Exposure, in simple terms, is the amount of light that is let into the camera to strike the film for a controlled amount of time. Exposure is probably the most important aspect of photography, since it has such a drastic effect on the quality of the picture taken.
To find out how you can get a correct exposure, you can use the light meter that is most likely built into your camera. This is probably one of the most powerful tools your camera has to help you take a good photograph.
The light meter inside your camera is made up of special light sensors that will give you an averaged reading ( meaning that it judges the light not only in the center of your photograph, but to the surrounding sides as well ) of the amount of light that a given target is reflecting.
There are four basic ways your camera's light meter
will show this reading, depending on the type of camera you have.
Cameras are a lot like cars in the fact that they all operate
in the same basic way, but the layout and options are just a little
different from one type to the next.
Looking through your view finder, you will most likely see one of the four types of light meters diagrammed here.
In Diagram A, the meter will show you the shutter speed and F-stop for a correct exposure, Just set your camera to those settings and you will have a correct exposure.
In Diagram B, the F-stop you camera is set to is shown in the small window above the frame and on the side is a small panel will the various shutter speeds available. A small light will flash next to the correct speed for the F-stop your camera is set to for the correct exposure. Just set your camera to that shutter speed and you'll have the correct exposure.
In Diagram C, there is what is called a match needle meter that has a scale with positive and negative sides to it. Balancing the needle in the center of the scale will give you a correct exposure. You balance the needle by adjusting the F-stop or shutter, or possibly both, until the needle rests in the center of the scale.
In Diagram D, the meter is much like Diagram B except that it will show the correct F-stop for the shutter speed your camera is set to. Simply adjust your camera to that F-stop and take a correctly exposed photograph.
In general, light meters help you select the correct settings of F-stop and shutter speed for the exposure. But keep in mind that these meters are averaging the reading, so much like averaging test scores for a curve, a high or low score can throw the curve. This is true for light meters too.
If there is a very bright or very dark area in your frame, the average will be thrown off. To learn more about the exposure process and to see the effects of under and over exposure, look to the "Two Halves of Exposure" chapter of this site.