exit pupil

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The smallest cross-section through the beam of light from an eyepiece through which all of the light from the eyepiece passes. The observer's eye should be at the exit pupil to see the full and brightest field of view. The diameter of the exit pupil is the focal length of the eyepiece divided by the focal ratio of the telescope. Thus an 18-mm eyepiece will have an exit pupil of 3 mm on an f/6 telescope. A simple way of finding the diameter of the exit pupil for binoculars is to divide the aperture by the magnification. For 7 × 50 binoculars, the size of the exit pupil will therefore be just over 7 mm. If the exit pupil is larger than the diameter of the dark-adapted eye (about 7 mm on average), some light will not enter the eye and will be wasted.

Subjects: Astronomy and Astrophysics.

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