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If it's Called "White Balance," Why Do People Use Gray Cards?

As digital photography becomes increasingly popular, photographers are finding a lot of new buzzwords to either ponder over or impress their friends with. Among such trendy new terms, nothing elicits more confusion than does "white balance."

Yet white balance isn't a new concept at all. Throughout the day, the color of daylight changes from the warm glow of sunrise, through the cooler colors of midday shade, then back to the golden tones of glorious sunset. While the average person is at least somewhat aware of this cycle, we are blessed with brains that automatically compensate for color temperature so non-photographers rarely feel a need to compensate in any way. Even photographers who work only with print film can practice their craft while mostly ignoring the phenomenon since the lab they take their negatives to will fix things for them when printing. Those shooting slide film learn to use color correction filters to avoid rude surprises when they get their film back. Take a picture on slide film indoors under incandescent light and you'll find out just what color temperature is all about. Most outdoor slide shooters carry at least an 81A warming filter to avoid the color temperature blues of overcast days.

Film has a fixed response to color, but in the digital world, there is little need to place a filter in front of your lens to correct for color temperature since the same thing can be done electronically. For it to do so though, it needs to have some frame of reference for what color things are supposed to be. Enter, white balance.

The concept goes something like this: if you point the camera at something that is supposed to be a known color, the camera can compare what it actually sees to what it knows it is supposed to be seeing, and determine what it needs to do to compensate. Since any shift of color is most noticeable when starting with a neutral hue, that's what is traditionally used for this task. If we could define and accurately manufacture a standard shade of any other color though, we could all agree to use "red balance" or perhaps "blue balance," but for a variety of reasons the world has standardized on "white balance."

In practice though, white itself isn't generally your best bet since any color sufficiently overexposed will yield pure white. It isn't really the brightness we are interested in measuring anyway but rather the color. For this reason, many photographers make use of a standard gray card instead. When you think about it, it makes perfect sense. It has a nice, medium tone and a neutral hue, and photographers frequently have one anyway for metering. If you point a camera at a gray card and shoot fully on auto exposure, you will get a medium toned result. We know how bright the picture will be, so the only thing in question is what color it will be. Since the gray card actually has a neutral hue, any variation from neutral that the camera records must have come from the color of the light shining on it. To do this, it is important that the frame be completely filled with the color we are measuring to prevent reflections from other objects influencing it.

This method is known as manual or custom white balance and is generally regarded as the most accurate for applications where color accuracy is critical. Fluorescent, halogen and other artificial light sources have a tint as well as a color temperature. Such light sources are notoriously difficult to compensate for but manual white balance is your best bet. Whatever the camera sees it makes neutral, shifting all other colors as needed to compensate.

Even cameras that don't support manual white balance though often provide a number of predefined choices with names such as "cloudy," "daylight," "flash" and "incandescent" for adjusting white balance. When color temperature measurement is less critical, these can be an effective alternative and are certainly easier to use. These are the digital equivalent of choosing between daylight and tungsten film but with a few extra choices thrown in for good measure.

Some cameras also support direct input in Kelvins. Photographers with a high degree of familiarity with color temperature may prefer this method since such terms as "flash" and "cloudy" are not defined consistently by all companies or even within all camera models made by a given manufacturer. Studio photographers used to working with a color temperature meter may also find this way of working easier.

For the sake of those unfamiliar with the Kelvin temperature color, allow me a brief digression. Baron Kelvin was the title of a guy whose real name was William Thomson who lived in nineteenth century Britain. Among his many accomplishments was supervising the laying of a trans-Atlantic cable, coining the term "thermodynamics," and the proposal of the scale of temperature that now bears his name. Zero Kelvin, equivalent to about -273 degrees Celsius, is the lowest possible temperature, below which atomic motion itself no longer occurs. If you start with a theoretical "black body" at absolute zero and heat it, at some point it will start to glow a dull red. If you continue heating it, it will change colors through orange, yellow, white and eventually blue. Color temperature is measured in Kelvins as a way of quantifying the temperature such a body would have to be to emit a specific color.

By directly entering color temperature in Kelvins, you are telling the camera the color of light striking your subject. Here are some sample color temperature values:

 
 Light Source   Color Temperature 
Candle Light 1500 K
Sunrise / Sunset 3200 K
Tungsten Light Bulb 3400 K
Overhead Sun at Noon 5500 K
Overcast Sky 6500 - 7500 K
Open Shade 9000 - 20,000 K

The term "preset white balance" is used by different camera manufacturers in different ways. Nikon for instance uses it to refer to manual white balance since the photographer "pre sets" the camera for the exact lighting conditions. Elsewhere you can find the term used to mean the various predefined color temperature choices such as "cloudy" or "incandescent."

If you shoot jpeg or tiff, it can be helpful to learn to adjust white balance on your camera. The situation is much the same as it is for film shooters in that the image is fixed at the point the shutter is pressed. If, however, you shoot raw you can be more liaises faire about in camera white balance since you can freely adjust it after the fact without any loss of quality. With raw files, the image doesn't get fully formed until you run it through Adobe Camera Raw, Nikon Capture or other raw conversion software.

Next week, we'll look more at how color temperature and white balance fit in with Photoshop, both for raw shooters and for those shooting other formats.


Date posted: July 10, 2005

 

Copyright © 2005 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Dodging and Burning in the Digital Darkroom Return to archives menu Next tip: Correcting White Balance in Photoshop and Adobe Camera Raw

Related articles:
Essential Filters: Warming Filters
File Formats for Digital Imaging
The Zen of White Balance
What Exactly is Accurate Color?
Just Say No to Exposure Compensation in Manual Exposure Mode
 

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