We humans have a remarkably forgiving brain. We can walk from a natural daylight into tungsten (traditional domestic lightbulbs) or even fluorescent lighting (the 'striplights' found in most offices) and our brains immediately compensate for the colour differences. Film and digital sensors are not so accommodating. If you move a camera from daylight to tungsten lighting without altering the colour temperature accordingly, that bluey-pink person that you photographed reading the Daily Mail has suddenly turned into Tango Man reading the Financial Times.
The problem is white light from a light source isn't strictly 'white': under tungsten light, it's orangey-red; under fluorescent light, it's sickly green; under most daylight it's blue.
The colour of light is actually referred to as its temperature. This is because it is measured as the radiation spectra of a notional 'black body' - a block of theoretical material that does not reflect or transfer radiation, merely absorbs it. If such a black body existed and was irradiated causing its temperature to rise to about 2,800degrees Kelvin, it would begin to glow, just like metal in a forge begins to glow 'red hot'. As the temperature rises further, so the colour of that glowing black body (not an oxymoron, honestly) will change, passing through the colours of the rainbow; by about 4,000°K, the colour will have changed to green (which is why we all look slightly sickly under fluorescent lighting) and by around 5,000°K, the black body emits a blue glow - and this is what we see in daylight (in fact, daylight extends up to around 11,000°K in the shade, where the mood is distinctly indigo).
Note that this colour temperature scale does not mean that it really is 11,000°K in the shade, merely that the light under shady or overcast conditions approximates the colour emitted by a black body heated to 11,000°K. If it were 11,000°K in the shade, you'd be too busy spontaneously combusting to be concerned by colour temperature issues.
Camera manufacturers know that most of us don't instinctually think “Hmm, a sunny day... looks like about 6,300°K to me!” So, cameras instead come with a preset series of white balance settings with pictorial representations of the scenario you might be photographing. Often these also come with adjustments to fine tune your daylight, for example. Cameras also include an auto white balance setting, that guesses (often guesses very accurately) the correct white balance and adjusts the camera's post processing accordingly.
Trouble is, both can be fooled, especially at the extremes; a camera may set tungsten white balance (c2,800°K) when photographing candlelight (c1,000°K) or default to daylight (c6,000°K) in cloudy (c9,000°K) conditions. It's also possible to fox the auto white balance when photographing a scene with a predominantly warm or cool colour - if your subject is standing in front of a pale pink wall, the auto white balance may try to correct for the wall, not the light hitting that wall. As even a couple of hundred Kelvin out can make for slightly odd results, it's often better to take a custom white balance. In addition, if you want to keep the results consistent, a set white balance is better than one that can change with every photograph.
Then, there's mixed lighting, which can bedevil many cameras. If you have daylight and tungsten light in the same shot, the camera might either sum the two with dreadful consequences, or turn its attention on one colour temperature at the expense of the other, which might be exactly the opposite of what you need.
Most custom white balance settings can be made by taking a reflected reading of the light through the camera lens off a white or grey card, or by placing something across the lens that sums the light and taking an incident reading of the light hitting the subject matter (see Expodisc review). You can also take a neutral grey point on an image (if you can find one) and use the eyedropper white balance tool in Adobe Camera Raw or similar. Of course, if your neutral grey point isn't exactly grey, your white balance adjustment can throw up some extremely odd conclusions.
Here's a guide, in the handy form of a table!:
|
Color Temperature (K)
|
Light
|
|
1000-2000
| Candlelight
|
|
2500-3500
| Tungsten Bulb (like lights in the home)
|
|
3000-4000
| Sunrise/Sunset (no clouds)
|
|
4000-5000
| Fluorescent Lights
|
|
5000-5500
| Flash
|
|
5000-6500
| Daylight (clear skies)
|
|
6500-8000
| Overcast
|
|
9000-11000
| Shade
|
