Photoshop CS3 Tutorial
How to Adjust Image with the Photoshop Histogram
The Histogram (aka Levels) is a visual display of your image luminosity values. It can be quite helpful in setting the overall range and adjusting the mid-point.
Open multiple images in Photoshop (File > Open...).
Select one of the images and add a Levels adjustment layer by clicking the Create Adjustment Layer icon and choosing Levels.
Notice the graphic representation of the image. The left side displays dark pixels and the right side displays the light pixels.
Move the Black slider to adjust the dark pixels.
Move the White slider to adjust the white pixels.
The mid-point can be adjusted with the Gamma slider.
Click OK to save your new adjustment layer.
To see the effect of your new layer, toggle the layer's Visibility icon. By double-clicking the adjustment layer thumbnail, you can return to the settings and make adjustments.
Add Levels adjustment layers to the remaining images. Notice that the histograms change based on the image content. Darker images have more pixels to the left and lighter images are right heavy.