An image sensor includes a two-dimensional array of active pixels, where each active pixel in the two-dimensional array of active pixels includes a respective photodiode. The image sensor also includes a plurality of lock-in pixels at two or more regions of the two-dimensional array. Each of the plurality of lock-in pixels includes two pixel elements that share a common photodiode. Each pixel element of the two pixel elements in a lock-in pixel includes a charge storage node and a switch configured to receive a respective control signal to selectively connect the respective charge storage node to the common photodiode.