Abstract: A testing frame supports a test lens unit below a test image piece. A housing of an imaging device is secured to the frame below the lens unit such that an aperture in the housing is registered with the lens unit along an optical axis. An optical sensing unit is loosely mounted on the housing, is adjustable relative to the aperture along transverse axes of a sensing plane, and is rotatable relative to the housing about a rotary axis that is generally aligned with the optical axis. Upon operation, the sensing unit generates image signals corresponding to a captured image of the image piece. The image signals are processed so as to determine a skew angle of the captured image. The angular orientation of the sensing unit relative to the housing is then corrected in accordance with the skew angle.