Light quantity adjusting apparatus, color-registration-deviation amount detecting apparatus light quantity adjusting method, and color-registration-deviation amount detecting method

Light quantity is adjusted in an image forming apparatus including an image carrier by forming a pattern on the image carrier, detecting a light quantity of a reflecting light from the pattern and outputting a voltage, reading the light quantity of the reflecting light from the pattern detected once based on a present light quantity set, calculating from the light quantity read, a set light quantity corresponding to a target output voltage for printing, and setting the set light quantity calculated.