You can adjust the exposure (in f-stop units) for previews with the Adjust Exposure control, which is located to the right of the Reset Exposure button  at the bottom of a Composition, Layer, or Footage panel. Each viewer can have its own Adjust Exposure setting.

When the Adjust Exposure control is set to a value other than zero, the center of the Reset Exposure button is orange .

The Adjust Exposure control doesn’t affect final output, only how video appears during previews. To make tonal adjustments to a layer that appear in final output, use the Exposure effect.

The Adjust Exposure control is useful for finding the black point or white point in an image. For example, drag the value control to the right (positive values) until the entire image is white except for one area; that area is the darkest area in the image.