Colour Constancy - "the ability to perceive an object as having relatively
the same colour under varying illumination conditions"
I've been saying for a while now, that being a good photographer requires a heightened sense of awareness - not just of patterns and themes within the landscape, but also of colour.
But colour is difficult to perceive accurately in the landscape, because our brains and visual system have evolved to allow us to perceive the same objects as having relatively the same colour under differing lighting conditions. This a is very useful evolutionary trick that allows us to identify objects under varying lighting conditions but it can be a problem for photographers when trying to visualise how the final image will turn out. This is because cameras don't have colour-constancy - they record the variances in colour that a subject goes through when the source of light changes.
As Wikipedia says:
"A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red."
Color constancy: The colours of a hot air balloon are perceived as being the same in sun and shade while we are on location, but does the final photograph allow us to perceive them as the same? (Image source: Wikipedia)
Another example would be to consider a white shirt under white sunlight. The shirt looks white to us, but if placed under a shaded green tree, the shirt has now taken on a green cast, except that we still perceive it as white and not green.
This is a real problem for us as photographers, because for many of us, we don't see the green cast until we get home and review the images. Colour constancy is not so useful to us as photographers when we wish to see the actual colour that the object will be rendered on our film / digital sensor. Our visual system hijacks us into believing that the apple still looks green, even though it has taken on a warmer hue, or that the white shirt is still white, even though it has now taken on a green cast.
It is important to understand that objects do not have colour, but instead, that colour is an 'event'. We need three things for us to see colour: a light source, a subject, and of course ourselves to witness the light being reflected of the subject. As the light source changes, the light reflected back of the subject changes and as a result, its colour changes. But because of colour constancy, we perceive the colour of the subject to be relatively stable as the light source changes.
Cameras do not see the way we see. They do not have colour constancy - if the apple takes on a different colour at sunset, then the camera sees and records the change in colour, but we in turn do not. Similarly, if the white shirt takes on a green cast whilst placed under a tree, then the camera is able to see this and record it also, whereas we do not.
The only caveat to this is when we set the white-balance of the camera to 'auto'. When we do this, we tell the camera to 'tune-out' any colour casts and try to render what it is recording as a mid-day temperature. So in effect, 'Auto-white-balance' is the camera's own way of obtaining colour-constancy. I don't believe we should use AWB (auto white balance) in cameras because we would effectively be tuning out the warm hues that are present at sunrise, or the cold hues that are present at twilight.
I see colour-constancy as a handicap though. For landscape photographers what we really need to see is how the colours change under varying light sources. Yet our visual system is doing everything in its power to 'tune-out' everything so we don't see these colour changes. You can consider colour-constancy as our own in-built 'auto-white-balance'.
Colour constancy allows us to perceive squares A and B as different, when they are actually the same luminance. (source Wikipedia). Try it out - open this image in Photoshop and use the eyedropper tool to check the RGB values of square A and B. You will find that they are both R:120, G:120, B:120.
Being aware of our own in-built 'white-balance' - our ability to tune out these colour changes is important. We need to be aware of the different colour temperatures that are present throughout the cycle of a day from twilight (cold, blue) to sunrise (magentas, warm) to midday (neutral) and how these will affect the subjects we photograph.
Over the years, I've learned to be more aware of how colour constancy is affecting my judgement.
About a year ago, I was standing on a beach with a group of workshop participants. There was a prominent red sky towards where the sun was rising, and I knew this would mean that if the light source is warm, the entire landscape would be bathed in the same warm tones. The first thing I notice about many photographers is that they want to shoot towards the sun because they perceive the red colour being present only in that direction. They don't perceive the rest of the landscape as being bathed in the same warm light, and this is because of colour-constancy. I asked my group to tell me what colour the clouds were during the sunrise. To my eye, they were magenta. It was interesting to note that half of the group said they were magenta while the remaining members said the clouds were grey. It was only when reviewing the work in our mid-day editing session that it was obvious the landscape was pink, and so too were the clouds, yet half of the group weren't aware of it at the time of capture.
Understanding our own visual limitations, of how we can be tricked into thinking that a subject's colour remains mainly constant under varying lighting conditions is a key awareness skill.