The importance of having the right tactical optics extends beyond rifle scopes. In a sniper team, the spotter’s optics are as important as those of the sniper. As every team knows, both the sniper and spotter must see as one. Having the right optics also can also be of vital importance for tactical reconnaissance and surveillance.
Binoculars come in two main types—porro prism and roof prism. Binoculars use prisms to turn images right side up. Without the prisms, the image that you would see through a binocular would be upside down. Porro prism and roof prism binoculars are easy to tell apart. The eyepiece lenses in porro prism binoculars are offset from the objective lenses. The eyepiece and objective lenses are aligned in roof prism binoculars.
There are pros and cons exist for each. The pros for porro prism binoculars are that they are generally less expensive for a given size than roof prism binoculars and lose slightly less light to reflection, everything else being equal. The pros for roof prism binoculars are that they are more compact and streamlined for a given size than porro prism binoculars.
All binocular sizes are expressed in two numbers, e.g., 8x28, 7x35 or 10x50. The first number is the magnification or power. The second number is the diameter of the objective lenses in millimeters (mm). Because the obvious purpose of a binocular is to magnify an object, one would thus think that the highest magnification would be best. This is often not the case. The appropriate magnification will be determined by the intended application(s) for which the binocular will be used.
As magnification increases, the depth of field decreases and the field of view (FOV) narrows. FOV is can be measured in either one of two ways—angular (degrees) or linear (a ratio of length, e.g., feet at 1,000 yards). Each degree of FOV equals 52.5 feet at 1,000 yards. Thus, a binocular with a 10-degree angular FOV would have a linear FOV of 525 feet at 1,000 yards.
At higher magnifications, image shake also becomes a problem. At lower magnifications, image shake (due to the inherent unsteadiness / tremor of handheld optics) isn’t noticeable to the eye. At magnifications of 8x or less, the image will appear steady when handheld. At 10x, the image will tend to show some shakiness, but it will still be manageable for most people. At over 10x, the binocular will generally need to be mounted on a tripod for steadiness.
Some binoculars are equipped with electronic image stabilization to minimize shake. Image stabilization eliminates or greatly reduces any shake, even at higher powers. However, aside from increased cost, the major downside to electronic image stabilization is that it increases the complexity of the binocular—it’s something else to break and go wrong at an inopportune moment.
When higher magnifications are desirable or necessary, spotting scopes are generally employed. Additionally, unlike binoculars, many spotting scopes can also be employed for photographic purposes with the use of camera adapters.
The size of the objective lenses of a binocular or scope is important. The size of the objective lens is known as the aperture. The larger the aperture, the more light that will be gathered by the binocular. For optimal nighttime use, an aperture of 50mm or better is generally recommended. However, a 50mm binocular is overkill for daylight use. Smaller aperture binoculars will provide more than enough brightness and resolution in daylight. As the aperture increases, so too does the size and weight of the binocular.
The size of the exit pupil of a binocular determines how much light actually reaches the retina of the eye through the optics. If the exit pupil is smaller than the pupil of the eye, some light will be lost in transmission. The pupil of the average human eye is 7mm at night and 4mm in daylight. The maximum pupil dilation decreases as we age.
Twilight factor is a number that is used to compare the brightness of the image seen through binoculars or scopes that are used in low light. It’s more accurate than exit pupil in this regard. The larger the twilight factor, the more detail you can see in low light, all things being equal. For binoculars and scopes, a twilight factor of 17 or better is recommended for reasonable detail in low light.
To determine the exit pupil, divide the diameter of the objective lens by the magnification. To determine the twilight factor, multiply the diameter of the objective lens by the magnification, and then find the square root of that number. The exit pupil and twilight factor don’t tell the whole story when it comes to brightness and resolution. The type of prism glass and anti-reflective coatings play an important role in light transmission.
Two types of glass are commonly used for optical prisms—BK-7 and BAK-4. BK-7 and BAK-4 were developed in Germany specifically for optical use. Most binoculars use prisms made from BK-7 glass. BK-7 is a borosilicate glass that provides good light transmission for most use. BAK-4 is more expensive and generally reserved for higher-end binoculars. BAK-4 is a barium crown glass that provides optimum light transmission. All things being equal, BAK-4 prisms yield brighter, shaper images than BK-7 prisms.
The lens coating is also very important. Lens coatings are used to reduce or largely eliminate reflections and increase the light capturing (phototropic) capability of the lens. Most binoculars today feature some sort of lens coatings.
There are different degrees of optical coating. “Multi-coated” means that multiple layers of coatings have been used. “Fully coated” means that all lens surfaces have been coated. “Fully multi-coated” means that all lenses surfaces have been multi-coated. For optimal performance, you want a binocular with fully multi-coated lenses. Fully multi-coated lenses provide the best light transmission for maximum brightness, clarity and contrast.
Reflections from optical surfaces have resulted in loss of personnel and the failure of missions. Reflection isn’t only a tactical issue during the daytime. Reflections can occur at night from a bright moon or other bright lights.
Anti-reflective lens coatings aren’t always a complete solution to the problem. The use of Killflash Anti-Reflection Devices (ARD) on the objective lenses of your binocular will provide a low-cost solution to any issues of reflections.
Killflash ARDs work by clustering numerous tiny tubes into a honeycomb grid to eliminate external reflection. The patented Killflash technology also reduces internal glare. While there’s no perceivable loss of resolution, nor any vignetting or loss of FOV that results from the employment of Killflash ARDs, they will reduce light transmission by approximately 15%, which may or may not be a consideration. A
nother factor to consider when buying a binocular is the eye relief. Eye relief is the distance between your eye and the eyepiece lens of the binocular when the entire FOV of the binocular is visible. Although the optical design of the binocular plays a role in the eye relief, as a general rule of thumb, eye relief decreases as magnification increases.
Too short an eye relief will restrict the FOV, resulting in a keyhole effect. Eye relief is especially important for eyeglass wearers. A 16mm or better eye relief will provide the maximum FOV for both eyeglass wearers and non-eyeglass wears.
The eye relief will also be affected by the eye cups of the binocular. Eye cups are designed to position your eye for the optimal eye relief. Less expensive binoculars generally have fold-down rubber eye cups. Fold-down rubber eye cups are OK, but they are not the best option. The better binoculars generally have adjustable eye cups. Adjustable eye cups will allow you to customize the eye relief to what is best for you, which can be especially important for eyeglass wearers.
Focusing capability is an important consideration when buying a binocular. Most binoculars are of the center focusing (both oculars are focused to together), although a few binoculars feature individual ocular focusing. So-called “permanently focused” binoculars have a fixed focus that’s set at infinity. Permanently-focused binoculars are a trade-off and don’t have the close focus capability of adjustable-focus binoculars.
Close focus is the closest distance that you can look at an object and still focus the binocular or have the object in focus. Some binocular models can be focused closer than others. Although the close focus capability of a binocular can be important for many users, such as naturalists, it’s of less importance to the tactical operator.
The right eyepiece on nearly all center-focus binoculars is adjustable to equalize differences in the vision between your eyes. Most are marked in +/- one-diopter adjustments. Better binoculars will have locking or click-adjustment capability.
Binoculars (and spotting scopes) with range-estimating reticles are available. The spotter and shooter should be using the same design reticle when viewing the target. For example, if the shooter has a Mil Dot reticle scope, the binocular (or spotting scope) of the spotter should also be so equipped.
A binocular with ranging reticles should to be correctable to the axis of the horizon after the interpupillary distance (IPD) has been set. Not all binoculars with reticles allow this. One that does is Leupold’s new high-performance 10x50mm Tactical binocular, which is available with your choice of a Mil Dot or Leupold Tactical Milling Reticle (TMR). The Leupold 10x50mm Tactical binocular is available without a reticle. Mil Dot and TMR reticles are also available on the Leupold Mark 4 12-40x60mm Tactical Spotting Scope.
Reticles that can be rotated allow different users with different IPDs to view the reticle horizontally. Another plus is that it allows the reticle to be rotated to range an object that is not perfectly vertical or horizontal without having to tilt the entire binocular.
The IPD is the distance between the pupils of the user’s eyes. This differs from individual to individual. Most binoculars can be adjusted to fit the IPD of most adults. However, the adjustment range should be checked before purchase, especially if the binocular will also be used by smaller people.
Some binoculars have an IPD lock. An IPD lock allows the IPD to be locked in place. This allows for instant, one-handed use by eliminating the need for readjustment every time the binocular is used. Tactical binoculars need to be rugged to withstand the rigors of field use. Rubber armoring is a plus. Rubber armoring is not just for looks, it does provide a degree of protection.
Tactical binoculars must also be fog-proof under all conditions. All of the better binoculars are nitrogen filled to eliminate fogging. Tactical binoculars also need to be weatherproof or waterproof, preferably the latter.
Weatherproof binoculars aren’t waterproof. Weatherproof binoculars are designed to withstand inclement weather, such as rain or snow. If they are dropped in a stream, they will likely be damaged. Waterproof binoculars are designed to be able to withstand submersion in water, as well as being able to withstand inclement weather.
The best binocular will depend on what the binocular will be used for, where it will be used, when it will be used, the level of performance that is desired, and by whom it will be used. And, finally, keep in mind that with optics, you generally get what you pay for.
Eugene Nielsen provides investigative and tactical consulting services and is a former officer. He can be reached at firstname.lastname@example.org.