Patent ID: 7263318

Claim:
An image forming apparatus comprising: electrostatic image forming means which charges an image bearing member and forms an electrostatic image; developing means which contacts and develops the electrostatic image with a developer including toner and carrier, wherein the developing means includes a developer bearing member which bears and carries the developer, the developer bearing member having a magnetic field generation means therein, wherein when a contact pressure of developer borne by said developer bearing member against the image bearing member is P (Pa); a circumferential velocity of the developer bearing member is V s1 (mm/s); a circumferential velocity of the image bearing member is V Dr (mm/s); and an elastic deformation ratio of the image bearing member is W (%), and wherein an index S defined by the following formula is within a range of 650 to 60500: S = P ⨯ {  v S1 - v Dr  v Dr } ⨯ { 8.50 ⨯ 10 5 ⨯ exp ⁡ ( - 0.32 ⁢ ⁢ W ) } .