|Using colour temperature to create a mood in a photo.|
Colour temperature can be really confusing, but if we keep everything simple then it should be quite easy to get a handle on what is happening with the photo at top. To be honest, keeping it simple is the only way I can get a handle on it!
For this image, all we need to know is that the colour temperature of daylight is around 5,500K (Kelvin), and a tungsten light is somewhere around 3,200K.
If the camera is set to daylight white balance, then anything less than 5,500K will turn increasingly orange and conversely anything above 5,500K will turn more blue. That's why, if we have the camera set to daylight (5,500K) and take a picture with a tungsten light in it (~3,200K), the image will be very warm with an orange colour cast to it. It's also why, back in the days of film, we'd stick a blue filter on the lens to counteract this orange cast, to bring it all back to normal.
If we reverse that situation and set our camera's white balance to tungsten, then take a photo in the ambient light, that colour cast will be blue, as the ambient colour temperature is higher than we have set in the camera.
The critical thing to remember here is that any colour temperature higher than we have set in the camera will cause a blue colour cast.
For the photo above, I set the white balance (colour temperature) on my camera to be about 3,400K if I remember rightly. I then exposed for the ambient light, which shifted the colour of the entire photo towards blue.
But, I don't want the subject to be blue as well, so I need to counteract that with an orange filter of some sort. It's no use me putting an orange filter on the lens as that will correct the entire image, which is not what I want to achieve (and would be kinda pointless as I could just change the white balance setting!).
Instead, I use a CTO (colour temperature orange) gel on the flash / strobe / speedlight that is lighting the subject. That way, I only affect the light hitting the subject and that light is now corrected as we have removed the blue colour cast by using an orange gel.
You may ask, why not just gel the background? Well, because the technique used in this post, and the last post, works if you are outside and your background is far away, too large to light or whatever. Try it for yourself and see just how extreme you can make it, then dial it back to suit your taste, or whatever it is you are trying to achieve.