An imaging device for evaluating focusing of light by one or more optical elements on an imaging sensor, the imaging device having one or more processors configured to compare a first value of a characteristic of a first signal from a first photoelectric conversion structure of the imaging sensor having a first light-receiving surface area and a second value of the characteristic of a second signal from a second photoelectric conversion structure of the imaging sensor having a second light-receiving surface area smaller than the first light-receiving surface area, and to determine whether focusing is achieved by the one or more optical elements.