Image forming apparatus and optical sensor detecting target formed on image carrier or recording medium using effective light-receiving elements

An image forming apparatus includes a light-receiving unit that includes light-receiving elements arranged along a movement direction of a target formed on an image carrier, and is arranged so as to receive light emitted from a light-emitting unit and specularly reflected off the image carrier; a member configured to limit a size, in the movement direction, of diffused reflection light that is incident on the light-receiving unit; a selection unit configured to select whether or not each of the light-receiving elements in the light-receiving unit is an effective light-receiving element; a generation unit configured to generate a detection signal from an output of the effective light-receiving element; and a detection unit configured to detect a target formed on the image carrier based on the detection signal.