Digital Photography 101

Chapter 1: Introduction to Film and Digital Cameras

1.9: White balance, colour casting, colour temperature resulting in warm and cool tones

Not more science to consider! As an artist you are probably thinking with a side of your brain not overly interested in physics, but please be patient here and do some basic learning. Photography is a wonderful mix of science and creativity and the most successful image makers will from time to time be thinking with both sides of the brain and analyzing the science required. This is very important when we consider what filters to use, when and why.

Colour temperature is a term that is borrowed from physics. In physics we learn that a so-called "black body" will radiate light when it is heated. The spectrum of this light, and therefore its colour, depends on the temperature of the body. You probably know this effect from everyday life: if you heat a steel bar it will eventually start to glow dark red ("red hot"). Continue to heat it and it turns yellow (like the filament in a light-bulb) and eventually blue-white. Be careful with the terminology here! The hotter the body gets (measured as the temperature in degrees Kelvin) the more the colour moves from red towards blue. But we say that red is a "warmer" colour than blue! So a warm body radiates a cold colour and a (comparatively) cold body radiates warm colours. I know, it's confusing. The photographic colour temperature is not the same as the colour temperature defined in physics. As mentioned above, the photographic colour temperature is measured only on the relative intensity of blue to red. However, we borrow the basic measurement scale from physics and we will measure the photographic colour temperature in degrees Kelvin (K).

The following table should give you some feeling for the scale.

This means that you will find photographers talking about "daylight balanced" film (nominally 5500K) and type A and B tungsten balanced films (3400K and 3200K). When used in the right situation it makes the subject take on natural colour casts similar to how our own eyes would see it.

Our eyes are very clever when it come to adjusting to colour temperatures and will make white look white under most conditions except for in darkness. If our white subjects look white then it stands to reason other colours will be consistent. A camera lens is not so clever and this is why we can't just shoot all situations in daylight balance film at 5,500 K. If we did the shadows at 9000K will take on a blue or cooler casting and the house hold light bulb at 2500K will make our photos seem orange or red.

This is specially so in portrait photography and weddings are a good example. Say we have daylight balance film and we are taking a photo of the bride and her face is in shadow, which is desirable to eliminate squinting into the sun. Because the colour temperature is in shade the resulting photos will give her face and veil a blue casting. In traditional professional photography we would compensate this by using a red warm up filter. What a hastle!

Light balancing filters are used to change the colour temperature of light. If you place a light-balancing filter in front of your lens, the overall temperature of the scene will change. These filters are sometimes called conversion filters because they may be used to "convert" daylight balanced film to use in different lighting conditions. This is additive colour mixing that Sir Isaac Newton discovered.

Along came Prosumer and SLR digital cameras and things got a little smarter in the white balance department. Now we can simply dial up how we want our cameras to react to colour temperature. We can do this on a simple scale we select or with the more expensive SLR and Prosumer cameras it will measure the colour temperature for us. Using the Automatic White Balance mode we can leave it to the camera and even choose to warm up the auto setting or cool it down with compensation. I leave my camera on auto white balance and warm it up a bit by -3 on the compensation. If I am photographing in shade I will choose the shade preset, as often the camera will get it slightly wrong in auto mode, which is typical. I have a greater understanding of what I want than my camera does. If I am shooting in RAW file format I can fine-tune the white balance later in postproduction or I can change the colour balance in Photoshop. More on this later.

With fast moving situations you are best to leave the white balance on auto as your light will be changing all the time and with it your colour temperatures. You don't want to be photographing under lights and then go outside into the sun with the same white balance. If we did, all our photos will have a strong red colour casting over them.

As with all camera systems the more expensive and professional the body of the camera the better it will be at auto white balance.

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