Have you ever looked at a photograph that you took and thought to yourself, "I don't remember seeing that red car in the background"? Or, "How did I miss that telephone pole that looks like it's growing out of Uncle Howard's head?"
It turns out that the eye and the brain work together to "see" the world around you. This cooperation is not objective. It's selective and easily fooled. I'll admit right now that this illustration is terrible, but it helps me make a point.
Suppose you were hiking up Diamond Head in Hawaii, and you looked down at this lighthouse. Unlike a camera-and-film system, the boundary of what your eyes see is not a rectangle. The center of your vision (greatly exagerrated here) records more detail and color than the periphery. Thus it's very easy for use to focus in on small bits of our world.
This "glance" is just a little part of the bigger picture. Your eyes constantly move around, and your brain adds up all of those little bits.
For some people (for me anyway), the eyes forget to look around when they're peering through the viewfinder. We select the subject and forget the background. If we're going to improve the composition of our pictures (or even just get closer), we have to become more aware of the framing and the background.
The way to do this is to consciously move your eye around the viewfinder until it becomes a habit. Here's a little routine I've tried to train my eye to do:
Exercise: Try it right now. Make a little viewfinder with your thumb and index finger. Hold it up to your eye, and select a subject in the room with you. Practice these steps. Repeat them in your mind. Pick another subject and do it again. If you make it a little ritual, it becomes hard to miss the telephone pole growing out of Uncle Howard's head, and it becomes easier to see if you're filling the frame as you get closer.
Remember the second stone head shot? You were probably surprised to learn that it was less than half of the photograph. This is another example of your eye homing in on a detail and overlooking the rest.
If you're fortunate enough to have one of those digital cameras with the LCD screen on the back, you might find that your eyes aren't seduced into laziness as they can be with a viewfinder. The framing is especially easy to see on one of those screens. Of course, those screens kill the batteries pretty quickly. But think of all of the money you're saving on film!
Awareness of Composition