October 2025 Column MARINE BINOCULARS GUIDE By Capt. John N.Raguso
Marine binoculars are handy, portable and useful onboard just about any type of boat, ranging from a multi-engine go-fast outboards, diesel sportfishers, sailboats, sport boats, yachts or workboats. This month’s column will take a closer look on how to choose the right binoculars for your boat and budget. Numbers Explained
No matter what type of binoculars you consider, for marine use, hunting, birding, or otherwise, they all share a common numerical identifier, which reads something like 7x50. The“7” part of the equation is the magnification power, which indi- cates that the image being observed will appear to be seven times closer to you than observed by the naked eye. In theory, lower magnification is ideal for use on the water, as it mini- mizes dreaded“shake and bake”from a rocking or moving boat and allows for a wide, stable field of view. The“50”part of the formula is the size of each objective lens, which in this case is 50mm in diameter. The larger lens (compared to a 40 or 42mm diameter model) gathers more light, which is crucial for a bright, clear image during pre-dawn, early morning, sundown or dusk use.
For most mariners, the key ingredient when purchasing marine binoculars is to prioritize steady magnification, waterproof and durable armored construction and good light-gathering capability. The industry-standard 7x50 model is recommended because it offers a wide, stable image and performs well in a variety of the conditions that you are likely to encounter, but especially in low-light situations where it can be a critical life-saver. The biggest problem with high- magnification binoculars is that they amplify the movement of a boat and can cause eye strain as you struggle to find the dancing image and focus on the moving target.
Yet another critical number when trying to find the ideal marine binoculars is the“exit pupil”rating for the optic. This is determined by taking the diameter of the objective lens and dividing it by the magnification power of the lens. Let’s take a theoretical comparison of two different binoculars in low light conditions (8x20 and 7x50 binoculars). According to Nikon, the average pupil diameter of a human eye is about 7mm. The exit pupil generated by the 8x20 binoculars is calculated by the formula 20÷8=2.5mm. Because the 2.5mm exit pupil of binoculars is smaller than the 7mm human eye pupil, the images you see will be dark.
For 7x50 binoculars, the exit pupil is calculated by 50÷7=7.1mm. Because the human pupil is about the same size as the binoculars' exit pupil, you will perceive images as bright as when seen with the naked eye, which is critical in low light conditions.
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