Take a sheet of white paper outside in to the daylight and it will appear bright white. Take the same sheet of paper in to an indoor environment lit by artificial lighting, and it will still appear white although probably less bright. However, do not be deceived. Indoors, the paper no longer really appears white. The colour temperature of the artificial lighting is reflected from the white surface of the paper and what the human eye sees is a warm yellowish tint (in the case of tungsten lighting). The reality is that the brain knows the sheet of paper is white and makes a very subtle and sophisticated adjustment to the signals received from the eyes. Hence the paper is still seen as white. Digital cameras, and indeed film cameras, are not as clever as the human brain. They record faithfully the wavelengths of light they receive. It may therefore be necessary for the photographer to use a film designed for artificial light, or tell a digital camera to make an appropriate adjustment. This is where white balance becomes important.
The white balance control on digital cameras can be used to adjust shooting parameters to suit the prevailing colour temperature of the light. It is relatively easy to set an appropriate white balance for most normal scenes. Standard modes include sunny, shade, cloudy or hazy, flash, fluorescent, tungsten and auto. Each setting implies a degree of compromise, but errors of less than about 250K are unlikely to be noticed.
Typical settings found on digital cameras include:
- Auto - This setting leave all the decision making to the camera.The electronics makes its best guess at the prevailing colour temperature, and in many cases gets the answer right. However, in complex mixed lighting situations the result may be less than ideal. Many photographers are tempted to leave their cameras in automatic white balance mode, but judicious selection of other apparently incorrect settings can be useful for creative purposes. Auto white balance systems typically provide adjustment between about 3,500K and 8,00K.
- Tungsten - Tungsten light, typically produced by incandescent filament bulbs, is warm and tends to give a yellow or orange tint to images. When using this setting, a camera will render images a little cooler, or more blue, to neutralize the warming effect of the tungsten light. An appropriate colour temperature might be about 3,500K.
- Fluorescent - The fluorescent setting is designed for use under fluorescent strip lighting which tends to be cool and can give a blue or green tint to images. The problem here is that lots of different fluorescent tubes exist. Some are designed to simulate daylight, whereas others a intentionally cold or warm. More sophisticated cameras may offer a menu of the various types so that more accurate correction can be made. Normal white fluorescent lights may have a colour temperature of around 3,700K, cold fluorescent tubes such as those found in supermarkets may operate at about 4,200K and the warmer varieties are likely to measure approximately 3,000K. Daylight fluorescent tubes are obviously closer to the colour temperature of normal daylight - about 5,500K - 6,000K.
- Direct sun - Many cameras do not feature this setting because sunny daylight conditions are regarded as normal. However, it can be useful to fix the white balance at a particular value and not have to worry about adjustments the Auto setting might make. A typical setting might be about 5,200K.
- Cloudy -This setting is intended for the somewhat cool colour temperatures found under cloudy conditions. Its effect is usually to warm the images slightly. The colour temperature measured under these conditions may be about 6,000K.
- Shade - This is normally a similar adjustment to that made by the Cloudy setting, but the adjustment is typically rather larger. Shade conditions are usually cool and hence rather blue or green, so images are likely to benefit from a subtle warming.The most appropriate colour temperatures typically about 8,000K.
- Flash - Flash guns tend to emit reasonably cool light - perhaps 5,400K.. If daylight fill-in flash is used, white balance should still normally be based upon ambient light because the colour temperature of the flash has only a small overall effect. When flash is used indoors, it is usually best to select the designated flash setting because the flash will provide a significant part of the illumination.
- Choose colour temperature - Sophisticated cameras may have a setting which allows a photographer to choose a specific colour temperature - ie 2,000K or 10,000K. This is very useful when the prevailing colour temperature is known, perhaps in a studio, or when a colour temperature meter is available to measure the prevailing light.
- Direct measurement - Top-of-the-range cameras may feature a measurement mode which is used by placing a standard grey card in the relevant lighting and using the camera itself to measure the colour temperature. A figure obtained in this manner can then be stored in the camera's memory and used for future work under the same lighting conditions.
Other sources may require different settings. Sodium vapour light, often found at sports venues, may require a colour temperature setting of 2,700K. High temperature mercury vapour light may measure as high as 7,200K.