1. An image forming apparatus for forming an image with toners of a plurality of colors, comprising: an image bearing member configured to bear a registration correction pattern that includes patch images in the respective colors; a sensor configured to direct light to the registration correction pattern formed over the image bearing member and receive light reflected therefrom; a pattern forming section configured to form the registration correction pattern over the image bearing member; and a position correction section configured to specify positions of the patch images of the respective colors included in the registration correction pattern based on outputs from the sensor corresponding to a surface of the image bearing member and to the registration correction pattern, and specify a correction amount to be applied to an image forming position for each of the plurality of colors based on the positions of the patch images of the respective colors, wherein the sensor includes a first photodetector configured to receive specular reflection components of the reflected light and a second photodetector configured to receive diffuse reflection components of the reflected light, the position correction section specifies the position of each of the patch images of the respective colors based on a difference between a sensor output by the first photodetector and a sensor output by the second photodetector, the position correction section specifies, as a first reference position, a position of each of the patch images of the respective colors included in the registration correction pattern formed by the pattern forming section when the image bearing member is in an initial state, the pattern forming section forms a black toner layer on the image bearing member to form the registration correction pattern on the black toner layer, the position correction section specifies, as a second reference position, a position of each of the patch images of the respective colors included in the registration correction pattern formed on the black toner layer, and in a registration correction carried out when the image bearing member is in a time-elapsed state that is a state after the initial state, the position correction section specifies the position of each of the patch images of the respective colors included in the registration correction pattern that is formed by the pattern forming section at the time of the registration correction and corrects the correction amount specified in the registration correction and to be applied to each of the image forming positions for the respective colors, the correction being performed based on the first reference position and the second reference position.