* Understanding White Balance Can Help
You Better Deal With Color In Your Photos
Since this issue is our annual how-to guide, I thought I’d discuss the subject of letters that I receive on a regular basis-white balance. Many readers new to digital have become confused when dealing with white-balance settings on their cameras. If you understand how to use white balance, it can be a helpful tool in capturing better images.
Any explanation of white balance has to start with a discussion of color temperature, which is an attribute of light and is a way of quantifying light’s overall, basic color.
The actual term “color temperature” is borrowed from physics, where color temperature relates to the spectrum of light that’s radiated from a theoretical object called a black body. As you heat this object, the color that it emits changes. Terms like “red-hot” and “white-hot” are a manifestation of this principle. As an example, if you heat a piece of metal, first it becomes dark red. As it becomes hotter, it turns yellow. Finally, as it’s hotter still, it turns bluish white.
The actual color temperature is measured in degrees Kelvin (K). Low temperatures make for warmer light; high temperatures make for cold light.
You also can think of color temperature as the ratio of blue light to red light. When more blue is present, the color temperature is higher; when more red is present, the temperature is lower. For the scientifically minded, the Kelvin scale starts at absolute zero—the temperature at which all classical molecular activity ceases. Zero Kelvin equals minus 273.16 degrees Celsius.
In photography, the terminology we use for the color of light (such as “warm” or “cool”) is just the opposite of its meaning in physics. In Kelvin terms, the higher the temperature, the bluer it becomes, and the lower the color temperature, the warmer it looks. In photography, we consider blue to be a cool light, however. In other words, when you’re hot, you’re not!
As you might expect, different light sources have different color temperatures. What’s surprising is that the color temperature of a single light source can fluctuate dramatically as well. Consider sunlight, for example. Its color varies as the sun changes position in the sky because the light traveling from the sun passes through several thicknesses of atmosphere that scatter light rays differently, depending on the wavelength. At noon, when the sun is directly overhead, the sun’s rays travel through the least amount of atmosphere. When the sun is setting, its rays travel at a sharp angle and are affected by more particles in the atmosphere. The blue is scattered, so we see more of the warmer wavelengths.
Artificial light sources vary in their color temperature, too. Depending on the process used to create the light, the color temperature could be on the lower end of the scale (warm), as with incandescent light bulbs. Halogen lamps, on the other hand, become more blue as the color temperature moves higher.
Color Temperature In Digital Photography
We all can make the comparison between the iris in our eye and the iris in a camera, but the comparison can go further. Just like the human eye, a digital camera has red, green and blue photoreceptors and a brain (or CPU) to control the signals coming from those photoreceptors.
A digital camera, which is essentially trying to mimic the human vision system, also tries to copy our approximate color consistency adjustment. Modern-day image processing gives us the ability to correct for color temperature differences without the use of lens filters. The processor in the digital camera takes the output of the sensor and attempts to make adjustments to the RGB values in order to make white look white.
This adjustment, called white balance, is found in all digital cameras today.
There are several settings for white balance in a digital camera that fall under the terms auto, preset and manual.
Automatic White Balance. This adjustment evaluates the overall field of view being captured by the image sensor, examining it for any color bias. If there’s any bias, it attempts to zero it out. Unfortunately, this averaging makes many assumptions, one of which might be that there’s a lot of white in the scene you’re photographing. If that’s the case, the image might be what you want, but if it isn’t (and experience proves this is usually the case), then the color-balance adjustment that the camera performs will be less than desirable. This can be a problem especially when it removes the warm light at sunset.
Preset White Balance. There are preprogrammed white-balance settings that are akin to loading up your digital camera with daylight or tungsten film, or adding a color-balancing filter to your lens. Some of these presets (such as those indicated by the ubiquitous sun icon or the ever-popular cloudy mode) are specifically designed for certain hours of the day. For both color correction and creative control purposes, you may decide a certain preset gives you the look you like in conditions it wasn’t designed for, however. Many photographers like to shoot with cloudy or flash settings outdoors in all conditions because it warms up the scene, for example.
Manual White Balance. In its simplest form, you point your camera to a white card (any white card or even a neutral gray card will do), engage the manual white-balance function, and the camera makes the necessary adjustments to the RGB values of the CCD to produce white. The steps to do this vary from camera to camera, so check your manual. This also is sometimes called custom white balance.
It’s important to note that the adjustment or correction to the captured image may not be as simple as adding the same amount of red to every pixel in the image in order to compensate for a cooler light source. This is why it can be more difficult to correct for poor white balance in an image-editing program than it is to get the white balance adjusted properly in the camera.
Using Manual White Balance
If you’re using manual white balance to achieve a true white, there are some things to consider. First, there are the more obvious suggestions, such as making sure you fill as much of the lens’ field of view with the card to minimize white-balance errors. Avoid placing the card near highly saturated surfaces (like a red tablecloth, for example). Also, be sure the card is in the same light as your subject.
A less obvious clue to successful white balance deals with exposure. When you’re using a white card, make sure it isn’t overexposed. If the camera’s CPU is attempting to adjust the red, green and blue channels, and those channels are oversaturated, the software in the camera won’t make the correct adjustment. If you use the typical white card and are viewing it through an LCD screen, consider drawing a line on the card so that you can see whether or not you’re overexposing it. A gray card has less of a problem for exposure.
There are times when you don’t want perfect white balance. In those cases, you’re just looking for white-balance control. Just as portrait photographers might use a warming filter, you can use different-colored cards to adjust the warmth or coolness of your image. Instead of using a neutral card, you might use a light-blue card when you want to warm up (in photography’s, not physics’, parlance) the picture, or a li
ght-pink card when you want to cool it down. There are cards you can purchase if you don’t want to make your own (check out www.warmcards.com/digital_camera.html).
While initially you might consider white balance a challenge in working with digital cameras, in reality, it’s an opportunity. Manual white-balance control offers the digital photographer an on-location tool that film photographers never had—an almost continuous color temperature adjustment.