1. A method for detecting change in retinal oxygen saturation over time, said method comprising the steps of: capturing a first group of retinal images of two or more narrow band wavelengths, wherein at least one of said narrow band wavelengths is at one or more isobestic wavelengths, by means of camera; measuring optical density of one or more areas of interest in the first group of retinal images; determining oxygen saturation in said first group of retinal images; storing said first group of images on a data storage device; capturing a second group of retinal images of two or more narrow band wavelengths, wherein at least one of said narrow band wavelengths is at one or more isobestic wavelengths, by means of camera; measuring the optical density of one or more areas of interest in the second group of retinal images; storing said second group of retinal images on a data storage device; determining oxygen saturation in said second group of retinal images, said first group and second group of retinal images, captured at different times, are spatially registered, by mapping features in the first group of images to the corresponding features of the second group of images; and detecting a change in retinal oxygen saturation between said first group of retinal images and said second group of retinal images; evaluating image comparability of said first and second groups of images captured at said one or more narrow band wavelengths, to determine if comparison between retinal oxygen saturation measurements at different point in time is valid, by comparing said optical density between said first and second groups of retinal images, wherein the step of evaluating image comparability comprises the steps of: measuring the optical density of said first and second groups of retinal images at one or more isobestic wavelength; scaling said optical density according to vessel width; and measuring the difference of said scaled optical density between said first and second groups of retinal images by calculating the difference of said optical densities.