Abstract: A method of reducing memory requirement in the compensation memory unit of a scanner. The method includes providing an even compensation data value and an odd compensation data value and averaging the two to produce an odd-even compensation data value. Only half as much memory space is required to hold the averaged odd-even compensation data values.

Abstract: A focus calibrating method for an image scanning device by testing a focus chart is disclosed. The focus chart has a base sheet and a plurality of black bars with different widths arranged in parallel on the base sheet and spaced from each others. The focus chart is adhered on a bottom surface of a top panel supporting the document positioning plate of the scanner and disposed facing the optical scanning module of the scanner. The focus method includes steps of moving the optical scanning module to read the image data of the focus chart, calculating a focal length value of the image data of the focus chart, judging whether a predetermined optimal focal length is reached or not according to the calculated focal length value, repeatedly moving the optical scanning module with respect to the focus chart to adjust the focal length until the optimal focal length is reached.