Question
"What does "vignetting" mean as it applies to optical design?"
Synopsis
Definition: vignetting
Solution
"An optical ray is vignetted when it passes through (or would have passed through) the limiting aperture of an optical system (the system Aperture Stop), but instead gets blocked by some other surface in the system. This surface is typically a physical baffle or obstruction, or a lens mount holding a smaller aperture lens. Vignetting is typically worse for off-axis field points and results in an image that is bright in the center but decreases in brightness nearer to the edges of the image. The more vignetting there is in an optical system, the more the image brightness varies from center to edge. In a more general sense, vignetting can also refer to the blocking of rays at the aperture stop itself. Imagine holding a ring or small hoop (or cylinder) in front of your eyes. Orient the ring so that anything you see through the hole in the ring is framed by the circular hole of the ring itself. Now tilt the top of the ring away from your face so that as you look through the ring, that aperture of the ring hole appears to be more elliptically shaped. This is equivalent to a circular bundle of rays that traverses through an optical system from an off-axis field point. The circular bundle of rays must now pass through the aperture stop at an oblique angle. A portion of the circular bundle of rays will be blocked or "vignetted" so that the circular bundle becomes a bundle that has a nearly elliptical cross section. "
