An image sensor includes an array of pixels. Each pixel has at least one photo-sensitive element. Readout circuitry receives an analog signal from each pixel at a first time and at a second time, between which the analog signal changes. The image sensor further includes associated support circuitry which is a source of time variant noise. The signal level at both first and second times includes pixel noise. Sample and hold circuitry is provided to maintain substantially level at least a proportion of this support circuitry noise time invariant at the sensor output between the first time and the second time.