The above image was shot during a talk I gave to the University of York photographic society (thanks to Jens for letting me volunteer him for this demo). The topic was light, more specifically how nothing to do with light (and therefore colour) is absolute - it’s all relative.

The rim light coming in from the right has a strong yellow coloured gel in front of it but what if you don’t happen to have a yellow gel handy? Well all is not lost, provided you have a blue gel. By putting a blue gel on main light, the rim light - despite being white* - is yellower than the main light:

White balance is the process of picking an arbitrary point on the spectrum and declaring it to be “white” (something our eyes do without us realising). By choosing the white balance in order to make the blue light white, the colours shift accordingly and we end up with this:

which is surprisingly like the first shot (especially since I grabbed a yellow and blue gel that looked about right, without actually testing!)

Ok, so this isn’t the most useful technique in the world, after all if you one colour gel you’re likely to have others as well! But what about if you can’t gel one of your lightsources, what if one of your lightsources is the sky?

MG

_{*as in the colour of daylight. Residents of another planet would probably have flashes which produce a different colour light as “white”.}