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Why is it so complicated to get the exposure right on my pictures? (Why doesn't the camera just capture what my eye sees?)

The exposure for a shot determines the amount of light that strikes the film or sensor. There are two variables the control this, the aperture and the shutter speed. These adjustments are required because no film or electronic sensor has yet been developed that can capture the full range of light intensities to which the eye responds. Of course, our eyes have help too. We have pupils which constrict in bright light and dilate in low light.

So, why don't our eyes ever expose things incorrectly? Our pupils tend to adjust to whatever we're focusing on, so we automatically compensate as our gaze moves. (Obviously, a camera can't do this since it must use a single exposure for the entire scene.) However, it is possible to get your eyes to expose things incorrectly: Have one of your friends stand with his back to a very brightly illuminated window in an otherwise dark room. Take a few steps back and try to concentrate on your friend's face. It should look dark to you and you may have trouble making out his or her facial expressions. The reason is that your eye is being tricked by the bright background.