Getting That Green Screen Done Right
When properly done, green screen cinematography is an excellent way to get spectacular shots at a minimal cost. When improperly done the result can be a nightmare for the editor and a financial headache for the producer.
Green Screen video production studio with lights set ready for filming.
Let's first answer the question: Should you use a green screen or a blue screen?
The overwhelming number of digital composition today is done on green screen.
There are two main reasons for this.
The sensors off today's video cameras are more sensitive to green and that gives a cleaner image against which to key. This is because the green channel has a higher luminescence than the red or blue channels.
Chroma green is the color furthest from human flesh tone.
Having said that you may want to use blue screen when any foreground element is green. Green clothing on a person will turn that part of their body transparent.
Perhaps the single most important element in getting a good, clean key on green is proper lighting.
You want to light the green screen with a flat even light.
Stand talent and place your props at least 5’ to 10’ from the green so that the green light bouncing from the screen does not fall on anything. The trouble you will have in post to eliminate the green spill could be enormous. So, take your time and look carefully at the monitor before your shoot. Make absolutely sure there's no green edges on any foreground element. If any green is there, try to identify and flag off the offending light source or wash out the green with a stronger, more directional lighting source.
Needless to say, your talent should never wear green and not wear white where it could reflect the surrounding green. So a white shirt worn under a dark jacket is fine but white stockings or pants, white shoes or legs are a source of trouble. The same is true for reflective surfaces such as jewelry, shiny shoes or glass objects.
You may solve a good deal of the green spill issue by simply using as little green as you really need. Many producers want a huge green screen no matter how little of it they will actually use. Lots of green can cause lots of problems.
Most professional green screen studios will have black curtains to close off all but the minimal amount of green needed for the shot.
Green Screen tips - Reflective Spill
To compensate for any reflective spill, side spill suppressor lights flanking the subject will warm up and wash out spill, giving you a good, clean a clean key.
And don't neglect the floor. If you are on a cyc and the floor is green you may have green light reflected upward onto the talent. If the shot does not include the floor, simply throw down a black sound blanket.
When ever possible incorporate a dark carpet into the set to minimize the upward green reflection.
The background image is very important. While it is possible to simply shot on green and insert any image later in post… it's usually a bad idea.
The background image usually has one or more light sources. You will want to match where the lighting in the image would logically fall on the foreground. So, if there is a high sun on camera left you will want sunlight over the left shoulder of your talent. And pay attention to color temperature. A lamp in the background image would require a 3200k light on the appropriate foreground area.
A very useful technique you might use in a large green screen space with a green cyc wall is a "China Ball". That's a light that looks like a big round white Chinese lantern. The advantage is that it doesn't cast any shadows.
You need to take very special care when the green light from the background illuminates fine blonde hair or hair or transparent items such as glass or liquid. As a side note: blue screen is a bit better for compositing fine blonde hair.
Its wise to use modelling lights to illuminate your talent. This is really just the same fixtures you would use to sculpt a figure in normal video or still shoots. Just be careful to separate your lighting. The lights on the green… just light the green. The lights on the foreground elements such as people, props, practical sets… are all reserved just for them.
Stick with the fundamentals: the three basic lights are the same as you always use: key light, fill light, and back light.
The back light creates a soft halo effect on the subject's hair and shoulders. It lends more definition from the background but isn't necessarily great for compositing into a background scene where you need to match lighting. In that case, you could use fill lights to help insure smooth, even lighting.
The best way to increase the illusion that both the background image and foreground are part of the same reality is to “mix them”. By this I mean, see if you can place an element from the background image into the foreground as a practical object.
Easy items would be a tree branch in the extreme foreground to match a tree in the background or a vase with the same flowers we see in the background image. It gets harder when you have to match background furniture to place up front but is often worth it in the increased credibility of the final shot.
Use Stock Shots ?
One way to do this is to not use stock shots for the background. If your green screen is set in say a living room. Photograph the real room and then take some of the actual elements such as lamps and chairs into studio as practical foreground elements.
This will easily sell the shot as real.
What About that Background Plate?
If your background plate has a window you might remove all curtains, drapes and obstructions leaving it a lean rectangle. Then in post insert a video image of a landscape or cityscape with movement in the shot. Clouds drifting, traffic moving in the distance, twinkling lights on buildings.
All of this plays well into our “willing suspension of disbelief” and we accept the shot as real.
So go for green. If done properly it will vastly expand your visual bag of tricks.