1. An image forming apparatus comprising: an image bearing member; image forming means for forming an image on said image bearing member in accordance with an image signal; wherein said apparatus is operable in a first mode in which the image is formed by said image forming means on said image bearing member which is rotated at a first peripheral speed and in a second mode in which the image is formed by said image forming means on said image bearing member which is rotated at a second peripheral speed different from said first peripheral speed; wherein a calibration for correcting a density level of the image formed on said image bearing member relative to a density level of the image signal, is executed for each of said modes.