It Matters if it's Black and White

Lately I've been trying to write a paper about black and white photography and the workings of the human eye, but its not quite coming together right so I thought I'd try the idea out here.

So, the back of the eye is covered in light-detecting cells called rods and cones. Rods are responsible for 'black and white' vision (cones sense colors.)

If you stare at a completely black surface long enough, your rods, which initially send the signal 'this is black' to your brain, will adjust and instead send the signal 'this is middle grey'. The same thing happens with a white surface - your eyes acclimate until the brain treats it like middle grey. This might seem strange, but it's a very useful phenomenon. It's actually the same thing that happens when your eyes adjust to a dark room, or a bright day. If you adjust to the amount of light coming in, you're better able to compare the things around you to each other, no matter how dark or light it is.

There is a way, though, to prevent the eye from treating either a black surface or a white surface as middle grey. If you place black and white side by side, your rods don't adjust. And, in fact, they send stronger 'this is black' and 'this is white' signals than they would without the juxtaposition. There is also a good reason for this - the brain is finely tuned to parse edges. It works hard to make edges salient so that you can tell where one object ends and another begins. As a result, your perceived contrast between a dark surface and a light surface is not just a function of how different the two surfaces naturally are, it involves your brain upping the perceived difference so that you can really pick it out.

(Incidentally, you also do this with color. A red surface and a green surface side by side will look redder and greener than they would individually. Ditto blue and yellow.)

How does this apply to black and white photography? Mmm, so, in intro photo classes, the teacher will make it abundantly, completely clear that when printing your photo you should always make sure that the brightest part of the picture is completely white, and the darkest part is as black as it can go - 'Get your blackest black and your whitest white.' This rule is so ubiquitous that when a professional photographer fails to do so, the choice usually makes a statement in itself. Photos without a high contrast don't quite...draw the eye. They don't look very snappy, they don't give you that excited-beautiful picture look.

Low contrast compared to...

High contrast -

My idea is that these phenomena are related - the reason black and white photographs look best with a high contrast is that black and white in juxtaposition look blacker and whiter than they do on their own. But there are a few problems: 1) even photographs printed in middle gray are 'fixed' by the brain so the edges look more salient 2) this idea doesn't explain why high-contrast photos are more appealing, just why they look so durn contrast-y.

Thoughts? Anyone?