Put simply, green screen is achieved by filming someone against a perfectly lit green screen, then subtracting the green colour and effectively making it transparent, and then merging another video behind to fill the place. Unfortunately with the introduction of cheaper cameras and cheaper green screen material bought in photographic shops there is a notion by some people the green screen is extremely simple.
There is nothing wrong with using a green pop-up screen as long as the person is completely surrounded by green and their arms don't go off the edge of the screen when they wave them around. A shot can be used as long as the legs of the presenter don't disappear offscreen. If the green doesn't cover the area, there will potentially be a problem at the post production stage.
This is unfortunately a perception by a lot of people who think they can shoot a whole movie on iPhone and edit it to be broadcast on a mainstream cinema. Whilst technically this is possible it would be very foolish way to go. I have even been asked on numerous occasions how to do green screen and when I've asked the person how they were doing it I was frankly amazed they got anything at all.
Green screen works by lighting a background in one single colour (normally green, it can also be blue) then afterwards using software to layer, or swap out images by eliminating that green colour. This creates an illusion of the subject appearing in the scene, and when it is done well it can look just like the subject in the foreground is actually at the location at the same time. Done poorly and it will be really obvious the subject is in a different location at a different time, so there are many considerations to make green screen effective and work properly.
Chroma keys are generally blue or green because these colours are the furthest colours to the opposite spectrum away from human skin tone. Green has now become a more popular choice as sensors in the latest digital cameras work a lot better with the pure saturated green colour, and the green channel in the camera is the cleanest channel with less electronic noise to potentially keep the colour as clean as possible.
Chromakey has a few different names in broadcasting and the film industry has different names for a similar effect. If you're an engineer in the BBC it is called colour separation overlay, (although I've not heard anybody call it that for quite some time now) and in the film industry in the past it was called a travelling matte, and is a very similar yet much more labour-intensive effect to achieve, with several layers of high contrast into negatives being printed from the original camera footage. Nowadays it will be done electronically as most feature films are now post reduced digitally and not on film.
Chromakey is often an effect achieved live for TV news, especially weather reporters. Often it is recorded separately and composited later, however and this is where any mistakes will not be seen until the next day or beyond, so you have to be extremely careful to get the effect right on the day.
There are several basic factors to remember for successful green screen work:
When setting up a green screen one needs to remove all possible wrinkles from it. Tightly pulling the ends of screen and positioning it with the help of a stand and tape or clamps helps in this process. Also, when storing, it is advisable to roll the screen.
There is actually a specific colour green paint designed for green screen work. this is used when the background is not necessarily a flat surface and other areas need to be coloured green. When I last looked this type of paint was hugely expensive for large areas, so most people tend to go for the green Infinity curve in a studio, but for smaller areas it is incredibly effective on a flat wall.
Either way green screen only works well when the shot is carefully constructed and when many different elements come together in harmony. It can be difficult to eliminate the colour fringing later if the green screen is not shot properly, and this can lead to your subject with a fringe of green completely around them, or potentially losing the edge detail of your subject – especially noticeable in their hair or other fine detail.
Most difficulties often arise when the green screen is too close behind the subject and the colour bounces back from the screen and affects the subject in front of it. It can happen when the main key light is used to illuminate the subject at the same time as the green screen, so avoid this when you are working with green screen.
Achieving a seamless and convincing perfect key in post production is still actually very difficult, so it is important to understand the elements needed and why they are all important.
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