Image forming apparatus

An image forming apparatus includes a controller. The controller is configured to switch between (i) first speed printing in which a rotation speed of the polygon mirror is a first speed and the power of a laser beam from a first and second laser light sources is a first laser power, and (ii) second speed printing in which the speed of the mirror is slower than the first speed and the power of laser beam from the light sources is a second power weaker than the first power. The controller is configured to control an output timing of the laser beam from the light sources in the second speed printing based on the timings at which the laser beam is detected by first photodetector and second photodetectors when the laser powers of the first laser light source and the second laser light source are changed, respectively.