According to a first aspect of the present invention there is provided a method of measuring a property of an eye of a subject using an apparatus which is: arranged to provide a bleaching light a background light ostensibly invisible to rods (e.g. substantially red and/or having or being a wavelength greater than 580nm) and a target light ostensibly visible to rods (e.g. substantially green and/or having or being a wavelength less than 580nm and greater than 400nm e.g. around 514nm +/ 50nm) the background light and/or target light (together) being suitable for rod dominated stimulation the background light and target light being superimposable upon one another and the target light being presentable to the subject such that the target light is incident on a parafoveal region of the subject s retina the method comprising: a) stimulating the subject s retina with a low level bleach using the bleaching light b) after the bleach has been undertaken modulating a target light at an initial luminance at which modulation of the target light is not perceivable by the subject whilst a background light remains substantially constant c) increasing the luminance of the modulated target light d) receiving an input from the subject when modulation of the target light is perceivable by the subject thus indicating a rod related threshold e) after said input decreasing the luminance of the modulated target light and f) repeating parts c) to e) of the method to determine a series of rod related thresholds over a period of time.
