The slider controls directly below the histogram are the Input Level controls. These controls are used to set the range of colour values used in the image:
You can use the Input Level controls to increase the contrast in the image. For example, if you set the minimum value to 50, pixels with colour values less than 50 are remapped to 0 (black). Pixels with colour values greater than 50 are left unchanged. This darkens the image and increases the contrast in the shadow areas.
If you set the maximum value for the range to 150, pixels with values greater than 150 are remapped to 255 (white). Pixels with colour values less than 150 are left unchanged. This brightens the image and increases the contrast in the highlight areas.
You can also set the maximum and minimum limits for the colour range by entering the values directly in the Minimum and Maximum Input fields on either side of the histogram.
To increase the contrast in an image:
The value of the lower limit for the colour range appears in the Minimum Input field.
The value of the upper limit for the colour range appears in the Maximum Input field.