1. A method of displaying images during a visual field test using a display device with a plurality of pixel elements, said elements generating a level of brightness which varies in response to the level of the drive signal supplied to the elements in accordance with a gamma curve, the method comprising: determining a first desired contrast and first range of brightness values for the images displayed during a first portion of the field test; setting the gamma curve to produce said first desired contrast and first range of brightness values in response to variations in the drive signals during said first portion of the field test; determining a second desired contrast and second range of brightness values for the images displayed during a second portion of the field test; and changing the gamma curve to a new setting to produce said second desired contrast and second range of brightness values in response to variations in the drive signals during said second portion of the field test.