Image forming apparatus including calculating portion configured to calculate surface potential of image bearing member

An image forming apparatus comprising:

• an image bearing member configured to bear an image;
• a charging member configured to charge said an image bearing member;
• an exposure portion configured to expose the image bearing member to light in order to form a latent image on a surface of the image bearing member;
• a developing member configured to form a toner image by developing the latent image, with a toner, formed on the surface of the image bearing member;
• a transfer member configured to transfer the toner image from the image bearing member onto a sheet;
• a current detecting portion configured to detect a current value of a current flowing between said transfer member and said image bearing member when a voltage is applied to said transfer member;
• a controller configured to discriminate, as a first discharge start voltage, a voltage applied to said transfer member when the current value detected by said current detecting portion under application of a positive voltage relative to a reference potential to said transfer member after the surface of said image bearing member is charged to the reference potential by said charging member reaches a predetermined value, and configured to discriminate, as a second discharge start voltage, a voltage applied to said transfer member when the current value detected by said current detecting portion under application of a negative voltage relative to the reference potential to said transfer member reaches the predetermined value;
• and
• a calculating portion configured to calculate, as a correction voltage, ½ of a sum of the first discharge start voltage and the second discharge start voltage, wherein
• said controller discriminates, as a third discharge start voltage, a voltage applied to said transfer member when the current value detected by said current detecting portion under application of a positive voltage to said transfer member relative to an exposed portion potential after the surface of said image bearing member is charged to a predetermined potential so as to be a target potential during image formation by said charging member and then the surface of said image bearing member is exposed to light by said exposure portion with an exposure amount during the image formation reaching a predetermined value, and discriminates, as a fourth discharge start voltage, a voltage applied to said transfer member when the current value detected by said current detecting portion under application of a negative-side voltage relative to the exposed portion potential to said transfer member reaches the predetermined value,
• said calculating portion calculates ½ of a sum of the third discharge start voltage and the fourth discharge start voltage as a surface potential of said image bearing member after the surface of said image bearing member is exposed to light by said exposure portion, and
• said controller sets an image forming condition on the basis of a result obtained by subtracting the correction voltage from the calculated surface potential.