Page 192 - Modern Optical Engineering The Design of Optical Systems

Chapter
9

Stops, Apertures, Pupils

and Diffraction

9.1 Introduction
In every optical system, there are apertures (or stops) which limit the
passage of energy through the system. These apertures are the clear
diameters of the lenses and diaphragms in the system. One of these
apertures will determine the diameter of the cone of energy which the
system will accept from an axial point on the object. This is termed
the aperture stop, and its size determines the illumination (irradiance)
at the image. Another stop may limit the size or angular extent of the
object which the system will image. This is called the field stop. The
importance of these stops to the photometry (radiometry) and perfor-
mance of the system cannot be overemphasized.
The elements of an inexpensive camera system are sketched in Fig. 9.1
and illustrate both aperture and field stops in their most basic forms.
The diaphragm in front of the lens limits the diameter of the bundle of
rays that the system can accept and is thus the aperture stop. The mask
adjacent to the film determines the angular field coverage of the system
and is quite apparently the field stop of the camera.
Not all systems are as obvious as this, however, and we will now
consider more complex arrangements. Because the theory of stops is
readily explained by the use of a concrete example, the following dis-
cussions will be with reference to Fig. 9.2, which is a highly exaggerated
sketch of a telescopic system focused on an object at a finite distance.
The system shown consists of an objective lens, erector lens, eyelens,
and two internal diaphragms. The objective forms an inverted image

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