What is Eye Relief?

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.

What is Field of View?

Field of view is the extent or the range of your visual area. In terms of optics, it is widest dimension an object is visible through the eyepiece of a scope or binocular. Binoculars are the most common form of optics purchased for sporting activities. It is important to choose the right features in a binocular for your activity. Field of view is an important factor when making that decision.

Field of view, or FOV, is expressed in two ways. It is either expressed as the width in feet at 1,000 yards or in degrees of field. When the field of view is expressed in feet, it is called the linear field of view. When it is shown in degrees, it is referred to as angular. It is simple to convert one measurement to the other. The linear field of view is measured in feet at 1,000 yards. One angular degree is equal to 52.5 feet. Once the angular field of view, or degrees, is known, multiply that figure by 52.5. In most cases, the field of view is often indicated in degrees on the outside of the binoculars. For example, if the angular field of view is six degrees, then multiply 6 by 52.5, for a total of 315 feet at 1,000 yards, or a linear field of view of 315 feet. Similarly, if the linear field of view is known, divide that number by 52.5 to obtain the angular field.

The term real field of view refers to angle of the visual field which can be seen without moving the binoculars. It is measured from a central point on the objective lens. A higher value will translate to a wider visual field. The term apparent field of view refers to the angle of the magnified view. A large apparent field of view will provide a wide field of view even at a higher magnification. Optics with a wide field of view will make it easier to follow moving objects.

Binoculars display two numbers. The first number, followed by an X, is the magnification. It is the degree by which an object will appear larger. Binoculars are manufactured in a range of magnification, but lower magnification binoculars, such as 5x to 8x, are the most popular and typically offer a wider field of view. Higher magnifications, 10x and above, are available for long distance viewing, but generally have a narrower field of view.

The second two-digit number is the aperture. This is the diameter of the objective lens measured in millimeters. The size of the objective lens determines the light gathering capability of the binocular. The greater the light gathering capability, the brighter an object will appear. However, the size of the objective lens will affect the physical size of the binocular. A larger lens diameter will mean a larger and heavier binocular.

The field of view, in addition to the magnification and aperture size, are important considerations when looking to purchase an optic. Binoculars with a wide field of view are popular for bird watching, wildlife watching, or sporting events, as it is easier to follow objects that move quickly. A general purpose binocular, such as 7×35 or 8×42, would be a good choice for these activities. Lightweight enough to carry hiking, these binoculars have a wide FOV yet are powerful enough to distinguish detail. However, if you intend to bird watch or wildlife watch near wet, marshy areas or in wet weather conditions, you may want to be sure that the binoculars you purchase are waterproof and nitrogen or argon purged for fog proofing.

In choosing binoculars for astronomy, the aperture will be an important specification. For this activity, a binocular such as 7×50 or 10×50 is preferred. The 10x magnification will aid in viewing the planets, but a larger aperture will be important in dim and low light conditions. A larger aperture and greater light gathering ability will provide better resolution and allow you to study the planets in better detail. Binoculars with magnifications of 16×70 or 20×80 will benefit from being mounted on a tripod for steady viewing.

For boating activities, the marine binocular traditionally purchased is the 7×50. The 7x magnification allows a wide field of view and makes steady viewing easier with the movement of the water. The 50mm lens offers color and detail when viewing the horizon. When choosing a binocular for hunting, a lower magnification, such as 7x or 8x, provides a wide field of view to scan fields and tree lines. A 42 or 50mm objective lens would be a good choice for low light conditions, as a hunter might expect game to be more active at dawn or nightfall.

Optics will open you to a variety of scenery and wildlife, but understanding the field of view and other features will aid in purchasing an optic that is right for you.