If cameras were so smart, they'd take pictures that looked like what you see, wouldn't they? And because you have probably picked up a camera at least once, you already know that cameras just can't do that.
Very simply, cameras are incapable of "seeing" the way people see. The fact that we have this stubborn idea that a photograph is THE TRUTH is just a testament to our own capacity for seeing what we want to see, or believing what we want to believe. (Stay tuned for a look at just how much "truth" is in a typical magazine photo these days.)
There are enough reasons why a photograph is not an accurate representation of reality to fill four years of college courses. I'll discuss a few of the most interesting ones as we go along, but in this post, I'm just going to discuss how cameras "see" light.
Problem #1: Exposure
Here's the first problem: your miraculous eyes have an astonishing ability to see in an extreme range of lighting conditions, from a star-lit night to a painfully bright sunny day. They are so sensitive and flexible that they can see in light a billion times dimmer than the brightest light they can handle. They change the amount of light that's admitted to your retinas by changing the size of their pupils. This happens so smoothly and automatically that you're not consciously aware of it, like the beating of your heart or your breathing. Camera lenses have a similar mechanism called an iris or aperture which can be adjusted in size to let more or less light into your camera, giving you a brighter or darker picture.
Unfortunately, cameras can only "see" a far more limited range of light than you can. Try shooting a picture in a dark situation, like at a candlelit dinner, or under a moonlit sky - both situations in which your eyes can see just fine. Under those conditions, a camera just panics, opens its iris as wide as it can, realizes that things are still too dark, then typically gives up and pops open a blindingly bright flash (or refuses to take a picture at all if it doesn't have a flash), and you end up with yet another deer-in-the-headlights picture.
50% Is The Only Brightness I Can Love
A camera is essentially a very simple device. Think of it as a bucket that gets filled with water through a hose attached to a faucet. How fast it gets filled with water depends on how big the hose is and how long you leave the faucet open.
The only difference is, a camera collects light instead of water. The size of your hose is called the aperture. And instead of a faucet, a camera has a "shutter," which springs open momentarily when you press the little "shutter release" button on your camera. How bright or dark your picture is depends on how big your aperture is and how long you leave the shutter open, letting light flow into your camera.
A camera needs a specific amount of light in order to form an image. (How much light it needs depends on the sensitivity of your film or your digital camera's sensor.) So when you tell your camera to estimate how much light it needs, it looks at the brightness of everything in your picture and adjusts its aperture and shutter speed so that the brightness of everything averages out to be right in the middle between pure black and pure white, i.e., 50% grey (setting aside color for the moment). It's OK with some bright stuff and some dark areas, but a camera on automatic exposure insists that the average brightness overall is 50% grey.
So what's wrong with that, you might ask?
The problem is, the average brightness of any given subject isn't necessarily halfway between black and white. But your camera is going to expose every single picture the same way. It's that dumb. So bright subjects will get darkened down to 50% brightness. Dark subjects will get lightened to 50% brightness. Remember that we're just talking about tones for now - how bright or dark something is - and ignoring its color.
Below is a picture taken of someone on a sled using a camera using automatic exposure. Since they're on a sled and wearing winter clothes, and the stuff around them is brighter than their skin, your brain concludes that that stuff must be snow. But in this picture, that stuff doesn't look anything like snow!
The camera has done what all cameras do (unless you tell it otherwise) and made the snow not the nice, bright white stuff we know and love, but a murky, 50% grey mess. And the funny thing is, unless you train yourself to really look at this picture objectively, the part of your brain that's only interested in the utilitarian, informational aspects of an image takes in the clues (hmmm... winter clothes, sled, hill), makes the very intelligent deduction that this dark, mushy grey stuff must be snow, overrides what you're actually seeing in the picture and tells you what you expect to see: "snow." This is just the tip of the iceberg of our astounding misperceptions, and the first of many examples.
Exposure is the term for how bright or dark a photograph is. And believe me, all cameras (left on automatic) make everything in the picture average to 50% brightness. So bright white snow becomes 50% grey mush, when it should look like the version below. They're that dumb.
The same problem occurs if you try to take a picture of someone standing in a bright beam of light in an otherwise dark room. Remember: cameras are dumb. They don't know what's important to you in the photo. They only know how to do one thing: make the average brightness of everything 50%. So your dark room becomes a not-so-dark room, and your spotlit subject becomes much too bright. (See the example below.) In the first version, the camera averaged everything to 50% brightness, which means that the brightly lit people are grossly overexposed. In the second version, the brightly lit people are well-exposed, but now the people in the shadows are extremely dark. Which brings us to our next limitation of cameras:
Problem #2: Contrast
Cameras simply can't handle as much contrast as your eye can - not by a very long shot. At a certain point, bright subjects just become pure white, and dark areas are rendered pure black. Thus in the picture at left, it's just impossible for the camera to "see" details in the shadows as well as the brightly lit areas. Whereas if you had been standing in this room, your eye would been able to see much more detail. Your eyes would have seen a much lower-contrast image.
Ultimately, exposure is an aesthetic decision, like everything else about making photographs. Since a camera can't see things the way your eyes do, you have to make decisions. That means when it comes to exposure, you have to give up something when you have more contrast than a camera can handle.
So what can you do about this? You've probably already guessed that you, the smart photographer, need to intervene to help your dumb camera, which conspires to make every picture look the same. Very simply, you have to override your camera's automatic exposure mechanism and tell it how bright or dark you want your picture.
Next time, we'll talk about how to do this in detail, and how to turn this liability into an asset to make dramatic and surprising photographs.