Eye relief

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The eye relief of a telescope, a microscope, or binoculars is the distance from the eyepiece the eye can be placed at which the exit pupil is approximately the same size as the eye's pupil. A poorly designed optical system may force the observer to press his or her eye close to the eyepiece in order to see an unvignetted image, or alternatively may have an exit pupil larger than the observer's pupil at a comfortable viewing position, resulting in wastage of light and a dimmer than optimum image.

Eye relief can be particularly important for eyeglass wearers and shooters. The eye of an eyeglass wearer is typically further from the eye piece which necessitates a longer eye relief in order to still see the entire field of view.

For a shooter, eye relief is an important safety consideration. An optic with too short of an eye relief can cause dangerous contact between the optic and the eye of the shooter during recoil.