A method of measuring a dark adaption curve of an eye of a subject using an apparatus which is: arranged to provide a bleaching light and a target light ostensibly visible to rods;a background light ostensibly invisible to rods, the background light and target light together being suitable for rod-dominated stimulation, the background light and target light being superimposable upon one another, the target light being located relative to a fixation point being presentable to the subject such that the target light is incident on a parafoveal region of the subject's retina when the fixation point is viewed by the subject; the method comprising: a) stimulating the subject's retina with a low level bleach using the bleaching light, wherein the low level bleach is sufficient to ensure a measured rod slope is independent of the bleach but not so high as to delay an onset of a rod-cone break; 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 the 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. arranged to provide a bleaching light and a target light ostensibly visible to rods;a background light ostensibly invisible to rods, the background light and target light together being suitable for rod-dominated stimulation, the background light and target light being superimposable upon one another, the target light being located relative to a fixation point being presentable to the subject such that the target light is incident on a parafoveal region of the subject's retina when the fixation point is viewed by the subject; the method comprising: a) stimulating the subject's retina with a low level bleach using the bleaching light, wherein the low level bleach is sufficient to ensure a measured rod slope is independent of the bleach but not so high as to delay an onset of a rod-cone break; 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 the 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. a) stimulating the subject's retina with a low level bleach using the bleaching light, wherein the low level bleach is sufficient to ensure a measured rod slope is independent of the bleach but not so high as to delay an onset of a rod-cone break; 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 the 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.