Below you see a bright and a dark stripe, each with 8 shades of grey. If your Monitor is calibrated correctly you should be able to distinguish all the 16 shades or at least the same amount of shades on the lower and upper stripe. Windows users should look for the official (.icm) Color Management Profile of their monitor manufacturer and get a system tool for gamma calibration like "Adobe Gamma". Mac users shouldn't have a problem at all as their hardware is usually correctly calibrated :D