The term “eye relief” can be somewhat confusing due to its unusual construction but it is one of the most important factors to consider when purchasing any kind of optics such as binoculars, spotting scopes, microscopes or telescopes. The simple definition of eye relief is an exact measurement of distance between the limit of the eyepiece and where the user can still see the complete picture of what is being magnified in the scope or binoculars.
Imagine that a pair of binoculars has two cups where the eyes can be placed. Should the user pull their face backward from that point, the distance where the full image can still be viewed is the eye relief.
How Eye Relief Is Measured
Although all modern rifle scopes, binoculars and sporting optic instruments have the eye relief provided by the manufacturer, it is important to understand how this critical distance is calculated.
All scopes and sporting optics work by magnifying light through the use of a lens or a series of lenses. In effect, incoming light is channeled and refracted into a cone of light. The cone of light rays that pass through the eye piece is officially known as the exit pupil, a term normally reserved for photography but also applicable when it comes to scopes and binoculars. Effectively, the measurement of the exit pupil is usually described in either inches or millimeters, referring to the diameter of the cone of light that could can reach the eye of the user of the instrument.
Due to the way modern scopes and binoculars are constructed, the exit pupil diameter is often deliberately designed to be larger than the user’s pupil. In effect, there is more of an image being channeled through the lenses than the eye can comfortable take in with one glance. This is a deliberate engineering strategy to allow for the user to move slightly to the side of the eye piece without losing any visual information, a term known in the business as vignetting or clipping. The eye relief is therefore the distance from the eyepiece that the entire image can be viewed in its entirety, and rarely matches the same measurements for the exit pupil.
To use a practical example, a binocular rated at 10 by 52 refers to an internal series of lenses that can magnify a given visual image by ten times with an exit pupil of 52 millimeters with no magnification being employed. Should the binoculars be used at full power (i.e. at 10 times power) then the exit pupil then becomes subsequently smaller, in this case 10 times smaller, therefore just 5.2 millimeters.
The eye relief is therefore calculated to determine the maximum amount of distance the eyes of the user can be from the scope or binoculars when used at its most powerful magnification and still retail a full image as focused through the lens(es).
Why Eye Relief Matters
Whether purchasing a rifle scope, a spotting scope, or binoculars, it is not always feasible or comfortable to place the eyes directly on the instrument. Instead, it may be advantageous to pull the face back from making direct contact with the instrument. Although this may be more comfortable or desirable, it is still essential to be able to get a full view of what the instrument is magnifying or if the user’s eyes move slightly to the left or right of the eyepiece.
Individuals wearing eyeglasses, goggles, or other forms of eye wear may find it uncomfortable or impractical to place the lenses of their goggles or glasses directly on the sporting optic device. Hunters and sharpshooters using a scope on a rifle may have practical reasons to keep their faces a short distance away from the eyepiece in order to avoid damage from recoil. Going by the informal nickname of “idiot cut”, it is definitely possible to sustain a laceration around the eye if the shooter presses their face too close to a weapon when it is fired.
What to Look For In Terms of Eye Relief When Buying Sporting Optics
The simplest way to understand eye relief when first using a scope or binoculars is that the larger the eye relief, the better. While this maxim is not always true under all circumstances, it is essential to always review the manufacturer’s information on eye relief before purchasing a piece of sporting optics.
For professional sportsmen and hunters, the eye relief becomes a critical factor especially when using the optics for large-scale magnification. Hunters and sportsmen who wear glasses or wish to wear goggles must be especially careful to make sure that they have a sufficiently large eye relief to still be able to see the entire image without risking damage to either the lenses of their glasses or goggles or to the skin and area around their eyes.
Many professional sporting optical devices have built-in eye relief adjustment mechanisms. Usually designed for wearers of corrective lenses, a rotating eye cup can be adjusted to provide additional eye relief.