Introduction

Every optical system contains a surface or surfaces which limit the width of the beam passing through the system from each object point. These surfaces may be a lens surface, a face of a prism, a mirror or simply a plate containing an opening of suitable size. Since the amount of light in the beam depends upon the beam width, they control the image brightness. They also affect image quality and to some extent the size of the field-of-view.

The surface that controls the width of the beam from the axial object point is called the aperture stop and because a beam cannot be infinitely wide, every system must have an aperture stop. Typical examples are the iris of the eye and the diaphragm of a camera lens. For off-axis object points, the beam width may be controlled by other surfaces.

Figure 9.1 shows a system with two components, a simple surface that acts as the aperture stop and a simple lens. For the on-axis object point and for object points some distance off-axis, the aperture stop limits the width of the beam. As one moves farther off-axis, the lens mount begins to limit the beam. The obstruction of the rays by a surface other than the aperture stop is called vignetting. As one moves even farther off-axis, the lens mount finally blocks the entire beam passing through the aperture stop and the vignetting has become complete. In this example, the width of the field-of-view is limited by vignetting. In complex optical systems, more than one surface may cause vignetting.