1. A digital optical comparator, comprising: a holder for holding a part, wherein said part is mounted on a moveable carriage; a light source for illuminating the part; a camera for capturing an image of the illuminated part and a lens placed in front of said camera; and a data processing system for receiving the image, for determining a position and orientation of said part, for obtaining a CAD drawing of said part, for displaying said image and said CAD drawing on a computer screen, and for digitally overlaying said CAD drawing on said image, said data processing system including means for aligning said CAD drawing with said image by translating and rotating said CAD drawing to line up the edges of the part in the CAD drawing with the edges of the part in the image to permit comparison of said part with said CAD drawing, wherein said means for aligning said drawing with said image automatically translates and rotates the CAD drawing to the part by minimizing the least squares distance between control points on said CAD drawing and the corresponding points on said part in said image; said data processing system displays tolerances for said part so that a determination whether said part is manufactured within tolerances can be visually made; and the movement of said part on said moveable carriage is tracked based on the displacement of said moveable carriage, thus enabling the comparison of the part to the CAD drawing beyond the system's optical field-of-view, across the entire travel of said moveable carriage.