HISTOGRAMS

http://luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

http://www.slrnikond50blog.com/
You can use the histogram to check if the exposure was correct.
When you review a picture and pull up the histogram, you’ll usually see a pattern that resembles a bell curve. If that curve is squished to the left of the screen, you’ve underexposed – a lot of shadow areas will be completely black. And if the curve is squished to the right of the screen, you’ve over-exposed – a lot of highlights are completely white.
If a portion of the histogram touches the top that means the highlights are blown out.

The common advice is to ‘expose to the right’. That means to expose the shot so that the curve is as far as possible to the right of the box, without actually blowing any highlights.


So take care to adjust the exposure so that the histogram doesn’t touch either of the sides or the top of the histogram doesn’t touch the top of the screen.


Here’s more on the reasoning behind that: http://luminous-landscape.com/tutorials/expose-right.shtml