TABLE OF CONTENTS In Fig. 2.10A we show a simple telescope. In order to cover the field of view indicated by the dashed rays, the eyelens would require a very large diameter. In Fig. 2.10B a field lens located at the internal image plane converges the light rays at the edge of the field toward the axis so that they will pass through a smaller eyelens. Note that this shortens the eye relief; a short eye relief and a small-diameter eyelens go hand in hand. One can lengthen the eye relief with a negative (diverging) field lens; this requires a large eyelens diameter. If A ? is the apparent field angle and R is the eye relief, the required diameter of the eyelens (for zero vignetting) is equal to the exit pupil diameter plus A ? R. Note that by placing the field lens exactly on the internal image location, the size of the image and the power of the telescope are unchanged. [Using Eq. (1.28), if we set d = f a , we find that f ab = f a .] In actual practice, field lenses are usually located away from the image plane, because otherwise any flaws, scratches, dirt, etc., on the lens would be seen, nicely in focus and magnified by the eyelens. Shifting the field lens away from the focus will cause only a small change in image size and system power.
