It is of great significance to digitize ancient paintings and calligraphy. A typical way to acquire them is using a linear CCD based large area table scanner. But this kind of solution has great drawbacks in terms of precision as well as scanning range, which prohibit its use in museums and libraries. Our lab has recently developed a new equipment to solve these drawbacks and hopefully it would shed new light on the documentation of ancient paintings and calligraphy. This paper will discuss a feasible way to improve image sharpness in theory including the theoretical optimal aperture determination, the maximum optical resolution as well as the advantage of using large image sensor. The acquiring experiment shows the method and the scanning hardware can achieve satisfactory results.