Light is colourful – even white light – but the colour of the light depends on the light source. The human eye and brain tend to adapt to colour changes, and so we see most white light as white, but a digital camera is not so capable when it comes to this. That is why digitial cameras have white balance controls – for compact cameras, the white balance might be chosen for you automatically, but for Digital SLRs, most cameras offer the photographer a range of controls to adjust the white balance. And when you adjust the white balance, you adjust the colour of the light that the camera sees.
The correct term for all of this is “colour temperature”, and the colour temperature of a light source is measured in Kelvin. Light sources which have lower colour temperatures tend towards orange, light sources which have higher colour temperatures tend towards blue.
Technically, the colour temperature of a light source is the temperature to which you would have to heat a black body to produce light of the the same colour as the light source.
For example, candlelight has a colour temperature of around 1500K. Sunrise is around 3200K, incandescent lightbulbs (tungsten) are about 3400K and daylight (sun in a clear sky) is about 5500K. An overcast sky is about 7500K and a blue twilight sky is closer to 10000K.
More often that not, if you take a photograph in a lit environment, you want white objects in the scene to appear white. You therefore need to set the camera’s white balance (manually or automatically) to match the colour temperature of the light source dominant in the scene.
Many photographers often remain blissfully unaware of all of this – at least until they add flash to a scene. Flash is daylight balanced, so it’s in the 5500K ball park for colour temperature. Use flash indoors in a room lit with incandescent bulbs and in the final image a white bedsheet in the background will appear orange while the subject’s white shirt will be white. Adjust the white balance for the incadescent light sources, and the white bedsheet in the background will now appear white, while the subject’s white shirt will appear blue.
If you don’t want to make use of this for artistic effect – which you often will by the way – you can add an orange gel to your flash which matches the colour of your flash to the incadenscent lightbulbs (or close enough anyway) meaning that all whites in the image appear white, regardless of whether the flash or the lightbulbs are lighting them. But open a curtain then and let in some daylight also, and things are more complicated again.
In summary, the fact that different light sources have different colours is something that is worth understanding for photographers – especially those who use flash, as you’re almost always guaranteed that you’ll have two different light sources in your image – the flash, and the ambient. I’ve just touched the surface, but as I post images where the colour temperature is significant, I’ll explain how I overcame and problems it caused, or indeed used it to artistic effect.
The image above is one where I used it to artistic effect – the stairway is lit by the ambient daylight and the corridoor is lit by the incandescent lightbulbs along the walls. I could have set the white balance so that the stairs, or the corridoor became grey, but I had no easy way of making the both grey. But when you have such vivid colours in a scene like this, who wants grey? Instead I manipulated the white balance in post production to be somewhere between daylight and tungsten – giving me the nice contrast of the blue to the left and the orange to the right.
Update: This image is one of 67 photographs on display at “Kilmainham Gaol in Focus 2009″ at the gaol from now until August 2nd, 2009.
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