1. A method of processing image data representing an image of a scene to generate an estimate of noise present in the image data, the method comprising carrying out the following steps using a processor: generating filtered image data from initial image data representing the image of the scene, said filtered image data comprising spatially filtered pixel values determined using pixel values of pixels from the initial image data; evaluating a function for different values of an estimate of noise present in the initial image data, said function incorporating the term p k −λ in the denominator, where P k is the spatially filtered pixel value of pixel k after application of a spatial filter and λ is the estimate of said noise; and determining from said function an estimate of said noise for which said function has an optimum value.