How to Choose the Right Marine Binoculars

Up Close and Personal

Binoculars are an indispensable asset at sea. Here’s what you need to know to find a pair that suits your needs.

The other day, I was hunting for old photographs of warships. I wasn’t interested in pictures of battlewagons launching shells the size of automobiles at each other. I was looking for people—haggard, unshaven faces after weeks at sea, or beaming with relief and pride after a battle. And it struck me that in all those grainy, black-and-white photos, you could easily pick out the captain, from the binoculars hanging around his neck.

Years ago, when radar and thermal imaging were in their infancy, the Mk1 Eyeball (unaided eyesight) was crucial. But today, the Mk1B—eyesight boosted with a pair of binoculars—is even better! But how do you choose the right binoculars?

Binoculars are classified according to two numbers, such as 8x30. The first number refers to the magnification, so a pair of 8x binoculars will make objects look eight times closer.

But bigger is not necessarily better. A 40x “observation scope” is fine atop a tripod on the balcony of an oceanside condo, but you’d never be able to hold it steady enough for use at sea. For handheld use, most experts recommend nothing more powerful than 8x.

The second number refers to the size of the objective lenses, in millimeters. All other things being equal, a 50 mm objective lens admits about twice as much light as a 35 mm objective. So the image you see through a pair of 7x50s should be roughly twice as bright as you would see through a pair of 7x35s.

Dachs or Porro?

The binoculars in those old photographs were all the classic binocular shape (see the photo above). But many binoculars nowadays have a much sleeker design that looks like two straight tubes. Both kinds use prisms to bend the path of the light, to fit the length of a 19th century sea-captain’s telescope into a few inches. The classic layout was patented by Ignazio Porro in 1854, and is still favored for most “marine” binoculars, while the more modern “Dachs” or “roof prism” design is often preferred for birdwatching and hunting. Porro binoculars have no internal moving parts, so they’re much more durable. It’s easier to make Porro binoculars waterproof/submersible—just a matter of sealing the lenses into the bodies. The Porro design has individual eyepiece focusing: Once adjusted to the user’s eyes, everything is in focus from 65 feet to infinity. The objective lenses of Porro binoculars are farther apart, giving a better sense of perspective. Apart from sleeker styling, the main advantage of the Dachs design is that it can focus to much shorter distances—perfect for watching birds in your backyard.

That’s only the beginning. The light also has to get out of the binoculars and into your eyes. That depends partly on what experts call the “exit pupil,” and partly on the user’s physical characteristics.

The exit pupil is the bright spot you see if you look into the eyepiece of a pair of binoculars from a distance of about a foot, but you can calculate it by dividing the size of the objective lens by the magnification. So 7x30s will have an exit pupil of just over 4 mm.

The pupils of a fit young adult can dilate to a maximum of about 7 mm—which makes 7x50 binoculars a good match, especially at night. So why do some companies offer marine binoculars with much smaller objective lenses?

I put that question to Dr. Bill Cross, senior optics engineer at Bushnell Outdoor Products (www.bushnell.com). “When we’re young, in low-light conditions, the pupil of our eye can open up to 7 mm,” Cross explained, “But as we get older, this drops down to perhaps 5 mm. And during the day, it’s typically only about 3 mm.” So someone in their 60s or who uses binoculars in daylight might not reap the full benefit of 7x50s.

Dennis Phillips of Steiner Optics (www.steiner-optics.com), reinforced this point, offering arguments in favor of smaller objectives. “A lot of women sailors like the 7x30 because they’re lighter and more comfortable in the hand.”

That’s not a trivial point: At 36.3 ounces, Steiner’s Navigator Pro 7x50 is almost twice the weight of its 7x30 version. But Phillips still favors 7x50s. “The 7x30s are often a second choice to have on board for land excursions or use in a dinghy” he said. “But the 7x50 is superior in light gathering, field of view, and clarity.”

When you’re using binoculars on a swaying boat, field of view (FOV) is significant. It’s quoted in two different ways—either the width of the field of view at 1,000 yards or 1,000 meters, or as an angle. A meter of viewing width at 1,000 meters is about equal to 3 feet of viewing width at 1,000 yards, and each degree corresponds to about 52.5 feet at 1,000 yards, or 17.5 meters at 1,000 meters. So an FOV of 7 degrees corresponds to about 360 feet at 1,000 yards, or about 123 meters at 1,000 meters.

But for anyone who wears glasses, the field of view can be seriously reduced by an effect known as “vignetting.” It’s like tunnel vision, cutting off the edges of the visible image with a dark ring. It happens when the image is created in front of the lens of your eye, rather than on it. To find out whether this is likely to be a problem, check the binoculars spec sheet for “Eye Relief.” This is the distance from the eyepiece lens of the binoculars to the lens of your eye. If the eyepiece is designed for use without glasses, the eye relief is about 3/8 inch (or 10 mm). If it’s designed for use with glasses, it’s more—at least 5/8 inch (or 16 mm).

Most binoculars with “long eye relief” incorporate some kind of adjustment, such as a sliding extension to the eyepiece or folding rubber eyecups, so they can be used equally well with or without glasses. If you’ve found a pair of binoculars that are perfect aside from insufficent eye relief, there’s a simple solution: Just take your glasses off!

Okay, so you’ve decided on a pair of 7x50s, should you pay $25 or $2,500? The quality of the glass makes a difference. “Cheap” glass simply isn’t as good as “expensive” glass: It may be imperceptibly cloudy, so when light passes through half a dozen or more lenses and prisms, it can make a distinct difference to image brightness or clarity.

Then there’s the way the glass is ground to create the lenses and prisms, and the way it’s treated. In good-quality binoculars, the edges of the prisms are given an opaque coating to prevent stray light from getting in where it’s not wanted, and the lenses are coated with other materials that allow light to enter the lens, rather than being reflected away.

The way the lenses and prisms are mounted affects the binoculars’ performance, as well as their life expectancy in the rough-and-tumble life afloat. And then there are the mechanical aspects of the binoculars’ construction: Waterproofing and nitrogen filling don’t just stop the binoculars from filling up with water if they fall overboard, but prevent them from fogging up on cold winter nights. Smooth controls help you get the perfect focus.

Steiner doesn’t make cheap binoculars. Its Marine 7x50 is a little over half the price of a basic thermal imager at $329.99, while its Commander 7x50 compares with an entry-level radar at $1,207.49. Why choose the Commander? “The Commander is a professional model, built to a military specification,” Steiner’s Phillips says. “Navy SEALs swim out of submarines with them, and they can be dropped 6 feet onto a steel deck multiple times without breaking. The coatings are the best we offer, delivering 98 percent light transmission, and an external hydrophobic coating makes water sheet off in rain, snow, or fog. The Marine 7x50 is less feature-rich, and although it’s waterproof, it’s not submersible. And its light transmission is around 89 percent.”

Phillips was slightly rueful as he told me he’d met a couple who had taken their Commander 7x50s around the world three times in 20 years. “I couldn’t sell them new ones,” he said, “because there wasn’t any problem with the original!”

VIDEO EXCLUSIVE: Check out our take on today’s hottest marine binoculars ▶

TESTED: A closer look at four marine-grade binoculars on the market today.

Presented with available features ranging from reduced weight to built-in compasses, it pays to choose your binoculars wisely.

1.Nikon OceanPro 7x50
$299.95; www.nikonsportoptics.com
Keep a grip on the Nikon OceanPro 7x50 binocs even when wet, thanks to their rubber armor. Sealed with O-rings and filled with nitrogen to prevent fogging and condensation, OceanPro binocs have multiple lens coatings made from antireflective compounds that sharpen the view and produce solid performance in the range of conditions you typically see on board a boat. A built-in compass helps you set course once you spot your target.

2.Steiner Navigator Pro 7x50
$517.99; www.steiner-optics.com
Looking for a great view in a slightly smaller size? With dimensions of 8 inches wide by 5  inches long, the Steiner Navigator Pro 7x50 has a key benefit: a reduced weight, just 2  pounds. Though not built to military spec (that model is the Commander 7x50, which lists for $1,207.49), the Navigator Pros feel rugged in the hand, and the integrated lens caps at both ends will help keep everything clear. Full disclosure: My old pair of Steiners have never let me down. 

3.Fujinon 7x50 FMTRC-SX
$849.95; www.fujifilmusa.com
With a built-in compass and reticle viewable in the left eyepiece, these high-grade spec binocs are a navigator’s partner in spotting that next landmark or navaid in poor light conditions. The optics are bright and the views are sharp. Nitrogen-filled chambers ensure no fogging or condensation. They’re solid to hold, weighing in at a shade under 3  pounds, and measure a bit less than 7 inches long and a bit more than 8  inches wide. 

4.Weems & Plath 7x50 Pro
$549.99; www.weems-plath.com
Such a pleasant surprise, these Weems & Plath 7x50 Pro binoculars performed comparably with other optic brands, featuring similar flat-field lenses for edge-to-edge image sharpness. While the large design felt solid in hand, I found the overlap in the seams of some of the rubber armoring to be a little narrow. Also the lens caps for the ocular lenses were a pair of loose, shallow, plastic cups that could get knocked off in a bag or filled with lint in a pocket. 

—Jason Y. Wood

This article originally appeared in the October 2016 issue of Power & Motoryacht magazine.