In the determination of the length of a sheet that has been inserted in a printer, an optical sensor scans the length of the sheet as it is fed into the printer, and a counter is controlled to provide a count corresponding to the length of the sheet. When the sensor encounters a mark on the sheet, such as a line, it provides an output indicating that the sheet is not present. This erroneous output is corrected by moving the sensor laterally of the sheet to a position displaced from the mark to enable further sensing of the presence of the sheet in a region that does not include the mark.