Level Adjustment
The Level Adjustment dialog box shows the distribution of brightness levels in your image. To adjust the histogram plot, drag the triangles, located below the horizontal bar to the desired values. The left (black) triangle changes the shadow levels. The right (white) triangle changes the highlight levels. The middle (gray) triangle controls the midtone levels.
Level Adjustment tab 
Shadow control
Highlight control
Midtone (Gamma) control
By varying the Shadow and Highlight points you can change the relative shades and highlights of the image. The Shadow point represents the darkest value and the Highlight point is the brightest value.
The Midtone point represents the Gamma setting for your particular screen. On the histogram, the Shadow and Highlight scales are divided into 255 units.
Therefore, all pixels in the original that are darker than the current shadow point are now completely black in the enhanced image. Conversely, all pixels in the original that are brighter than the current highlight point are now completely white in the processed image.
Setting the highlight point to 200 renders all pixels greater than or equal to 200, on the original, completely white in the processed image. Similarly setting the Shadow point to 200 renders all points less than or equal to 200, on the original, completely black in the enhanced image. Photographers often refer to these effects as
3.10 Scanning
