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The eye relief of an optical instrument (such as a telescope, a microscope, or binoculars) is the distance from the last surface of an eyepiece within which the user's eye can obtain the full viewing angle. If a viewer's eye is outside this distance, a reduced field of view will be obtained.
Eye relief typically ranges from about 2 mm to 20 mm, depending on the construction of the eyepiece. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic. Until recently, and still quite commonly, eyepieces of a short-focal length have had a short eye relief.
Eye relief – Eye relief is the distance from the rear eyepiece lens to the exit pupil or eye point. [15] It is the optimal distance the observer must position their eye behind the eyepiece to see a non-vignetted image. The longer the focal length of the eyepiece, the greater the eye relief.
Since the eye's pupil varies in diameter with viewing conditions, the ideal exit pupil diameter depends on the application. [1] An astronomical telescope requires a large exit pupil because it is designed to be used for looking at dim objects at night, while a microscope will require a much smaller exit pupil since an object being observed will be brightly illuminated.
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Still, “Erfles remain quite popular owing to their large eye lenses and good eye relief.” [5] “The basic Erfle,” according to an overview of “Common Telescope Eyepiece Designs,” “uses five elements in three groups and delivers a wide apparent field of view of about 60-degrees.
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The sharp pain is combined with eye redness, tearing, facial flushing and nasal congestion. The pain can be so intense that it can be mistaken for a brain aneurysm, Cohen noted.
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