Brightness/Contrast is especially useful if you've never performed tonal corrections before, and on many images, it's all the control you'll ever need. However, Photoshop's Levels adjustment provides a more sophisticated tool that offers a much finer degree of control (Figure 4.5).
Figure 4.5 Understanding the Levels sliders.
With Levels you have five different sliders that you can adjust, as well as a histogram (sort of like a bar chart) (Figure 4.6) that indicates exactly what is happening to the image.
Figure 4.6 This histogram indicates that the shades between around 90% and 75% gray take up a lot of space (tall bars), and the shades between around 5% and 15% take up little space (short bars).