page_title: Exit pupil
section_title: Telescopes
hierarchical_section_title: Exit pupil / Telescopes
caption_attribution_description: English: Photograph of binoculars showing the 'exit pupil' as a white disc visible through the eyepiece lens.
context_page_description: In optics, the exit pupil is a virtual aperture in an optical system. Only rays which pass through this virtual aperture can exit the system. The exit pupil is the image of the aperture stop in the optics that follow it. In a telescope or compound microscope, this image is the image of the objective element as produced by the eyepiece. The size and shape of this disc is crucial to the instrument's performance, because the observer's eye can see light only if it passes through this tiny aperture. The term exit pupil is also sometimes used to refer to the diameter of the virtual aperture. Older literature on optics sometimes refers to the exit pupil as the Ramsden disc, named after English instrument-maker Jesse Ramsden.
context_section_description: For a telescope, the diameter of the exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio (f-number) of the telescope. In all but the cheapest telescopes, the eyepieces are interchangeable, and for this reason, the magnification is not written on the scope, as it will change with different eyepieces. Instead, the f-number f=L/D of the telescope is typically written on the scope, as well as the objective diameter D and focal length L. The individual eyepieces have their focal lengths written on them as well.
  In the case of binoculars however, the two eyepieces are usually permanently attached, and the magnification and objective diameter (in mm) is typically written on the binoculars in the form, e.g., 7×50. In that case, the exit pupil can be easily calculated as the diameter of the objective lens divided by the magnification. The two formulas are of course equivalent and it is simply a matter of which information one starts with as to which formula to use.
caption: The exit pupil appears as a white disc on the eyepiece lens of these 8×30 binoculars. Its diameter is 30 ÷ 8 = 3.75 mm.
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