1. An apparatus for characterising a subject's visual response, comprising a data processor, a display and an input device, wherein the data processor is arranged to present evaluation images at different positions on the display such that they occur at different positions within the subject's field of view, and wherein each evaluation image comprises a contribution of at least two items selected from a list comprising: a base image a test image, and a noise image, and wherein the data processor is responsive to the input device such that the subject can indicate whether they can see the test image in the evaluation image, and the data processor is further arranged to evaluate the subject's responses so as to give an indication of one or more of visual efficiency, internal noise and visual sensitivity as a function of position within the subjects field of view.