Tutorials2D GraphicsGIMPGIMP Tutorial: How to adjust your photos using Levels.

GIMP Tutorial: How to adjust your photos using Levels.

Written by Marc Roberge | 11 April 2010

GIMP Adjusting Levels 1

The levels tool allows near limitless control over your red, green and blue channel information which can help you breathe life into your washed out or flat digital photos.

In our last tutorial, we saw how to adjust exposure using the Auto White balance filter in GIMP. Again, if you don’t know what GIMP is, read our article: What is the GIMP?. In this follow up, we will learn how to edit your digital photos using the Levels tool.

What is the Levels tool and what does it do?

The levels tool allows you to interactively tweak color levels for each of the Red, Green and Blue channels using the input level histogram as a base line. When your camera captures an image, it is actually recording color and light concentrations across the spectrum for each individual color channel. For illustration, consider the following image. In it we have the Red color channel for the barn photo above.

To the left of this histogram, we have the dark end of the red spectrum starting at near black through all tones of red towards near white at the right end. You’ll note that the bulk of our reds register in the middle of the spectrum as shown by the graph spikes. Where the graph falls off and flattens on the left and right is where the camera measured little to no red which in this case the dark and light ends of the spectrum. What’s important to consider here is that these flat areas, which represent little to no information, makes up part of your image’s information and often translates to washed out our grayish tones.

Open up your photo in GIMP and launch the levels tool by clicking “Colors>Levels”.

When the Levels window opens, click on the drop down from the Channels field to select your Red channel. You will also adjust your Green and Blue channels in the manner described below. Begin by adjusting the black and white sliders (small triangles below the Input Levels graph) by sliding the left or right towards the start and end of your graph curve from this:

…to this…

Your green channel graph…

…and your blue channel graph…

Already, we begin to see how trimming our levels eliminates color washout…

You can further adjust color values by lightly tweaking the mid-tone slider (gray triangle) on each of the Red, green and Blue color level graphs. While the levels tool requires a bit more fiddling to correct the exposure, tone and contrast of your digital photos, it affords you near limitless control.