Different perception

Color pictures are different for perhaps unexpected reasons.

A choice that comes up whenever our photographer pulls out one of his film cameras is which type of film to use.  There are many possibilities, even now in the afterlife of film; but the basic one is black-and-white versus color.  And it remains the important choice.

It might seem surprising that anyone would load B&W film these days.  We see the world in color, and color film is now as fast, fine-grained and stable as monochrome for almost all purposes.  We suppose there is some lingering prejudice among those who learned their trade in the mid-20th century, but among practicing photographers they are few.  It is possible to get a monochrome digital camera, but they are a tiny niche market.  Why B&W at all?

There are, as it turns out, important human reasons for it.  One is that sometimes we do see in monochrome.  Some of the sensors in our eyes, especially in low light situations. give no color information.  Moonlight seems silvery because it’s too faint for us to see color in it; it’s actually the same color as sunlight.  More importantly, though, monochrome information is processed in an entirely different way by our brains.  It seems to give more of the fine detail of a picture, to which the color-processors give a sort of watercolor wash.  And indeed we can recognize scenes and objects from black-and-white alone.  Otherwise early photography would have been a non-starter, and artists could never work in charcoal or ink.

Let us go the other way: our eyes use three color pigments, which are conveniently labeled as blue, green and red.  (We will leave aside color-blindness, in which some people lack one or more of these pigments; as well as those few individuals who have four, a fascinating idea we won’t pursue here.)  Their sensitivity curves don’t quite match what a scientist would prefer and which correspond to manufactured filters, but onboard processing takes care of that mostly.  Anyway, it is possible to recreate a scene visually identical to the original by breaking it up into the three colors and recombining them.  That’s what color film does and what digital cameras do.

The restriction to three channels is what makes all color imaging possible.  It means that we need not reproduce the entire complicated spectrum of a tree leaf, say, in order to depict a tree; we only need to get three numbers right and everyone recognizes not only a tree, but a specific type and even the season of the year.  Supposing we were to take the visual spectrum and break it up into five channels (an experiment our photographer and astronomer want to do sometime), we would still need only three to match what the eye sees.  We cannot get more realistic by adding colors.  (There is more scientific information to be derived; that’s what “hyperspectral imaging” is all about.  But that’s a different question.)

So it’s the way our brains process visual information that means monochrome imaging is still viable, and three colors is all that’s needed for realism.  What you actually do with them is another matter.