Abstract: A method of diagnosing glaucomatous field defects of particular use in diagnosing the early stages of glaucoma and ocular hypertension is disclosed. The method comprises displaying an image comprising an annulus of a first color on a background of a second color to the patient, the annulus having a gap in it at a variable position. The patient is asked to identify the location of the gap and the tonal contrast between the first color and the second color is varied to determined the color contrast sensitivity threshold of the patient. Also disclosed is an apparatus for performing the method.