Proper exposure is critical to the success of any photograph. The "proper" exposure is the exposure that yields the desired result in the final image. There may be 5, 6, or even 7 correct exposures, but only one really renders the scene as you , the photographer, visualized it. Understanding exposure and how it works allow you to make better decisions in your image capture process, which subsequently lessens the post-capture processing in Photoshop or your favorite image processing program.
One site that I've looked at over time and often referred my students to is http://www.photonhead.com . This site offers several interesting and useful functions, particularly its exposure demonstration module called The SimCam.
Take a few minutes to look at this exposure calculator and the various results you can get by changing the settings for aperture and shutter speed. A great tool and its all free!
Have a great day and Keep Shooting!
