Modern Lens Design, 2nd Edition

Chapter 7Eyepieces and Magnifiers

7.1 Eyepieces

The eyepiece of a telescope or microscope is an unusual optical system in that its pupil must be located completely outside the system. The aperture stop is usually located at the objective lens. The exit pupil is the image of the stop, and it must lie a suitable distance away from the eyepiece so that the eye can be placed at the pupil (to see the full field of view). This distance, or eye relief, must be at least 9 or 10 mm, just long enough to clear the eyelashes; an additional 5 mm eases the situation considerably. An eye relief of 20 mm or so is about the minimum necessary for comfortable use by spectacle wearers. Many systems depart from these guidelines. Any system subject to sudden motion ...