The eye relief in binoculars is the distance between the eyepiece lens and your eye. More precisely, it is the distance at which the projected virtual image “hovers” in the front of the eyepiece.
When the observer’s eye is exactly at this point then the projected virtual image will be perceived optimally and completely in its entirety.
If the eye of an observer is beyond this distance, the field of view is reduced and the projected image appears cropped.
If the eye is too closes, dark shadows (in the shape of kidney beans) appear around the outer edges of the image disturbing the field of view.
The eye relief is expressed in millimeters (mm). For comfortable viewing, the eye relief of binoculars should be around 15mm. You may need a longer eye relief if you have to wear eyeglasses when using binoculars.
Eye Relief And Wearing Glasses
Many people suffer from eye defects and need to wear glasses. Mild ametropia up to 1.5 dioptres can be corrected with the diopter adjustment. Stronger refractive defects or astigmatism requires wearing glasses when using binoculars.
When using eyeglasses, the distance between the eye and eyepiece increases. As a result, the eye may be outside the eye relief and the entire field of view may not be seen.
For eyeglass wearers, thus, a long eye relief of approximately 17 to 20 mm is required. Only then can the entire field of vision be seen without restriction.
Older and inexpensive binoculars are often not suitable for eyeglass wearers.
Eyecups and wearing glasses
Attached to the ocular barrel are the eyecups. They are intended to prevent stray light from entering the eyepiece, great for people that don’t need glasses. To see the entire field of view without shading, eyeglass wearers need to fold them in, thus shortening the distance between eye and eyepiece.
Binoculars Eye Relief is related to The Magnification
The calculation of eye relief of an optical instrument is complex. Generally, the higher the magnification the shorter the eye relief. A larger magnification reduces also the field of view. So a smaller field of view results in shorter eye relief too.
Extra Long Eye Relief
Long eye relief is a critical safety feature with riflescopes. Here the eye relief may be as far as 120 mm (4 inches). This makes sure your eye is not too close to the scope so it does not get hit when the rifle recoils and you end up with a nasty “scope bite”.
Minimum Eye Relief
Most binoculars that are available today, have an eye relief of at least 12 to 15 mm, which is enough if one does not wear glasses when viewing nature or observing wildlife. When wearing glasses while using binoculars a minimum of 16 to 20 mm or even more is recommended. Premium binoculars usually have eyepieces that have long eye relief suitable for wearers of glasses.