After a lens has been entered into CODE V and the location of the image plane for an axial object point has been found using a paraxial image solve, we need to determine how the lens performs at off-axis object points. We begin with a small number of points in the object plane, called fields, that are used to represent extended objects. Then we’ll examine the passage of light through the lens by introducing the concepts of the aperture stop and field stop of the system, as well as its pupils and windows. These definitions may seem to be a needless complication ("C’mon, you’ve found the image, haven’t you?"). But the purpose of a lens is to transfer radiation (light) from an object to form an image. If the image is too dim or the object is not completely imaged, the design is a failure no matter how well resolved the image might be on axis. Students encountering the stops, pupils, and windows of a lens for the first time may find them bewildering. Thus, the title of this chapter. The objective for this chapter is to present stops, pupils, and windows as clearly as possible, so that you feel comfortable using them in the construction and evaluation of optical systems.
Thus far, the object to be imaged has been a single point on the optical axis of the lens. We could add many points covering the entire scene so that the object would be a 2D picture. This type of object is available in CODE V, but the approach takes significant computing power and time. By taking advantage of the symmetry of the lens, we could determine the quality of the image by tracing rays from many points along a line in the object plane. But even that may be too complicated. To start, we simplify the evaluation by using three object points (see Fig. 5.1).