Using the levels tool in photoshop
The level tool in Photoshop lets the user move and stretch the brightness of image histograms. When considering adjusting the black and white point levels of your histogram refer to this. It’s really amazing how much someone can improve their images by only changing contrast levels of certain parts of the image. With very good examples and explanations, this tutorial simplifies using the powerful level tool.
>> http://www.cambridgeincolour.com/tutorials/levels.htm