1.3.3.1 Histogram Level [1]

The Photoshop command given by selecting menu item Image, select Adjust select Auto Levels will automatically give the maximum range of contrast and brightness. This is usually what you want, but sometimes this needs fine tuning by manually choosing the transform levels. This is done by the "Levels" command described in the next paragraphs.

In Photoshop, the menu item Image, select Adjust, select Levels, gives the following pop dialog:

which show the pixel histogram and defines the following default pixel value translation:
which does not change the image in any way - because each input pixel value is mapped to the same pixel value. Each of the 5 parameters in the form of either the 5 triangles (3 for input and 2 for output) can by clicked on and dragged by the mouse (or the numbers in the 5 text fields can be edited).