A document camera is used to show image of an object on a display. In the document camera, a first image data of the object obtained by a video-capturing device is derived into a second image data and a third image data by an image processor in response to a zoom operation, wherein the second image data results in a zoomed image of the object and the third image data results in a scaled image of the object. In response to the zoom operation, there is also a mark frame data generated by a position information controller, which results in a mark frame. The zoomed image, scaled image and mark frame are then synthesized into a combined image by an image synthesizer, and the mark frame superimposes the scaled image to indicate a result of the zoom operation.