An image sensor array of active pixel elements is arranged in rows and columns. Each column has an output circuit for reading out pixel image signals. The output circuit includes a pair of sample capacitors, a switching circuit operable based upon pixel switches for applying pixel voltages to the pair of sample capacitors. At least one optically masked pixel is provided so that output image signals obtained therefrom represent substantially only the column fixed pattern noise (FPN). An image processing circuit records the column FPN for each column from the optically masked pixels, records the image signal from the sensor array of active pixels, and subtracts the column FPN column-wise from the image signal.