Exposure

In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph .
Exposure is measured in lux seconds , and can be computed from exposure value (EV) and scene luminance.
The "correct" exposure for a photograph is determined by the sensitivity of the medium used. For photographic film, sensitivity is referred to as film speed and is measured on a scale published by the International Organization of Standardization (ISO). Faster film requires less exposure and has a higher ISO rating. Exposure is a combination of the length of time and the level of illumination received by the photosensitive material.
Exposure time is controlled in a camera by shutter speed and the illumination level by the lens aperture.

A photograph with an exposure time of 25 seconds

A photograph of a night-time sky with an exposure time of 8 seconds

A two second exposure of a fire poi ball dance

The shutter and aperture control how dark or light your pictures will come out and how focused they are.

To mess with the shutter and aperture your camera needs to have a manual setting


Sample shutter settings:
15" to 1/1600

Sample aperture settings
F2.7 to F8.0

What is a "Stop"?
A stop is making your aperture or shutter brighter.
examples: moving from f5.6 to f8 is:
ONE STOP darker and
moving from f4 to f2.8 is:
ONE STOP brighter




http://en.wikipedia.org/wiki/Exposure_(photography)