The fill factor of an image sensor array is the ratio of a pixel's light sensitive area to its total area. For pixels without microlenses, the fill factor is the ratio of photodiode area to total pixel area,^[1] but the use of microlenses increases the effective fill factor, often to nearly 100%, by converging light from the whole pixel area into the photodiode.^[2]

Another case that reduces the fill factor of an image is to add additional memory beside each pixel, so as to achieve a global shutter on CMOS sensors.^[3]