1. A document camera for showing image of an object on an image display, comprising: a video-capturing device for picking up an initial image of the object to realize a first image data; an image processor electrically connected to said video-capturing device for deriving a second image data and a third image data from said first image data in response to a zoom operation, wherein said second image data results in a zoomed image of the object and said third image data results in a scaled image of the object; a position information controller electrically connected to said image processor for generating a mark frame data in response to said zoom operation, wherein said mark frame data results in a mark frame; and an image synthesizer electrically connected to said position information controller and said image processor for synthesizing said zoomed image, said scaled image and said mark frame to generate a combined image shown on the image display, wherein said mark frame superimposes said scaled image to indicate a result of said zoom operation.