Lesson 02c: Key Points

The Levels Command

The Levels command is ideally suited to increasing or decreasing brightness values. Where contrast is concerned, Levels is better at increasing contrast than decreasing it.

The Levels command allows you to adjust the shadows, highlights and midtones of an image independently. It is available from the Image > Adjustments menu.

 

How to Read and Respond to a Histogram

• A histogram is a kind of bar graph that shows 256 vertical bars.
• Each bar represents one brightness value (far left), from black to white (far right).
• The height of each bar indicates how many pixels in your image correspond to that particular brightness value.
• Think of the histogram as a series of sand dunes. Consider where the majority of the sand resides.
• To manually adjust the histogram:
  • Move the black slider triangle to the point at which the dunes begin on the left.
  • Move the white slider triangle to the point at white the dunes begin on the right.
  • Take care not to make too many colors black or white. This will result in clipping, in which Photoshop renders whole regions of your image flat black or white.
  • When you move the central gamma slider, think of "center of gravity"; imagine you need to balance all the sand in the histogram on a teeter-totter. See more about the gamma value below.

 

Gamma Value

In the Levels command, the middle Input Levels number (which corresponds to the gray slider below the histogram) is calculated differently than black and white points. Expressed as an exponent, this gamma value multiplies all colors in a way that affects midtones more dramatically than shadows or highlights.

• The default gamma value of 1.00 raises the colors to the first power, thus no change.
• Higher gamma values make the midtones brighter.
• Lower gamma values make the midtones darker.

 

Source: "Adobe Photoshop CS3 One-On-One" by Deke McClelland