1. An image forming apparatus comprising: a plurality of image forming units; an image carrier on which images formed by said plurality of image forming units are transferred; a sensor configured to irradiate said image carrier with light, and detect reflected light; a detection unit configured to detect, based on an output value of said sensor, a feature value of an edge of a patch image which has been formed by said image forming unit and transferred to said image carrier; a determination unit configured to determine a position of the patch image using the output value of said sensor corresponding to the patch image and the feature value of the edge; and a color registration control unit configured to perform color registration control based on the determined position of the patch image.