Consumer's Guide to Boat BinocularsBy Tom Neale
Published: February/March 2012
Look deeper than magnification when you're buying binoculars. There are models that stabilize, relieve eye strain, and have a built-in compass. Here's what to look for, from an experienced boater's point of view.
The first thing you notice when you're buying binoculars is a "value" such as 7x50. Those numbers give the degree of magnification and the diameter of the objective lenses. In 7x50 binoculars, the "7" tells you that the target object is magnified to seven times its size. More isn't necessarily better. For example, higher magnification reduces the field of view, making it more difficult to find a small object at sea. The "50" gives you the diameter of the objective (front or larger) lens. The larger that lens, the more light it will gather, a quality that's extremely important on a boat at night or in other low-light conditions. Doubling the size of the objective lenses quadruples the light-gathering ability of the binoculars.
Many feel that 7x50 is the best compromise for boat binoculars, but it's important to consider other features. One is the "exit pupil." This is the diameter, in millimeters, of the beam of light that leaves the eyepiece. A larger beam gives a brighter image, a great advantage at night. Theoretically the exit pupil should approximate the diameter of your eye's pupil after it has adapted to the dark. This decreases with age, but it's typically between 5 mm and 7.5 mm. Divide the size of the objective lens by the magnification to get the exit pupil in millimeters. If you cruise in low-light conditions regularly, say pre-dawn fishing trips, this is another reason not to buy a set with extremely high magnification.
Not So Fast
From these basics, you may think you have enough to make a good choice, but there's more. For example, "eye relief" is the distance that binoculars can be held from the eye without straining your eye while still seeing the full field of view. If you wear eyeglasses, this is especially important and you'll probably want at least 15-mm eye relief. Some better binoculars allow you to fold or otherwise adjust the rubber eyecup to help with this issue. Some, such as the Steiner XP Commanders, come with several different sets of eyecups.
Oh, Say, Can You See?
Light-gathering ability is critical for a boater. When I purchased my first set of high-quality binoculars many years ago, I was amazed at what I could see out in the ocean at night with starlight providing the only ambient light. It was very different from the cheap sets I'd used before. The size of the objective lens is important for this, but there's more involved. Color differentiation and sharpness are also concerns. We want not only to see that buoy at night but also to hopefully determine its color and shape. Many factors contribute to enhanced night vision, including quality of the glass in the objective and eyepiece lens, quality and type of prisms used (we'll get to prisms below), and lens coatings. The light transmission as well as color differentiation can be objectively measured by the manufacturer as the beam exits the eyepiece and the makers of more expensive binoculars strive to excel here as well as with other issues. Although the subject of lens coatings seems somewhat esoteric, it's not just smoke and mirrors. Coatings can do things such as reduce internal glare and light loss, which can yield better light transmission and brighter images. Coatings may also reduce internal fogging, helpful even if the binoculars are nitrogen filled. There are different levels of lens coatings, ranging from coated to fully coated, multicoated, and fully multicoated, the latter being the best. It means multiple coatings are on each lens.
Quality manufacturers go to great lengths to develop and apply proprietary coatings. One example would be what Fujinon refers to as its patented EBC (electronic beam coating). This is another reason why I wouldn't buy cheap binoculars. Resolution and contrast are important on the water. Poor quality here could make you miss a distant buoy or mistake its shape.
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How Do They Fit Your Face?
We're all built differently, and the differences include the distance between our eyes. Good manufacturers therefore address the interpupillary distance (IPD), the amount of lens separation that allows both eyes to see a complete viewing field without overlapping or other distortion. This changes from person to person, so a range from 58 to 72 mm is typically built in. The wider the range, the more people can use the set. You adjust the set by looking at a distant target and moving the barrels on their hinge until you see one image properly. Markings on the hinge help you remember your setting.