David Pogue's Digital Photography: The Missing Manual

Open the Aperture

Remember, the shutter and the aperture are yin and yang. You can speed up the shutter without changing the amount of light—if you make the aperture larger. You can make the aperture smaller and keep the same amount of light—if you keep the shutter open longer.

Adjusting the aperture size does more than just admit more or less light, though. It also affects the depth of field—that is, how much of the scene is in focus, front to back. A larger aperture gives you a shallow depth of field; in this picture, for example, both the foreground and the background are blurry. Only the middle ground is sharply in focus.

You can read more about how the aperture controls the depth of field on The Flower Shot.

For now, it's enough to note that if your shutter offers modes for shutter speed and aperture, the camera handles the adjustment for you. That is, if you choose a faster shutter speed, the camera enlarges the aperture automatically to compensate. And vice versa. In other words, the camera takes care of soaking in the amount of light it needs.

All bets are off when you choose the M mode, though. That's full automatic: You can fiddle with aperture and shutter speed yourself, with no help from the camera. Until you know what you're doing, you can wind up with a lot of too-dark and too-bright photos.

Where to Find It

On SLRs, look for an A or Av setting on the Mode dial. In this mode—called ...