A light-sensitive pixel array has an active area and an additional area of optically shielded pixels. A “noise figure” is derived as a measure of the prevalence of noise in the image signal without being affected by the presence of moving detail in the image. The noise figure is derived by measuring the difference in output between the pixels of at least one pair of pixels in the optically shielded area in one frame, repeating this measurement in a subsequent frame, and then subtracting one pixel pair difference from the other to give a difference of differences. In a preferred form, four pairs of pixels are used. The noise figure may be used to control digital signal processing of the image signal, such as be smoothing.