They consider me to have sharp and penetrating vision because I see them through the mesh of a sieve

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Kahlil Gibran, "A Handful of Sand on the Shore"

WE all know we shouldn't trust our own eyes. Mirages lead us from the path. The instinct to avoid shadows has been buried inside us since our Cro-Magnon days. Magicians and our eyes conspire to make fools of our perceptions. Colors fade and melt and blend from dawn to midnight. Out of the corners of our eyes, we see someone pass by fleetingly and turn to find no one's there.

One reason our eyes are untrustworthy is that, combined with the wetware processor that sits right behind them, they are highly efficient image processors. Our eyes would embarrass the megapixel counts of most digital cameras. Combine both eyes' light-detecting cells—cones for color and rods for night vision — and you have an 18-megapixel set of optics constantly taking new frames. Fortunately, there is no need to save exact pixel-for-pixel records of each ocular snapshot, or else our brain would need to swell into some grotesquely large bulbous organism, leaving our body dangling from it like the head of an engorged tick.

But the human visual system — our eyes and brain — has to do something even more difficult, if nowhere as icky. Our vision has to make sense of what it's seeing, a chore a camera rarely is called on to perform. (Canon has developed a technology that allows a camera to recognize a face in a crowd of other perceptions so the camera's focusing mechanism has a likely target.) For the first year of life, a baby spends much of the time trying to make sense of the changing patterns of light, color, and brightness that constantly play on his retinas. He must figure out, pretty much on his own, how things like size, distance, direction, brightness, patterns, irregularities, and a host of other visual sensations work together to represent that world outside the crib. Strictly speaking, he doesn't see a mobile of cartoon characters hanging over his head; he sees patterns of light, color, and movement that the brain processes to represent a physical thing, like a dangling, dancing toy. The baby laughs at the mobile, not because it is inherently funny, but because the act of making sense of a small part of the existence around him invokes its own merriment.

It is also this need to process what we see that allows magicians to fool us, optical illusions to puzzle us, and digital cameras to offer much needed aid when we're trying to make sense out of the light, shadow, color, brightness, movement, distance, and focus that must be processed into a image on paper or screen that looks like the vision we had in our head.

Seeing is believing because it certainly isn't knowing. You believe you see the Grand Canyon in front of you — or in a photo—because your brain processes all that visual data and tells you it's the Grand Canyon. Has your brain ever lied to you? Of course it has. In this chapter, with "circles of confusion," "Airy discs," and other concepts with simpler names but which are no less daunting, we'll tackle some of the most basic but slippery factors in photography. The attributes of a photo we take for granted will be revealed as subjective qualities we agree on because there is no real objective alternative. We'll explain how tricks of the light affect cameras and photographs and how you can use the slipperiest of them to pull a fast one on the unsuspecting people who see your photographs.

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