Eye relief

The eye relief of a telescope, a microscope, or binoculars is the distance from the last surface of an eyepiece at which the eye can be placed to match the eyepiece exit pupil to the eye's entrance pupil. Short eye relief requires the observer to press his or her eye close to the eyepiece in order to see an unvignetted image. An exit pupil larger than the observer's pupil wastes some light, but allows for movement between eye and eyepiece without vignetting. Conversely, an exit pupil smaller than the eye's pupil results in a vignetted image. The optical designer must also consider that the pupil of the human eye varies in size with lighting conditions and the age of the observer.

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. Exit pupils in sights are designed to be larger than the eye's pupil, to allow for a range of motion without vignetting.

See also

*Telescope
*Binoculars
*Monocular
*Telescopic sight
*Red dot sight
*Spotting scope

References

* [http://www.zeiss.com/c1256bcf0020be5f/Contents-Frame/59fc8b054116cb7c85256bcf00613215 Reference from Carl Zeiss Optical]
* [http://www.bushnell.com/products/tech_talk/binoculars.cfm#13 Definition from Bushnell]

Further reading

* [http://www.gunsandammomag.com/gun_columns/notes/relief_1021/ The essentials of eye relief]
* [http://www.eagleoptics.com/Default/Buying+Guide/pid2508 Information for eyeglass wearers]