How does Auto White Balance work?

[kirbinster]

This is from the school of a thermos bottle keeps hot things hot and cold things cold - But How Does It Know?

How does the camera know how to adjust white balance when it is in auto mode? Why does it sometimes do a perfect jobs, while other times it is really off. What does it actually do to estimate the white balance when in auto mode?

[Jim Bryant]

All sorts of good info here: how does auto white balance work - Google Search

[vandergus]

Originally Posted by Jim Bryant

All sorts of good info here: how does auto white balance work - Google Search

Ironic that the first hit on the search is somebody in a forum asking how white balance works.

[RLucas]

Here is a good article from Cambridge in Colour. Read specifically the "Notes on AWB" to answer you question on why it is hit or miss.

[Mr Guy]

For those of you wanting the short answer, and wanting to know why different makers treat exposure completely different, it's basically a matter of measuring pieces of the picture and attempting to determine what color the world really is. The picture is divided into sections, and each section has a loose influence on the sections around it. This is how cameras prevent destroying shadows to protect highlights, and so on. Different makes and models have different numbers of zones. These zones are then individually measured and compared to "typical" situations. The camera then adjusts the raw values on the sensor to predict what the lighting actually was. For RAW, those adjustments are saved along with the original values. For other image formats, that processing is saved and the original values were discarded. The Pentax K-7 actually has a specific additional sensor that measures the wavelength of the actual light, to help shift the colors correctly AND to autofocus more accurately.

The assumptions the balancing makes are heavily dependent on how the processing is done. Pentax is well known for protecting highlights fiercely. People used to other systems often complain that Pentax underexposes by default because their camera underexposed to protect a highlight. This behavior is, of course, highly adjustable.