Levels: Adjusting image contrast and grayscale range

One of the most useful tools in Photoshop is the Levels tool and dialog box. With this you can increase (or decrease) the range of grayscale values of an image to enhance contrast. The Levels tool displays the histogram of the gray values of the image. You use the histogram to modify the output values. This is a Histogram operation described in Lecture 3.

Procedure

1. Select Image:Adjust:Levels
2. The dialog box shows the histogram of grayscale values in the image (or selection). Make sure you check the 'Preview' box.
3. Input levels (number fields AND histogram slider triangles) dictate what image file range of values are to be mapped linearly to the output image.
4. Slide the black and/or white triangles inward to change the gray values mapped to the output image.
   
   Constraining the input levels to the new limits shown above results in this higher contrast:

   

5. The middle (gray) triangle sets the 'Gamma', or contrast level. In this image I've moved it to the left to output a gamma of 2.90.

• Auto: The 'Auto' button will automatically map the darkest and lightest pixel to black and white (respectively). This is the same as Image:Adjust:Auto Levels.
• Eyedroppers: Use the eyedroppers to select an image gray value. The dark dropper maps the selected value and all darker pixels to black. The light dropper does the inverse. The middle, gray tool maps the selected pixel intensity to medium gray throughout the image.