1. A method of measuring the contrast sensitivity of a subject, comprising the steps of: displaying chains of grating stimuli at randomly interleaved spatial frequencies, each of said grating stimuli consisting of dark and light bars and having one of two alternative orientations; prompting the subject for the orientation of each said grating stimulus from said one of two alternative orientations; varying the contrast level of the next grating stimulus according to whether the subject correctly identified the orientation of the previous grating stimulus, wherein a correct response decreases the contrast level of the next grating stimulus in the chain by a first amount, S_down, and whereas an incorrect response increases the contrast level by a second amount, S_up; varying said first and second amounts by which the contrast level is decreased or increased as the measurement progresses, while keeping the ratio between said first and second amounts the same; and finding the best estimation of the contrast threshold at each measured spatial frequency from the subject responses to the grating stimulus at the known contrast levels.