1. A method of assessing the prevalence of noise in an image signal forming the output of an image sensor having an array of pixels, comprising: providing a reference area including a pixel array which is shielded from incident light; defining at least one pair of pixels within the reference area; measuring a first difference in output between the pixels of said pair in a first image frame; measuring a second difference in output between the pixels of said pair in a second image frame displaced in time from said first image frame; and deriving a noise value from a determined change in said first and second measured differences between the first and second image frames.