Most of the time the camera's meter does a good job of determining the correct exposure. Basically though, it can easily be fooled by subjects and backgrounds that are either too light or too dark.
The metering system, with a few clever refinements, measures the light in a scene and tries to take an average of all the tones. I say 'with a few refinements' because there are different meter modes like centre weighted average or spot metering to try and compensate for the shortcomings of the system, but every now and then you will have to resort to a bit of manual intervention to get the best exposure.
Then it's time to hit the exposure compensation slider, most cameras have a similar display to the one on the right where you can set the camera to over or under expose the subject by up to 3 f-stops in 1/3rd stop intervals.
Using the slider is very simple, turn it to the left and the exposure will decrease (the picture will be darker), turn it to the right and the exposure will increase, making the picture lighter.
Each number on the scale (1,2,3) represents one f-stop, decreasing the exposure by one f-stop will halve the amount of light reaching the sensor. The dots in between the numbers represent 1/3rd of an f-stop.
One really important thing to remember is to put the slider back to the middle when you have finished, otherwise all your pictures will be incorrectly exposed, the settings are not self cancelling.
Below are a couple of examples of situations where you need to use exposure compensation.
Nearly all pictures taken in the snow will be too dark because the snowy background is uncommonly bright. When taking skiing pictures I always set the exposure compensation at least one stop over, don't go too crazy as you don't want to burn out the highlights. The best thing is to take a couple of shots and then have a look at the histograms in the camera to see if you are on the right setting. To read more about histograms click on the link.
In the picture, above left, of the stork the highlights are burned out. The detail has been lost and no amount of post processing will restore the texture of the feathers. In the picture on the right I have applied a negative setting on the dial to under expose the shot and retain the detail in the feathers.
If you're not sure what settings to apply, it is possible to set up a series of exposures at different settings, this is called bracketing and is discussed on another page.
Getting away from the auto settings.
An explanation of the mechanics of exposure and the side effects of choosing different aperture/shutter speed combinations.
Overriding the automatic metering system.
What they are trying to tell you.
Setting up your camera to take a series of pictures at different exposures.
Another piece of the exposure puzzle.
An explanation, strictly for the jargon heads.
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