Why do cameras need white balance if our eyes already adapt to different lighting?

Question

I understand that white balance compensates for the color of the light source so objects look more natural. But if our eyes and brain already adapt to warm incandescent light, cool fluorescent light, etc., why does a camera need to do its own white-balance correction?

What confuses me is this: in a room lit by very warm incandescent light, I can still tell the room has a yellow cast, but a camera on auto white balance often looks more “correct” than the same scene shot at a daylight setting. Also, when I look at a photo or screen, my eyes do not seem to cancel the color cast the same way they do when I look at the real scene.

Why doesn’t visual adaptation make a photograph look right regardless of camera white balance? Is there something different about viewing prints/screens versus viewing the original scene directly?

user7977 · Accepted Answer

The perceived color of an object depends on two elements: the intrinsic color of the object, and the color spectrum of the light shining on it.

A red apple for example, will appear nearly black with a pure blue light shining on it. Depending on the difference in spectral density of different lights, the absolute perceived color of the red apple will change, it isn't constant. But because we have knowledge of what color the apple really is, our brain adjusts our color perception so the red apple is what we expect.

White balance is the tool to make the output of the camera reflect the post-processing our brains do.

When we look at a photograph or a screen, our visual cortex applies its white balance depending on the lights in the room and your knowledge and preconceptions of what the intrinsic colors of items should be, but it's not equipped to make extra, special adjustment knowing it's looking at a photograph. When the white balance of the photo or screen is different than the environment you are in, the resulting colors look strange, e.g. the red apple's perceived color is different than what your brain expects in your rooms lighting.

You say everyone can see that incandescent lamps are yellow, but that's not strictly true. You have knowledge of the lights in comparison to other light sources which is why you think it's yellow, but that can easily be fooled. I could put you in a room, new to you, with only incandescent bulbs, and I could make the lights seem virtually any color, by carefully selecting the colors of the paint and other objects to trick your visual cortex into applying an incorrect white balance. If I had a bunch of objects in the room that are ordinarily white, but are in fact tinted a particular way, your brain will adjust its white balance correction so they are, and that can result in perceiving the incandescent lights a different color. The infamous blue/gold dress is an example of the phenomenon at work.

The Wiki page on Color Constancy has more explanation as well as some sample images that can further illustrate the concept.

Why do cameras need white balance if our eyes already adapt to different lighting?

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