1-2-3 of Photoshop Levels: This is a great article on using Levels with special attention given to what happens when you make your adjustments with the ALT (Command) key pressed. Well worth the read.
Using Photoshop Levels: This is a very detailed article that goes into a good bit of depth including a list of very important precautions when working with Levels. It also alludes to “Luminance Histograms” a subject we will cover more fully when we get to sharpening.
Output Levels: At some point you may wonder what the little sliders below the graph do; the ones just under the black to white box. This is the Output Levels adjustment and it is important for reducing contrast.
All of these videos cover ways to use Levels, though each one does it in a different way and using slightly different techniques. Watch all three and you’ll be using Levels like a pro in no time.