Editing Photos - Editing Levels with Dropper Tool

Some image editing software offer the option of using an eye-dropper tool to set levels when editing an image's histogram.  In GIMP (the software I use) the tool for editing levels offers both slider and dropper options at the same time.  The dropper tool tends to be less precise than using the slider option, but can be quicker when trying multiple settings.  The effects of using the dropper tool can be fine-tuned using the slider.

Gimp offers two options when using the dropper tool.

1)  Editing values only.

This option is used by utilizing the two dropper buttons located directly under the image's histogram.  Clicking on the left button and then clicking on a spot in the image sets the dark value to the same value as the location chosen.  Clicking on the right button does the same for the light value.

There is no option for setting just the mid-tone value in my version of GIMP using a dropper tool.  This has to be set with the slider.

Here's an image edited with the value dropper:

Unedited Image

Light Value Set With Dropper

Both Values Set With Dropper

  The top image is the original with no changes made.  The second image shows the effect of setting the light value using the dropper tool.  I chose a section in a lighter area of the post.  Using the sky would have had no effect.  The sky is white as a result of being overexposed in order to keep the rest of the image within the proper exposure range.  The bottom image is after both values have been set.  I used the spot in the lower right corner where the paint has flecked off the post.

Setting the dark value first had very little impact so I chose to omit an example of doing so.


2) Editing All Channels

The dropper buttons on the bottom of the Levels window will effect the color channels instead of just value.  This means these buttons will change the image's color as well as altering lightness, darkness and contrast.

The exact effect of using these droppers will depend on the area selected when the dropper is used.  Selecting an area other than white, black or gray will effect color.  This effect is more pronounced the farther away the selected area is from true neutral.

Examples, using the middle button choosing mid-range value areas.

In the top image, I selected an area located in the white spot on the cicada's thorax.  A spot in the post was used in the second image.  An area located in the background tree was used in the third.  The yellow stripe on the cicada's back was used in the bottom image.

The bottom two images clearly show the color change that can occur when using the eye-dropper tool in coordination with color channels to edit levels.

Using the bottom dropper tools can be useful when dealing with an image that needs color correction.  Photos taken using artificial lighting can need correction as can scanned images of old photographs or slides.  Using the bottom tool and selecting an area that should be white, black or neutral gray can automatically correct the color.

You might have noticed that the histogram did not change using the bottom option.  To see the effect of using this option, you need to have the view set to see one of the channels.


The auto button does not use a dropper tool.  This button results in the software altering the color channels using built-in algorithms.