Seven by fifty. That is all you need to know. Almost. The classic size of binoculars for marine use is 7x50. This means that they have 7x magnification and the objective lenses are 50mm in diameter. There are three characteristics of the 7x50 binoculars that make them attractive for marine use: conservative magnification, large objective size, and generous exit pupil size.
Many binocular shoppers, regardless of the planned use for their optics, start off by looking for the maximum magnification available. The problem is that when you increase magnification, you also increase image shake due to unsteady hands. No matter how steady your ace-at-the-board-game-Operation hands are, you cannot avoid image shake in high-powered binoculars. Now, compound that shaking with the motion of a vessel floating upon the water. Are we not starting to paint a picture here? A blurry picture? Regardless of the type of vessel you are on, movement will occur while underway, and the best way to make your viewing reasonably steady is to limit the binocular’s power to somewhere around 7x.
The 50mm objective is also essential, as the relatively large opening is ideal for gathering a lot of light and permitting viewing in less than ideal lighting conditions that many mariners find themselves facing on a daily basis, especially when entering and leaving port in the early or late hours of the day.
Another advantage of the classic 7x50 binocular is that the combination of power and objective size produces a large exit pupil—the size of the image at the eyepiece. If your eye’s pupil is larger than the exit pupil, you will have the unfortunate experience of looking at a small image surrounded by darkness. When it is larger, you eyes will see the image from edge to edge. The larger exit pupil allows you to view an image even when the motion of the vessel is changing the alignment of your eyes and the binoculars.
Marine Binocular Feature Considerations
My B&H colleague, Christopher Witt, has written a comprehensive guide to binoculars and I will not reiterate every part of his piece, but if you want to dive deep into the world of these optics, check out his article.
But, if you have mouse-click fatigue, I will talk about a few things that specifically apply to binoculars designed for marine use, here.
Porro Prism versus Roof Prism Most marine binoculars are of the Porro prism genre that features the more traditional look of these devices. The objective lenses on the Porro prism binoculars are more widely spaced, and this provides greater field of view and depth of field for the viewer. The roof prism binoculars are more streamlined and can be more compact laterally, while being generally longer than their Porro counterparts. This makes them ergonomically preferable, to some users.
BAK4 versus BK7 Prisms Without prisms, binoculars would show a reversed and inverted image. To “correct” the image, light passes through prisms before exiting the rear of the binoculars. BK7 prisms are not as round as the BAK4 prisms and do not have light-transmission properties that are as efficient as the BAK4 prisms. This is not to say that BK7 prisms are poor; in general, however, they are of lower quality than the BAK4. An easy way of identifying the prism of a binocular, if you’re unsure, is to hold them away from your face and look at the shape of the exit pupil: if it is round it’s a BAK4, if it’s square, BK7. This becomes more important if your eye is moving around a lot, because you may hit the edge of the exit pupil and see a vignetting effect. Of course, prisms from different manufacturers will have differences in quality and clarity regardless of their configuration.
Water- and fog proofing is often an option among similar types of binoculars. I shall be stating the obvious when I say that, when shopping for binoculars to be used in the marine environment, please make sure you buy a water- and fog-proof pair. Some binoculars designed for marine use will literally float if dropped in the water, and some may use polycarbonate chassis for enhanced corrosion resistance.
Eye Relief This is the optimal distance from the eyepiece to your eye for viewing. Eye relief is certainly something to be considered if you are a wearer of eyeglasses. Many binoculars offer diopter adjustments to allow eyeglass-free viewing, but if you want to leave your glasses on, make sure your chosen binoculars have collapsing eyecups to accommodate eyeglasses.
Field of View This is the angular width of your image at 1,000 yards (or meters). Each degree of view is equal to 52.5' at 1,000 yards or 17.5 m at 1,000 m. The narrower the field of view, the more difficult it is to locate what you are trying to view. Field of view also might effect the eye relief, so keep those numbers in mind when comparing binoculars. The wider the view, the easier tracking is, but that may come at the expense of image sharpness, especially on the edges. Most 7x50 marine binoculars have fields of view in the 7º neighborhood. However, some pairs are relatively narrow at less than 6º and others are as wide as 8.3º, so, when shopping, keep an eye on this figure and keep the pros and cons in mind. On the B&H website, you will often see the specification for Apparent Field of View or Apparent Angle of View. The AAoV or AFoV is the angle of the magnified field when you look through the optics. This figure is approximated by multiplying the actual field of view by the binocular’s power. Binoculars with AAoV greater than 65º are considered “wide angle.”
Individual Focus versus Center Focus This is a matter of preference. Most marine binoculars are of the individual focus variety; meaning the user focuses each eyepiece separately, negating the need to focus every time you use the binoculars—once adjusted, everything past a certain distance will be in focus. Center focus is more traditional, where a focus knob changes the focus on both eyepieces. If you are sharing binoculars with your crew, center focus is advantageous and easier to use for most people. Individual focus, on the other hand, is a good way to keep unwanted grimy bilge-soaked hands off of your nice binoculars, as no one enjoys a blurry image, even a scallywag!
Coatings Lens coatings exist to improve the performance of the lens by reducing reflections and increasing contrast. Lenses are multicoated, coated, or not coated. Prisms get the same treatment, this time to increase reflectivity, and Porro binoculars may get treated with phase-correction coatings. The other goal of the coatings on both the lenses and prisms is to increase light transmission. On marine-specific binoculars, hydrophobic coatings serve to help bead and remove water spray on the optics.
Armoring On smaller vessels, stuff gets knocked around while underway. Many marine binoculars feature a suit of armor made of rubber, or the tubes themselves are made out of a rubber-coated polycarbonate, to help protect the binoculars. This rubber treatment also can help provide a surer grip for your hands or for whatever surface you place your binos on.
Marine Binocular Specific Features
Compass One hallmark of many marine binoculars, but not all, is the built-in compass that projects a magnetic bearing into the image. Some of today’s marine binoculars feature digital compasses. Some compass systems are illuminated and some are not. And, for those world voyagers, be advised that some marine binocular compasses will only work in the Northern Hemisphere.
Rangefinder Reticle Several marine binoculars include a rangefinder reticle that consists of a vertical scale and horizon line projected across the image. When viewing an object of a known height (many fixed navigational aids and landmarks have heights printed on nautical charts) you can use the scale to determine your distance from the object using simple geometry. Some rangefinding binoculars feature a laser system to determine distance accurately.
Floating straps Many marine binoculars ship with floating straps, but you can also add one after purchase. If your optics decide to go swimming, a floating strap may be a great investment! And, since you purchased a waterproof pair of binoculars, rinse them off with fresh water and keep on going!
There are a number of stabilized binoculars on the market today. Using optical image stabilization technology that first appeared in camera lenses, they allow the boater to achieve a steady view at magnifications that are far past the traditional 7x marine binocular.
Even though they are packed with electronics, some are waterproof. You will likely want to avoid pairs that are only “weatherproof” for marine use. One disadvantage is that they are battery powered. If you run out of juice, you will be left with a pair of binoculars that are far too powerful, and that may have an objective that is too small for comfortable and effective marine use.
Manufacturers also have developed stabilized binoculars that integrate night vision technology into the package. Some allow day use and then nighttime viewing with the addition of NV eyepieces.
7x50 is the traditional marine binocular, due to its comfortable magnification power, very good light-gathering capabilities, and large exit pupil. In reality, any binocular can be used on the water regardless of size, features, or power. There are alternatives that you can explore as long as you know the limitations.
The mainstream birding binocular is 8x and has an objective around 42mm. The difference between 8x and 7x is not huge, but it can make a difference when underway. I have personally used 8x42 binoculars on the bridge of a large containership and they worked fine while transiting the relatively placid Western Caribbean and Gulf of Mexico, but I can tell you that using those same binoculars on a small sailboat in a seaway is challenging.
Smaller and lighter 7x35 or 7x42 binoculars might be an attractive option, especially on smaller craft when space is tight, or for smaller hands. However, the 35mm objective will simply not let as much light in as a 50mm pair, and that can make a huge difference when searching for that unlit nun while underway at night or around dawn and dusk.
There are several pairs of 7x50 binoculars that are not specifically made for marine use. Some 7x50 astronomical binoculars, generally devoid of a compass, rangefinder reticle, or armoring, can make for very clear and bright viewing while underway. Many astronomical binoculars have first-class optics and coatings, are rugged, and are water- and fog proof. I have found a pair of these 7x50s ideal, not only for maritime use, but for stargazing nebulae from the bridge wing while on a night watch at sea.
B&H Optics Department
The B&H Photo optics department has nearly 100 different models of 7x50 and 7x50 marine binoculars that cover all features and a wide range of budgets. Many are available for hands-on testing in the SuperStore, so come by and try out a pair before you get underway on your next nautical adventure!