Image forming apparatus and method of detecting amount of residual developer

An image forming apparatus which can change an AC voltage to be applied to a developer carrying member for carrying developer stored in a developer container, said apparatus comprising:

• a first detecting member configured and positioned to detect a first value corresponding to the amount of developer in the developer container;
• a second detecting member configured and positioned to detect a second value corresponding to the amount of developer in the developer container;
• a processing portion configured and positioned to determine the amount of developer in the developer container according to the value detected by said first detecting member or said second detecting member when the AC voltage is applied to the developer carrying member;
• and
• an image forming portion configured and positioned to form an image with the developer stored in the developing container,
• wherein said apparatus is configured to change the AC voltage to a first AC voltage and a second AC voltage,
• wherein, when the AC voltage is the first AC voltage, said processing portion selects one of said first detecting member and said second detecting member and determines the amount of developer in the developer container according to the value detected by selected detecting member,
• wherein, when the AC voltage is changed to the second AC voltage, said processing portion selects one of said first detecting member and said second detecting member and corrects the value detected by selected detecting member, and determines the amount of developer in the developer container according to the corrected value, and
• wherein a value to be used for correcting the first value detected by said first detecting member and a value to be used for correcting the second value detected by said second detecting member are different from each other.