Patent ID: 8185003

Claim:
An image forming apparatus comprising: a rotation member which is used for image forming or carries a printing medium; a detector adapted to detect light from said rotation member; a first acquisition unit adapted to acquire first waveform data from a surface of said rotation member based on detection by said detector; a second acquisition unit adapted to acquire second waveform data from the surface of said rotation member based on detection by said detector, the second waveform data being detected from at least part of a detected section of surface of said rotation member on which the first waveform data has been detected; a calculator adapted to calculate information on a circumference of said rotation member based on matching between the acquired first waveform data and second waveform data; and a determination unit adapted to determine whether or not to use the calculated information on the circumference by comparing the acquired first waveform data and second waveform data, wherein when said determination unit determines not to use the calculated information on the circumference, said calculator recalculates information on the circumference of said rotation member, and wherein said calculator calculates the information on the circumference of said rotation member based on prestored information before calculation of information based on matching by said calculator.