A collection of information on the shooting of greenscreen for film and television.
[Step 1. The Color] [Step 2. The Light] [Step 3. The Stock] [Step 4. The Lens] [Step 5. The Transfer]
550 NanometersWith the power of today's digital keyers, just about any color will work for "greenscreening." However, paying close attention to the color choice can make the difference between just an "ok" matteing job and a great composite.
Film emulsion is organized into three layers that each have a different sensitivity to the three primary colors blue, green and red. Each of these layers has a "peak" where it is the most sensitive to a specific wavelength of light and least sensitive to all other wavelengths. Utilizing this peak will allow you to record an easily isolated, pure color into one layer without much contamination into the others. (See chart.)
(This is a detail of the actual sensitivity chart for Kodak Vision 200T film from the Kodak website)
Normally pigmented paints and fabrics, etc. don't have enough saturation to achieve this level of exposure density. However there are several manufacturers (The Composite Components Company, Rosco, Kino Flo) making products specifically for blue, red and greenscreen* work. These products are designed to return a very saturated color that corresponds to these peak wavelengths. This color can then be easily filtered and isolated in post resulting in a very accurate matte.
*550nm green is most often used because its strong peak lies well within the digital colorspace and its naturally brighter "dayglow" pigment is an easier color to saturate to keylight levels. 650nm red is usually reserved for a specialized matteing technique for miniatures and motion control, and 440nm blue, although great for traditional film composites, tends to be right on the edge of the electronic range and not always a great candidate for digital keying systems. (Although there are specific conditions where blue is preferred over green- such as when a talent's clothing contains strong shades of green.)
Flat, Green, and At Key
As with the color, if extra care is taken with the lighting of the greenscreen, the final composite can be improved remarkably.
As mentioned in Step 1. The Color, the more specific the green color, the better... and naturally this applies to the lights as well. Using traditional incandescent lighting can add too many other color wavelengths (most notably "yellow") to the screen making keying more difficult. A more accurate method would be to simply light the screen using colored gels to filter your light to as close to the color of your screen as possible. Another would be to use a light source that only emits the desired wavelength. Kino Flo makes a lamp called the Kino Flo Super Green that does just that.
The screen itself should be lit in the way that it is desired to appear in the composite. In otherwords, with the newer, more powerful keyers of today, all of the shadows, falloff, hotspots and color variations will translate to the composite. (See images below.)
(Notice that the subtle falloff in light on the left side of the greenscreen can become a nearly opaque area on the matte. This will appear as a darkening of the background image in the composite.)Consequently, in most cases it is desirable to light the green screen as flatly as possible, i.e. the screen should not vary in brightness or color across its entire active surface, including the floor. The screen should be lit at or just under the subject, and no more than a full stop under, when measured with a spot meter. This ensures a correctly exposed screen.
Conversely, the subject lighting does not need to be flat and can be done in any manner befitting the final composite, i.e. night lighting, side lighting, etc.
It is important to keep the subject physically separated from the screen, when possible, to minimize green spill on the subject, unnatural shadows falling on the screen, and the like. It may also prove easier to separate the two lighting schemes if there is physical distance between them.
Also of note, there are many outdated tricks to lighting for Chroma Keying such as rim lighting, backlighting or adding minus green to the foreground lights, etc. None of this is applicable for today's digital keyers, and should only be done if it produces the lighting desired for the shot. Most current effects systems have no problem separating the subject from the background and can easily remove blue or green spill falling on the subject.
SFX 200T or Vision 200T.
Here is a listing of the available Kodak 35mm film stocks* and their usefulness in greenscreening:
|
Stock
Number
|
-no
way-
|
- ok -
|
-better-
|
-best-
|
| KODAK VISION 5284 500T |
x
|
-
|
-
|
-
|
| KODAK VISION 5289 800T |
x
|
-
|
-
|
-
|
|
-
|
 |
-
|
-
|
|
| KODAK VISION 5274 200T |
-
|
-
|
|
-
|
| KODAK VISION 5277 320T |
x
|
-
|
-
|
-
|
| KODAK VISION 5279 500T |
x
|
-
|
-
|
-
|
| KODAK SFX 200T |
-
|
-
|
-
|
|
| EASTMAN 5245 EXR 50D |
-
|
|
-
|
-
|
| EASTMAN 5248 EXR 100T |
-
|
|
-
|
-
|
| EASTMAN 5293 EXR 200T |
-
|
|
-
|
-
|
| EASTMAN 5298 EXR 500T |
x
|
-
|
-
|
-
|
- As a rule, faster stocks will have too much grain in the the green and blue layer to make good keying stocks.
- Daylight stocks, although generally finer grained, need to be filtered for indoor use and therefore are not always a prime choice for studio effects work. (However, they do work very well in exterior daylight.)
- EXR stocks, although very good for keying, can sometimes introduce an undesirable "edge" enhancement along highlight edges when using greenscreen. (However, this problem seems to be nonexistent when using blue.)
- And, although the SFX 200T stock was designed specifically for blue and greenscreen effects work, at NTSC resolution the less expensive Vision 200T can often work just as well.
*16mm stocks are often poor effects stocks for several reasons (grain, image float, etc.) and therefore aren't represented here.
Prime, No Filters
Since the maximum resolving power of most lenses resides at about 2 stops down from wide open it makes sense to expose at that setting. Along similar lines, prime lenses are often optically superior to zooms and are preferred for greenscreen work. However, at NTSC resolution, there is some debate as to the validity of either of these concerns.
Certainly the most important item to remember here is to not use any sort of filtering on the lens. Filters often affect the color or the edges of an image and either would play havoc with a greenscreen shoot. (One exception to this rule could be the use of a polarizer for removing glare from the floor of a green cyc.)
No Processing, No Secondaries
Even the most marvelously photographed blue or green-screen plate can be rendered unusable by the wrong approach in telecine. It is important to leave off any sort of processing in the transfer stage. If needed, items like noise reduction, filtering, extreme color corrections, etc. can often be handled in alternate passes or in post.
One of the most damaging techniques, however, is the "boosting" of the green during color correction. It is the telecine operator's good intention to help provide the best possible key color for the effects artist, yet the manner in which this is usually achieved can destroy the delicate edge between the subject and the screen. The process is known as a "secondary" and utilizes the telecine's rather limited keyer to separate the green from the rest of the image. The end result is a degradation in the fine detail around the edges of the subject. This can result in a number of inaccuracies and other problems in the composite that ultimately have to be repaired by the effects artist, often to the detriment of the composite.
Contact me for corrections, additions: mark@fischeredit.com
(More urgent, on-set questions or short messages can be sent to my text pager here.)
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