Abstract: The optical unit of an imaging system is disposed between a target object and an image plane, and permits the passage of cones of light from different point sources on the target object therethrough to form an image on the image plane. The optical unit is provided with an aperture stop to control sizes of the cones of light passing through the optical unit. The aperture stop forms an entrance pupil in relation to the optical unit. A rectifying mask, which is formed as an upright opaque plate, is disposed between the target object and the image plane. The aperture stop forms a corresponding aperture stop image on a plane of the rectifying mask, and the rectifying mask has a mask width which is narrower than the width of the corresponding aperture stop image, and a mask height which is greater than the mask width.