Manual adjustment of brightness and contrast in the Levels tool

Manual adjustment of the Levels tool to improve contrast and brightness is possible instead of an Auto Levels correction. Alternatively, adjustment can be made as a part of refining the settings based on the intended output or creative intent. Adjustment of the combined RGB Levels is not only confined to color but it is especially useful for improving the tonality of black and white images.

First of all, set the black point slider to lessen clipping or blocking and then set the white point slider while checking on-screen for clipping of highlights. Once selected, adjust the middle slider to the left to lighten or to the right to darken the image without clipping either end-point. In most cases, this will be subjective, depending on the image content. 

If the final destination is in print and the image contains nuanced detail in the highlights that you would like to retain, adjust the highlight clipping point slider to just less than pure white at 255 (e.g., 250), making the image slightly darker.

If the image contains specular highlights (without any detail), it is common practice to leave these to clip and adopt the same hue of the paper.

1. Select an image, go to the Levels tool, and select RGB (combined channel mode).
   
   
2. Optionally, click on the tool’s Action menu (...) and select Preferences… The Preferences window will open. Add a checkmark to the Enable Shadow Warning option.
   
   
   
   
3. Select Show Exposure Warnings from the main menu or toolbar or use the shortcut Cmd/Ctrl+E.
   
   
   
   
4. Set the black point in the image by moving the left end-point slider beneath the histogram to the point where the histogram begins to rise on the left (i.e. to define the darkest shadow). Stop when the blue-colored shadow warning is displayed in areas of the image that are being clipped. Zoom into the shadow areas to confirm.
   
   
5. Adjust the white point slider to the point where the histogram rises on the right (i.e., the brightest highlight). Areas denoting the on-screen highlight warning should be minimal, with the exception of specular highlights (light sources or reflections) which can be left to clip.
   
   
6. Optionally, adjust the High Dynamic Range tool’s Highlight slider to the right to reduce clipping.
   
   
7. Modify brightness with the middle (gamma) slider, moving to the left to lighten or to the right to darken. 

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