Project: white balance and overall colour

An essential setting on the camera, why is this?

Most of the time we don't think about the colour of the overall lighting. We do this because our eyes adapt very well and quickly to changes in the colour of the light. There are changes within sunlight depending on whether the sun is high or near the horizon, and between sunlight and, say, tungsten lighting in the home. Our eyes and brain process the view so well that we 'neutralise' the difference. The camera's sensor, however, is literal. It records all the differences in colour, and makes no adaption. For this reason, the camera needs to be set to an appropriate colour balance.

Sunlight is our visual standard for colour. In the middle of the day it seems colourless, we usually call it 'white', because we are so used to seeing it. This is our benchmark for normal, neutral lighting. Sunlight ranges from 'white' to red in a colourful sunset or sunrise. In the shade on a clear sunlit day the light takes on the bluish cast of the sky. This progression of colours is called the 'colour temperature scale'. This is because if you heat something that will not easily burn, like iron, it becomes first reddish, then yellowish, then white ('white hot'). At even higher temperatures, materials that can stand it turn blue. Because real materials melt or vaporise at sufficiently high temperatures, and also give off certain colours depending on their composition, the colour temperature scale assumes in inert, theoretical substance - a 'black body'.

In photography, the most important colour temperature is that of the sun. On this scale it can be given a value, in degrees of  temperature. The temperature is measured in degrees Kelvin - similar to Celsius but starting at the lowest possible temperature - called absolute zero.

Noon sunlight is generally accepted to be between 5400 and 5500 Kelvin. As the sun becomes lower in the sky in the late afternoon, the colour temperature also lowers, to perhaps 400k, and as sunset, depending on the sky conditions, can be around 3000 K. In contrast, the reflected light from a blue sky (in shade, in other words) can be higher than 6500K. The actual figures vary depending on the time of year, latitude and clarity of the sky.

There is possible confusion here, because it seems natural to describe reddish colours as 'warm' and the bluish tones as 'cool', whereas the colour temperatures are, in fact, the opposite. When using the 'natural' terms of 'warm' and 'cool' there is a difference between the visual effect and the colour temperature.

There are 3 ways to adjust the colour balance in-camera:
1. let the process sort it out - auto white balance
2. use the pre-determined settings for known kinds of lighting
3. help the camera measure a white patch (white card), usually called the pre-set

A further adjustment is 'hue', which allows fine tuning of the settings, if shooting in raw, all of these settings can be changed. In a raw converter, there are two sliders, the first follows the temperature scale (bluish to yellowish) and the second changes an opposite colour axis from red to green.