Consumer-type cameras typically have automatic white balance circuitry that continuously monitors the video and attempts to keep colors true.
In white balancing, a sensor on or within the camera averages the light in the scene and automatically adjusts the camera's internal color circuitry to zero out any generalized color bias. The assumption is that when all colors and light sources in the scene are averaged, the result will be a neutral (light) gray or white (i.e., all colors will "zero out".)
As with all automatic circuitry, however, automatic white balance is based on certain assumptions, that may or may not be valid.
A problem arises if there are strong, dominant colors in the scene or (with some cameras) if different light sources illuminate the camera and subject matter.
Automatic white balance circuitry will work reasonably well under the proper conditions, and for the typical videographer with simple equipment, this is certainly better than nothing.
But in the professional realm where consistent color balance is expected automatic circuitry cannot be relied on to always produce accurate color. In this case, no substitute exists for a knowledgeable camera operator equipped with a white card or piece of white paper. (This has to be the cheapest technical aid in the entire video field!)
White Balancing On a White Card
Since we know from our earlier discussions that red, green, and blue must be present in certain proportions to create white, it's relatively easy to white balance a professional camera to produce accurate color.
With the camera zoomed in full frame on a pure white card, the operator pushes a white balance button and the camera's chroma channels will automatically adjust to produce pure white. The camera in effect says, "Okay, if you say that's white, I'll balance my electronics so that it will be white."
Focus is not critical, but you must place the card full frame within the dominant light source of the scene.
This illustration shows color balance that is (from the left) too reddish, normal, and too blue (if your computer monitor correctly shows these differences).
When the dominant light source in a scene changes in any way, you must again white balance your camera.
Going from sunlight to shadow will necessitate white balancing the camera again, as will moving from outside to inside. When shooting outside, even the passing of a few hours will result in a slight color shift in illumination.
If you do not white or color balance your camera, you risk scene-to-scene color changes. This is especially noticeable with skin tones in multiple-camera productions.
You can also "lie to the camera" during the white balancing process to create interesting effects.
White balancing the camera on a blue card can create a warm red color bias in a scene; white balancing on a yellow card will create a blue effect (below).
In an effort to compensate for colors presented as "white" the camera's circuitry will push the camera's color balance toward the complement (opposite) of the color on the card.
Note the different effect in the two photos above
Although an editor can electronically try to alter white balance in postproduction, starting out with proper white balance at the camera is always best. Otherwise, it may not be possible to perfectly match sequential scenes during editing.
Sometimes directors skew colors to create certain effects. For example, in the award-winning film Traffic, director Steven Soderbergh gave different locations specific color tones, suggesting different feelings. He gave scenes in Washington, D.C. cold blue tones, and made scenes in the San Diego area warm with gold overtones.
Often, we see commercials skewed strongly toward yellow-gold to impart a "warm" feeling. As we will note in an upcoming module on composition, colors suggest moods.
Black Level and Black Balance
video cameras also have black level and black balance adjustments.
These are typically set by capping the lens so that no light enters
and allowing automatic circuitry to appropriately balance the three
colors for optimum black.
Color Balancing Multiple Cameras
Color balancing a single camera is relatively easy, especially since the editor can often fix minor problems in postproduction.
The problem comes when you have to match multiple cameras--either in the studio or in the field. If you don't get everything just right, you may see an annoying shift in color, brightness, contrast, or sharpness as you switch from one camera to another.
The camera's internal digital signal processor (DSP) controls camera setup adjustments.
Some studio and field cameras are designed to use a "smart card." About the size of a credit card, it records all the parameters on the first camera you (carefully) set up. Then when you insert this card into successive cameras, they will adjust them to conform to the first camera's parameters.
Sometimes it's necessary to store these settings for use later or even to send them to another location where a different crew is doing segments for the same production.
You can check camera color match by focusing two cameras on the same subject and doing a split-screen -- putting the two images side-by-side on a single TV monitor, as shown here.
Assuming that the camera on the left is correct, then all other cameras (placed on the right) can be adjusted to match it. In this example note that the camera on the right has too much magenta.
To match cameras for skin tones (generally, the critical part of a scene) you can use a mannequin, a large color photo, or "real, live person."
If the split screen approach isn't available, you can quickly switch from one camera to another while viewing the results on a single, high quality monitor.
As we've noted, if you are after a particular "film look," you can even duplicate different film stocks (types of motion picture film) by manipulating the video camera's ¥ gamma curve (gray scale response).
You can also create sophisticated film effects, such as fogging, push or pull processing (over- or underdeveloping the film). Unlike the case of film, however, you can immediately see the effect.
Studio engineers use a central CCU (camera control unit) or DSP, to adjust all studio cameras at a central location. Note photo on the right.
DSP or CCU adjustments include iris, which controls the video gain or brightness; pedestal or black level; the subcarrier phase or SC control, which is similar to the hue control on your TV; and the gamma curve or the relative response to the various tones from white to black.
You'll find a more in-depth look at the various camera settings here.
Color Is Subjective
Sunlight contains a roughly equal mixture of all colors of light. The color of light is measured in Kelvin (K) degrees. On the Kelvin scale, the lower the color temperature the redder the light and, as you might assume, the higher the color temperature, the bluer the color.
Compared to sunlight with a color temperature of about 5,500K, the light from a standard 100-watt light bulb is only about 2,800K. The light from standard portable lights used in video field production measures 3,200K. (We'll discuss the color temperature of light in more detail in the chapter on lighting.)
For now, we can see this difference by looking at the photo on the right. The woman is lit on our right side by sunlight coming through a window and from the left side by standard indoor (incandescent) light.
Through a process called approximate color consistency the human eye can automatically adjust to color temperature changes in the 2,800 to 5,500K range.
Daylight color temperature varies, depending on location, time of day, and other factors, so normal daylight color temperature is considered to be between 5,400 and 6,000K.
If you look at a piece of white paper in sunlight, you should have no trouble verifying it's white.
When you take the same piece of white paper inside under the illumination of a normal incandescent light, it still looks white. By any scientific measure, however, the paper seen under a standard light bulb is now reflecting much more yellow light. A yellow (2,800 to 3,200K) light falling on a white object creates a yellowish object.
But your mind says, "I know that paper is white." And so (through approximate color consistency), you unconsciously adjust your internal color balance to make the paper seem white.
In so doing, you're able to shift other colors slightly so that you perceive them in their proper perspective also.
Although we make such color corrections for "real-world scenes," we tend not to make them when viewing television or color photos. In the latter case, we generally have a color standard within our view (for example, sunlight or an artificial light source) that influences our perception.
know human color perception is quite subjective, it's important to use
some objective measure or standard to white balance and color
balance video equipment accurately and consistently. That measuring
instrument, which was introduced earlier,
is the vectorscope.
Good Color vs. Real Color
You might assume that television viewers want to see colors reproduced as accurately and faithfully as possible. Not necessarily.
Studies show that people generally prefer their TV colors more saturated (exaggerated) than in "real life."
Color saturation preferences even differ in different countries. Compared to European countries, U.S. viewers, seem to prefer to see skin tones "healthier" than they actually are -- as well as grass greener, and the sky bluer.
In terms of the vectorscope, this preference does not mean that hues are inaccurate, only that they are stronger and more saturated.
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