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.
Color Temperature
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.
White Balance
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 pre-programmed 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 light-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.
