Exposure Compensation

So far, we've focused mainly on accepting your camera's light meter reading and balancing your camera's automatic exposure calculations.  This approach works well most of the time.  Current cameras take light meter readings from all over the frame and calculate what it believes will give you proper exposure.  But as good as these calculations are, they cannot give you the results you want all the time.  There are generally three kinds of situations that can can require you to adjust your camera's calculations to suit your purposes.

1. Light or Dark Scenes: When your scene has average tonal values that are either very dark or very light, your camera will try to make them average the tonal value of 18% grey.  This means that in very dark scenes, your camera will be likely to overexpose your image.  In very light scenes, your camera will be likely to underexpose your image.  You will have to adjust your camera's reading to get the results you want.
2. High Contrast Scenes: Your camera simply cannot see as much light as you can see.  You can see about twelve stops of light.  Your camera only sees about five to nine stops. This means that your camera cannot record many scenes like you can see them.  High contrast scenes, like scenes with direct sunlight and shade, will simply have more contrast than your camera can record.  In scenes like this, you must choose which parts of the scene are most important to you and expose for that portion of your image.  In a sunrise situation, you usually want to have the sky look brilliant and colorful, so you expose for the sky and let the tree in the foreground become dark, perhaps even a silhouette.  If you are shooting a bald eagle flying overhead on a sunny day, you probably want details in the dark feathers, so you'll expose for the dark feathers and let the sky turn to a lighter blue or even white if necessary.
3. Artistic Vision: Remember, proper exposure is what  you want, not what the camera calculates.  Cameras have no artistic vision and no aesthetic convictions.  You may want your image to look different from what the camera calculates, even if the camera's calculations produce an image with detail throughout the scene.

In scenes like these, the easiest way to ensure you are getting properly exposed photos is to shoot in what's called aperture priority mode.  With this mode, you use one  dial to choose the aperture you want, and the camera calculates what it considers to be the proper shutter speed.  Then you use another dial (or hold down a button with the same dial) to adjust that exposure.  In your camera's view finder, you will probably see a scale that looks something like this:

There will likely be a marker indicating your exposure compensation.  Unless you changed it, it will likely be on 0.  As you turn your exposure compensation dial, you'll see the marker move to the left or to the right on the scale.  If you move the scale to -1, you're telling the camera to shorten the shutter speed by 1 stop, making your image half as bright.  If you move it to +2, you're telling the camera to slow the shutter speed by 2 stops, making your image 4 times as bright.  Check your camera's user guide to find the controls that that let you set aperture and exposure compensation in aperture priority mode.

You'll find that you will achieve better results in the types of situations that can fool your light meter if you consider the following:

1. Shoot in RAW mode.  If your camera lets you (almost all SLRs do) and if you  have software that can process RAW files (Lightroom, Aperture, and many others), RAW files retain a more color and dynamic range than a JPEG file.  I shoot in RAW almost exclusively because it's just as easy and better.
2. Learn about HDR.  I'll have posts on this in the future.  In very high contrast situations, you can take 5 or even 7 photographs at different exposure values (separated by 1 stop or more) and use software to combine them. HDR photography can give you very realistic or surrealistic effects.
3. Minimize Contrast.  There are techniques you can use to minimize excessive contrast and harsh shadows, including shooting in the early morning before the sun is high in the sky and shooting with the sun to your back.