Image forming apparatus to acquire image bearing member surface potentials

An image forming apparatus comprising:

• a first image forming unit including a rotatable first image bearing member, a first charging member configured to come into contact with the first image bearing member to form a first charging portion and to charge a surface of the first image bearing member at the first charging portion, a first development member configured to supply a developer to the surface of the first image bearing member, and a first transfer member configured to transfer an image formed with a developer on a surface of the first image bearing member to a transfer material at a first transfer portion;
• a second image forming unit including a rotatable second image bearing member, a second charging member configured to come into contact with the second image bearing member to form a second charging portion and to charge a surface of the second image bearing member at the second charging portion, a second development member configured to supply a developer to the surface of the second image bearing member, and a second transfer member configured to transfer an image formed with a developer on a surface of the second image bearing member to a transfer material at a second transfer portion;
• an exposing unit configured to expose the surface of the first image bearing member charged by the first charging member and the surface of the second image bearing member charged by the second charging member;
• a transfer voltage applying unit configured to apply transfer voltage to the first transfer member and the second transfer member;
• a detecting unit configured to detect a first current value of electric current flowing from the first transfer member to the first image bearing member and a second current value of electric current flowing from the second transfer member to the second image bearing member while the transfer voltage is applied;
• and
• a control unit configured to control the transfer voltage applying unit and the exposing unit,
• wherein in a case where the detecting unit detects the first current value and the second current value, the control unit is configured to perform control so that timing when the surface of the first image bearing member exposed by the exposing unit reaches the first transfer portion is different from timing when the surface of the second image bearing member exposed by the exposing unit reaches the second transfer portion while the transfer voltage applying unit applies the transfer voltage having an absolute value smaller than an absolute value of discharge start voltage.