The Primary Numbers
Binoculars are classed according to magnification power and the diameter of the front lenses. An 8x40 pair would magnify objects by a factor of eight ftreon and have front lenses measuring 40 mm. Higher magnification is not always better, as high-powered binoculars are harder to hold steady on target because they show a smaller section of the landscape. Larger-diameter front lenses gather more light, resulting in a sharper, brighter image and better resolution in low-light conditions. A 7x50 binocular is optimum for marine use.
Light Transmission and Coatings
Light is lost as it passes through binocular optics. Inexpensive pairs might pass as little as 50 percent of the light to the viewer, while quality glasses will provide light transmission in the 80 percent to mid-90 percent range. To reduce reflection and increase light transmission, lenses are coated, typically with a magnesium fluoride, and higher-end manufacturers might add proprietary scratchresistant compounds. “Fully coated” lenses might sound good but are actually less desirable and usually found on lower-end products. Binoculars labeled “multicoated” will gather more light, and the best products are “fully multicoated” — sometimes abbreviated as FMC.
Field of View
This is the area you’ll see through the binoculars, expressed either in degrees or in the width of the area seen at a thousand-yard range. A field of view of seven degrees (or about 375 feet) is usually considered optimum for general marine use.
Binoculars use two common focusing systems: center focus and individual eyepiece. The single wheel of the center-focus system is fast and easy to use. With individual-eyepiece focusing systems, once adjusted, anything beyond 150 feet or so will always be in focus. Less-expensive versions, sometimes called “focus free,” use fixed eyepieces and don’t do well with objects closer than 30 to 40 yards.
Eye Relief and Exit Pupil
Eye relief is the maximum distance your eyes can be from the eyepieces and still take in the full field of view. This becomes more important if you wear glasses or sunglasses. A good pair of binoculars will have at least an inch of relief. The diameter of the exit pupil determines how much light is transmitted to your eyes. Less expensive glasses designed for daylight viewing might have an exit pupil of 3 mm, but for marine use in low light and on a moving boat, 7 mm or more is preferred.
Binoculars use one of two types of prism to invert the image you see: The Porro prism is bulkier but simpler in its design; the roof prism is more compact and complex. For the same money, a Porro prism will typically have better optical quality. The best prisms are phase corrected (sometimes abbreviated PC) and are made from BK7 glass as opposed to the lower grade BAK4.
Extra Features for Boaters
If the budget allows, look for features that enhance binocular performance and durability in the marine environment: Waterproof seals and nitrogen-filled optics that prevent fogging help, a rubberized outer coating is mandatory, and the ability to float is useful. Some even integrate compasses and, in recent years, electronic image stabilization, which comes in handy when boating in rough seas but can be a fairly costly option.