You can use the Posterize effect to reduce the number of brightness levels in your image. Select or change the Posterize options to apply this effect to your image.
You can save your settings as a preset for future use.
To Apply a Posterize Effect:
You can also access the Special Effects menu from the Filter drop-down menu or Filter toolbar.
You can right-click a slider to reset to the default value.
You can use the Edit Brush to paint this effect onto specific areas of your image.
You can use the Gradient tool to transition this effect across specific areas of your image.
You can use the Radial Gradient tool to apply effects around, or directly to, a center point.