Setting the correct white balance in a photograph is one of the more subtle aspects of photography, especially in nature and landscape photographs where the “wrong" white balance may produce a perfectly realistic looking image and its adjustment is often a creative choice. Before delving into such intricacies, let’s consider what white balance means and why we may need to adjust it.
Among its other innovations that are difficult to incorporate into a camera, the human visual system tends to perceive an object as the same color under many different types of incident light, an psychological effect called Chromatic Adaptation. This is at first surprising because we see everyday objects by simply seeing the light they reflect into our eyes, so you might think that an object seen under different light would look quite different.
However consider a green apple; what does it mean to say it is green? Due to their exact shapes, the molecules in its skin can be excited strongly by green light much as a child on a swing can be pushed sky high when pushed at the right frequency. The other frequencies of the incident light pass through these molecules, and are eventually absorbed into heating the apple. If the incident light is white (ie, approximately equal amounts of every color), most of the green light is reflected while most of the other frequencies are absorbed. But if the incident light is not white, (such as the blueish light of a fluorescent bulb or the reddish light of a sunset), it is not necessarily true that there will be more green light reflected than other colors because there is simply so much less green light to begin with. However we will typically still perceive the object to still be green due to Chromatic Adaptation in the brain.
As a sidenote, objects also emit some light, but at room temperature, that emission is a very low energy light called Infrared, which our eyes can’t see. However very hot objects do emit higher energy light we can see: think of a flame or its embers, and of course the Sun.
Our chromatic adaptation typically kicks in when we are surrounded by the light source, so everything is tinted, say by a red sunset, and our brain simply corrects for the tint so it can still recognize familiar objects. However when inspecting a photograph in an office, we are never in the same lighting conditions as the original scene. It this case, the tilt of the light illuminating the photographed scene often appears unnatural, even though it is a completely faithful representation of the original scene.
If all of the photographed scene is lighted by the same, common type of light, such as tungsten or fluorescent bulbs, or the direct or indirect mid-day sun, camera or post-processing settings can correct for the overall tint and to make the scene appear as if lighted by white light. Even if the light source doesn’t fit one of these presets, post-processing software can typically back out the correct white balance correction if you identify a neutral color in the image, say a white shirt or a gray stone. A neutral gray card can be helpful too; take one photograph with the gray card in the frame, and another after removing it, and set the white balance on the gray card.
The above hints at why white balance might be problematic in landscape photography. Sometimes the correct scene looks wrong, the wrong one looks right, and there is a creative spectrum in the middle! Indeed consider these images of the San Francisco skyline through the fog. The first in the sequence has no white balance adjustment, and in the following ones I set the white balance on the buildings, the sailboat, and the water. The first is my favorite as I find it the most visually compelling and its mood best matches what I remember of the scene. But others will disagree, and the dirty truth is that there is no right white balance correction of an image. If anything, I think there’s a case to be made that the most faithful correction is none at all. Even if you desire the final image to look perfectly realistic, there will be many white balances which subtly change the mood of the scene, and this particular adjustment becomes just another part of the creative process.
Below is the google street view image of this scene.