1. A method of displaying images for testing the vision of a patient 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 and wherein the gamma curve can be adjusted, said method comprising the steps of: determining the brightness level of the image to be displayed; and during the vision test, adjusting the gamma curve and selecting the level of drive signals supplied to the elements to produce an image with the desired brightness.