How to Isolate Midtones Using Threshold in GIMP

Isolating midtones in GIMP allows for precise tonal adjustments, specialized layer masks, and targeted color grading. This guide explains how to use GIMP’s Threshold tool to simultaneously define lower and upper luminance limits, enabling you to extract an exact midtone range from any image without affecting the deepest shadows or brightest highlights.

Step 1: Prepare Your Layer

  1. Open your image in GIMP.
  2. Duplicate the target layer by right-clicking it in the Layers panel and selecting Duplicate Layer (or pressing Shift + Ctrl + D). This ensures your original image remains intact while you generate your midtone map.

Step 2: Open the Threshold Tool

  1. Select the duplicated layer.
  2. Navigate to the top menu and click Colors > Threshold....
  3. The Threshold dialog will open, displaying a histogram that represents the distribution of tonal values from 0 (pure black) to 255 (pure white).

Step 3: Adjust the Low and High Sliders

The Threshold tool features two adjustment markers directly below the histogram: the lower threshold on the left and the upper threshold on the right.

  1. Set the Lower Limit: Click and drag the left slider toward the center to cut out deep shadows. Any luminance value below this number will be excluded.
  2. Set the Upper Limit: Click and drag the right slider toward the center to cut out pure highlights. Any luminance value above this number will be excluded.
  3. Isolate the Range: The area highlighted between the two sliders represents your isolated midtones. In the image preview, all pixels falling within this defined window will turn white, while pixels falling below the low threshold or above the high threshold will turn black.
  4. Enter exact numeric values in the input boxes below the histogram if you need precise mathematical limits.
  5. Click OK to apply the threshold.

Step 4: Convert the Isolated Midtones into a Mask or Selection

Once the threshold is applied, the high-contrast black-and-white output can be used to control edits on your original image:

  1. Go to Select > By Color (or press Shift + O).
  2. Click on the white area of the thresholded layer to select the isolated midtones.
  3. Hide or delete the threshold layer.
  4. Select your original image layer.
  5. Apply adjustments (such as Curves, Levels, or Color Balance) directly within the active selection, or click Layer > Mask > Add Layer Mask... and choose Selection to permanently restrict subsequent edits to that isolated midtone band.