Images on this site are best viewed in 24 or 32 bit display mode. If you cannot see all 13 steps in the image below or if bottom gradient is not smooth (posterized), then you'll need to adjust your monitor and display settings.

Adjust monitor contrast to maximum and adjust monitor brightness such that every step of the greyscale step wedge is visible. The two leftmost (darkest) and two rightmost (lightest) steps should be barely discernible.