Exposure Basics

Exposure is the amount of light collected by the sensor in your camera during a single picture. If a shot is exposed too long, the photo will be washed out. If it is not exposed long enough, the photo will be too dark. Almost all camera today have light meters which light in a given shot and set the ideal focus automatically.
The primary controls for exposure on a camera are aperture and shutter speed:
Aperture-the size of the lens opening that lets light into the camera. Apertures are measured in "f/stops" ex. f/2.8 is a very wide aperture, f/19 is very small.
Shutter Speed-the amount of time the light is exposed to the sensor. Shutter speed is measured in seconds (or fractions of a second). 1/2000 of a second is very fast and 8' seconds is extremely slow.



This is an example of a photo with a very low shutter speed, and a slightly narrow aperture

The relationship between shutter speed and aperture:

To maintain the same exposure value, if you change one of the two settings, you must change the other value in the opposite direction. Take this situation for example: The camera meter is 1/60th second and f8.0 and you wanted a faster shutter of 1/250th second. To have a faster shutter speed of 1/250th second it is underexposing three stops. So to keep this the same, the aperture must be overexposed three stops. This would be f/2.0. The exposure of 1/60 and f8.0 has an equivalent exposure to 1/250 and f/2.0. This is a table showing the relationship between shutter speed and aperture.