Gadget Bag: Situational Awareness

Throughout history, binoculars have been associated with explorers, spies and field marshals. They’re useful to anyone who wants to take a closer look at something far away. We can thank an Italian gentleman named Ignazio Porro for inventing the traditional style of binoculars that’s most commonly recognizable—and for lending his name to an optical component design that’s still in use today, some 160 years later. Navigating the maze of binocular nomenclature can be more difficult than focusing on a Serinus canaria 25 yards away. So we’ve created a concise glossary of the terms you need to know to make an informed buying decision. We’re also taking a deep-dive look at one of the most popular configurations for outdoor photographers, the 10×42.

Power And Objective Diameter. Binoculars are classified by their power and objective diameter, two specifications that always appear together with an “x” between them. For example, 10×42 are 10 power glasses with a 42mm objective diameter. The power number (sometimes called magnification) indicates how many times closer the subject will appear. Ten power (10x) makes things appear 10 times closer. Eight power equates to eight times closer. It’s that simple. But from here forward, things become a bit more complicated.

Exit Pupil. The exit pupil is a calculation that’s derived by dividing the binoculars’ objective diameter (in mm) by its magnification. A 10×42 pair of binoculars has an exit pupil of 4.2mm. Similarly, all 7×35 glasses measure 5.0mm. This measurement is used to compare the potential brightness of binoculars, but is often used incorrectly. The pupil (aperture) in a human eye closes to about 3mm in bright light and can open to larger than 7mm in dim light. In normal daylight, the pupil diameter of most adults’ eyes is about 4mm. Therefore, the effective objective diameter of any binoculars is limited to 4mm when used in daylight, regardless of its specification.

Relative Brightness. To calculate the theoretical relative brightness, square the diameter of the exit pupil; 10×42 glasses have a relative brightness of 17.64 (4.2² = 17.64). Compared to a pair of 7x35s, which have a relative brightness of 25, our 10x42s seem dim. To further blur the analysis, some reviewers and retailers specify twilight factor, a hypothetical estimate of binoculars’ performance under low-light conditions. The number is obtained by multiplying the binocs’ objective diameter by its magnification, then finding the square root of that number. For any 10×42, the twilight factor is 20.5. Although a higher number is generally better, the evaluation isn’t that simple, as we’ll soon see.

We can’t trust relative brightness or twilight factor when comparing products from two different manufacturers. The style and type of prism, type of glass and surface coatings, and the optical design all have more influence on brightness than the calculated number. So a pair of high-quality, well-made 7x35s with a twilight factor of 15.7 may outperform poorly assembled 8x40s with a twilight factor of 17.9.

Roof Versus Porro Prisms. Compact binoculars are typically built around roof prisms. The diameter of the optical tubes is about the same on either end. Chunkier traditional binocs have eyepieces that are offset from the axis of the objective lens. They use porro prisms (thanks to the aforementioned Ignazio Porro) to bend the light and make the magnified image appear upright.

The type of glass used to construct the prism is critical. Higher-quality binoculars use BaK4 glass prisms, whereas budget binocs may use BK7. In terms of practical benefits, BaK4 prisms cause less eye fatigue and produce crisper images.

When an optical beam is split by a prism or other means, the two light paths can become out of phase. Better-quality binoculars are more accurately collimated (aligned) and use special lens coatings or other methods to correct this aberration. For the best quality, look for binoculars that have phase-corrected BaK4 prisms.

Eye Relief. Eye relief describes the distance the binoculars may be positioned away from the eye and still be used comfortably. If you wear eyeglasses, look for eye relief numbers in the 16mm to 20mm range. Also look for diopter correction with click stops. Diopter correction allows you to adjust the binocs to match your eyesight. Having click stops allows you to reset quickly when your kids borrow the binoculars and change the setting. Clicks also make it harder to move the dial accidentally.

Field Of View. Some binocular makers publish the angle of coverage in degrees, but this is nearly impossible to understand in concrete terms. A better spec is the field of view at 1,000 yards. High numbers indicate that the binoculars are wide angle. At a given power, you’ll be able to see more edge to edge, and that’s beneficial when scanning the horizon or when following wildlife that moves in and out of your field of view.

Focus Adjustment. Nearly all binoculars have a focusing wheel somewhere in the middle and are referred to as center focus style. If you suspect you’ll be one-handing the glasses much of the time, look for a pair that has a center rocker to facilitate focus. Also important is the near-focus specification. As the name implies, this spec reveals how close you can be to an object and still be able to accurately focus. Birders should scrutinize this characteristic carefully.

Size. Size and weight matter, and are obviously subjective. Binoculars that fold neatly and slip into a jacket pocket or bag are likely to see more action than a two-pounder that’s the size of a toaster. Modern, lightweight materials, including metal alloys and superstrong plastic resins, have allowed manufacturers to keep optics quality high while improving portability.

Environmental Seals. Water resistance is a great plus, and being fully waterproof can be a lifesaver. If you see a pair of glasses marked nitrogen-purged, that means the individual optical tubes have been flooded with nitrogen gas and sealed, locking out water vapor that could condense on the inside glass surfaces and render the binoculars temporarily unusable. The added benefit is that sealing in the nitrogen seals out dust permanently. Binoculars with this feature are sometimes designated fog-proof.

Here’s a selection of popular 10x42s in alphabetical order.

Bushnell 10×42 Legend Ultra HD
These nitrogen-filled, waterproof and fogproof glasses feature BaK4 roof prisms and fully multicoated optics. Built in a lightweight magnesium housing, the optical design includes Extra-Low Dispersion (ED) glass, while the exterior features Bushnell’s proprietary RainGuard HD water-repellent coating for added protection. www.bushnell.com

Campco Humvee 10×42 Compact

Campco Humvee 10×42 Compact
The low-price leader that’s big on value, the Humvees feature ruby-red antireflective lenses and comfortable non-slip rubber armor coating. Their budget price and features make them a popular choice as primary glasses or as that second pair kept in the truck at all times. www.humvee.com

Canon 10×42 L IS WP
Perhaps the ultimate in professional binoculars, this Canon model offers image stabilization, an invaluable feature if you use glasses for extended periods of time. Built using Canon L-series optics with four elements of Ultra-Low Dispersion (UD) glass, they accept 52mm screw-in filters, lens caps and lens hoods. Suitable for any environment, they’re waterproof and fogproof. www.usa.canon.com

Celestron Nature DX 10×42
Compact and lightweight, the Nature DX model includes high-quality features like BaK4 prisms with phase coating, fully multicoated optics, wide-angle field of view and a very close minimum focusing distance (6.5 feet, sure to be appreciated by birders). They pack easily into any camera bag, backpack or jacket pocket. www.celestron.com

Leica 10×42 Trinovid

Leica 10×42 Trinovid
Legendary Leica binoculars in the popular 10×42 configuration feature a cast magnesium housing for strength and lightweight phase-correction prism coating, HDC multilayer optic coating and a wide field of view. They’re nitrogen-filled for fogproof performance and are waterproof. us.leica-camera.com

Leupold 10x42mm BX-3 Mojave
These armored, waterproof, nitrogen-filled glasses are built for the toughest conditions and are backed by the Leupold Limited Lifetime Warranty. They feature a lightweight and easy-to-carry open-bridge design, fully multicoated lenses, L-coated BaK4 prisms and long eye relief (16mm). Available in black or camouflage pattern. www.leupold.com

Nikon 10×42 Monarch 7 ATB

Nikon 10×42 Monarch 7 ATB
These popular Nikon roof-prism-design binoculars utilize Extra-Low Dispersion (ED) glass elements and offer phase correction, fully multicoated optics, wide-angle viewing and handy twist-up eyecups. They’re nitrogen-filled, waterproof and fogproof, and are protected by an armored rubber housing. www.nikonsportoptics.com

Pentax DCF CS 10×42
Eyeglass wearers (and sunglass wearers) will like the extra-long 18mm eye relief. This roof-prism design is waterproof and fogproof, and incorporates a standard ¼”-20 tripod socket. For easy individual eyesight correction, they feature a center diopter ring with click stops. www.us.ricoh-imaging.com

Steiner 10×42 Tactical
Professional glasses designed for heavy daily use, Steiner Tactical 10x42s include all high-end features (phase-corrected roof-prism design, fully multicoated optics, wide-angle viewing, nitrogen-filled fogproof and waterproof, fast focus center wheel, twist-up angled rubber eyecups, checkered rubber armor coating and solid, closed bridge construction), plus the unique 2-Way Valve Technology that allows advanced gas purging, inspection and maintenance. Usable just about anywhere on the planet, they’re built to withstand operating temperatures of -4º F to 158° F and are submersible to six feet. www.steiner-optics.com

Vanguard 10×42 Spirit XF 1042
Featuring BaK4 roof prisms, a wide viewing angle and full multicoating, these waterproof/fogproof binocs offer a solid range of features at a modest price. The sturdy open-bridge design is complemented by textured rubber armoring for a secure, slip-proof grip. www.vanguardworld.com

Zeiss Terra ED 10×42
Constructed using Schott ED glass with Zeiss MC multicoating, the Terra series of binoculars provides an incredible close-focus distance of just 514 feet, allowing you to zero in on subjects near and far. Use of a Schmidt-Pechan prism enables a more compact design, while the large focusing wheel ensures easy handling. Nitrogen-purged and waterproof, you’ll find these glasses comfortable for all-day use in any environment. www.zeiss.com/sports-optics