"Curves" is the most powerful adjustment command in Photoshop. Although its functionality and work principles are very similar to those of "Levels", this method lets us edit virtually any tonal range, or even adjust several different ranges in different ways.
So, how to adjust the image tone:
- For a layered document, select the front (uppermost) layer in the Layers palette.
- Open the "Color and Tone" dialog box and apply the "Curves" command.
A dialog box will appear allowing you to adjust image tone by editing the graph. Generally speaking, the sharper your graph is the more contrasting the image will be.
- Very carefully pull some middle point of the graph to brighten or darken the image midtones.
- Pull the graph at some lower point rightward to decrease brightness in the darker areas (let's say, in "mid-shadows").
- Uplift the upper segment to brighten the "mid-highlights".
- To define tonal range more precisely, [Ctrl]-click a specific spot on the image. Then pull the graph at the added point to adjust the selected range.
- Click "OK" to apply the correction.
If you need to modify the created adjustment layer's settings use Edit Adjustment Layer command.
A video-tutorial on this topic is available here.
NB Using the Curves for color adjustments is explained here.