In an auto-fitting mode, a scanner prescans an input document to determine the location and size of an image within the input document. The area of the image is defined in terms of a rectangular area encompassing all area of the input document where the reflectance of the image data is less than 20% of the reflectance value of the input documents background. Once the image area has been defined, a magnification factor is calculated. The input document is then rescanned applying the calculated magnification factor and properly registered so that the image area can be centered when it is rendered on to the desired output paper. If the image area detected is such that the longer side of the rectangular area does not have the same orientation as the desired output paper, the image is automatically rotated prior to the magnification and registering processes. The magnification factor can be calculated either anamorphically or isotropically.