A solid state image sensor has an array of pixels in which each column has a reset voltage line and a read line. The sensor is reset and read a row at a time, with reset-related values held in a frame buffer for subsequent subtraction from read values. Reset-related values are derived in each column by sampling the voltage during reset on one capacitor and the voltage on release of reset on a second capacitor, and differencing these values to provide an output for the frame buffer. This provides a reduction in the size of frame buffer which would otherwise be required.