Page 313 - Modern Optical Engineering The Design of Optical Systems

292 Chapter Thirteen

Figure 13.2 The action of the field lens in increasing the field of view.

strength of the field lens that can be used. In practice, field lenses are
rarely located exactly at the image plane, but either ahead of or behind
the image, so that imperfections in the field lens are out of focus and
are not visible.

Periscopes and endoscopes
When it is desired to carry an image through a relatively long distance
and the available space limits the diameter of the lenses which can
be used, a system of relay lenses can be effective. In Fig. 13.3, the
objective lens forms its image in field lens A. The image is then relayed
to field lens C by lens B which functions like an erector lens. The image
is then relayed again by lens D. The power of field lens A is chosen so
that it forms an image of the objective at lens B; similarly, field lens
C forms an image of lens B in lens D. In this way, the entrance pupil
(which, in this example, is at the objective) is imaged at each of the
relay lenses in turn and the image of the object is passed through the

Figure 13.3 A system of relay lenses.