In Adobe Photoshop, the levels command can be used to improve a documents tonal range and color balance by adjusting the output levels of shadows, midtones, and highlights. By increasing an images tonal range, we can increase its contrast, sometimes bringing out much more in an image as shown in the previous part of this tutorial. Lets do an experiment to see how Levels work in Adobe Photoshop. Open up a photograph, or simply use the one provided below:

As you can see, there are some huge problems in this photography. First off, this is a picture of someone catching air while sledding in snow. Snow should be much whiter than it is in this picture! We can use a levels adjustment layer to make the image appear correct. Add a Levels Adjustment Layer to your Document in Adobe Photoshop:

When the following box appears, click "OK" to proceed. If you want to use a layers mask to only apply the levels adjustment to part of the image, check the box.

After clicking OK, the Levels Settings box will appear. This is where we can make adjustments to our documents levels by making changes to the intensity levels of the images highlights, midtones, and shadows. One of the most important things about the levels adjustment box is being able to read the levels histogram:

This is the histogram of our image. This histogram gives a brief description of our images tonal range. If there is a large concentration in the left region of the histogram, this means that it has lots of details in the shadows, whereas a large concentration in the right of the histogram would mean details in the highlights. When it is concentrated to the left like shown above, it often means that we have an underexposed photograph. Likewise, one concentrated to the right could be over exposed. It is a good practice to try and have the tonal range spread across the histogram.

Section 4 "Auto Levels"