lighting is like in your intended image background, and light the subject accordingly. In general, you want to have your subject slightly brighter than the background. Keep the subject a good distance in front of the green screen to avoid shadows cast by the subject onto the screen. The green screen can cast a greenish hue on the back of a subject if it's too close to the screen. Using a portrait type lens can help keep the subject in focus while having the background blurry, this technique will assist in the removal process later. A perfect way to get quality and consistent lighting is to move outdoors. Shoot your subject in front of a green screen in direct sunlight. Then you will have many potential outdoor scenic images to use as a background. Shoot your subject outdoors on a cloudy day, and you will have a diffusely lit subject that can be placed in many indoor scenic images.
There are many different image editing programs available to remove a colored background from your subject. Below are general tips for image editing, not application specific methods. See the link at the end of this article for a tutorial using Adobe Photoshop. Use a color selective tool in the software that allows real time adjustment to the hue selection. Never permanently destroy the green background, use masks instead of deleting. If you delete, you will most likely notice that you have removed something that was apart of the subject. You will have to undo and repeat the process. Masks can add and subtract image information very easily allowing you to fine tune the removal process. Once the background has been completely selected, add a one or two pixel feather or fuzziness to the selected area before masking or deleting. The feather will keep your subject from having a jagged edge that may be noticeable when placing onto your image background. After your subject is completely separated from the green background, you may notice a slight green hue around the perimeter of your subject. This is due to some color bleed through hair or light reflected from the green screen and highlighting your subject. Use a color correction tool to desaturated the residual green hue left over from this effect.
Using chroma key techniques can open the door to creativity. Artists can use these new tools to create realistic images once only seen in his or her own mind's eye. Professional photo studios can offer their clients a greater selection of products. There is no need to keep several backdrops on hand. All a photographer needs is a nice single colored background, and with the digital chroma key process, has access to literally millions of background choices. You can photograph your own backgrounds, use 3d rendering programs, and even purchase ready made digital backgrounds from specialist artists and photographers.