Although many don't appreciate the fact, choosing a pair of binoculars is a complex subject, and just looking through a set in the store cannot enable you to make a wise choice. Consider this a primer. For more information, you can also take a look at the article "Consumer's Guide to Boat Binoculars" by Tom Neale in the BoatUS magazine archives.

Magnification (Power)

Power is determined by the size, curvature, and placement of the lens systems. The "7" in 7 x 50 indicates a magnification power of seven. This means that an image 700 yards away will appear to be only 100 yards away and seven times larger than with the naked eye. The "50" is the diameter of the objective (front) lens in millimeters. Most boaters prefer 7 x 50 binoculars because the 7x magnification allows you to locate objects and keep them within your field of view, and the 50mm lens admits enough light to make objects visible under low light conditions. Using binoculars with greater magnification makes finding and holding objects difficult, especially on a pitching deck, because small movements of the binoculars translate into sweeping arcs in the field of view. Greater magnification also reduces the clarity and definition of the image. Larger objective lenses gather more light, but the trade-off is heavier, cumbersome binoculars.

Field of View

This is the width of a scene that can be viewed at a distance of 1,000 yards, expressed either in feet or degrees of arc. Each degree equals 52.5 feet of width at 1,000 yards. The greater the magnification, the smaller the field of view. If you increase the magnification from 7x to 8x, a buoy will appear larger and closer, but the smaller field of view will make it harder to scan the horizon to spot the buoy quickly. A wide field of view is better for following fast moving action or scanning for moving objects.

There are two types of prism systems. Roof prisms place the lenses in line for compactness, the result is a less bulky shape. With porro-prisms, the eyepieces are offset from the front lenses, and generally provide better depth perception and contrast. They also use offset optics to produce a greater separation of foreground and background images, for a more pleasing view due to the stereoscopic perception created.

Relative Brightness

Relative brightness indicates how well binoculars will perform in dim light, and is the square of the exit pupil diameter. Exit pupil diameter. is the objective lens divided by the magnification. For 7 x 50s, that's 7.14. If the exit pupil diameter is as large or larger than your pupil (about 7mm in darkness), you'll see an image that's almost as bright as the image viewed with the unaided eye. That's why 50mm objective lenses are so popular with boaters. The larger the objective lens diameter the larger the exit pupil diameter and thus the greater the relative brightness. Compare the exit pupil diameters of 8 x 50s (6.25) and 7 x 35s (5).

There is a formula to determine relative brightness:

( Diameter of Objective Lens / Power ) 2

For example, a 7 x 35 binocular would have a relative brightness of: (35/7)2 =25. If the optics are fully coated, the relative brightness is increased by 50% (so in the above example, the relative brightness becomes 37.5.

Relative Light Efficiency

This is a measure of the amount of light reaching your eye through the ocular (rear) lens relative to the amount of light striking the front (objective) lens. The greater the quality of the lens glass and the optical coatings applied, the better the light transmission, and the brighter the image. RLE is an empirical figure consumers can't measure; it must be provided by the manufacturer.


Contrary to popular belief, optical coatings are not only applied to protect the lens, but to also reduce reflection and increase light transmission. Steiner and Bushnell use magnesium fluoride coatings to improve light transmission and reduce eye strain. Some binoculars are coated for UV protection and to provide a clearer view in bright sunlight. In coated lenses, only selected lens and prism surfaces are coated. Fully coated binoculars have coatings applied to all light transmitting lens and prism surfaces. Coatings should not be apparent when looking through the lens. You can check the quality of the coatings by looking through the glasses in bright light: glare, ghosting, flares, halos, or darkened images should not be evident.


Center eye focus is more convenient than individual eye focus.

If you opt for center eye focus, choose a pair with one adjustable eyepiece to compensate for the difference in visual acuity between your eyes. Fast-, auto- and perma-focus features allow binoculars to remain in focus, or to stay focused from a particular point-say 40'-to infinity, once set. This allows you to pass the same binocular from crew member to crew member without having to refocus. They are recommended for people with corrected vision. If your eyes have different prescriptions, you may not be able to get a clear image.

Some binoculars incorporate Image Stabilization technology. These binoculars have internal sensors that detect any vibration or motion and make the necessary corrections. The result is smooth and seamless, allowing rapid scanning both horizontally and vertically without lags. But binoculars with bearing compasses may not be as accurate with a stabilization feature because the sighting aren’t precisely real time.  


Rubber armor provides a sure grip when wet, and better skid resistance when lying on deck. The rubber helps to protect against shock and other onboard abuses.

Nitrogen Purging

To prevent oxidation and fogging, some manufacturers replace the oxygen in the body with dry nitrogen to reduce the possibility of interior corrosion. They're then sealed with secure, internal O-rings to ensure waterproofness. To be labeled waterproof, a binocular must be able to withstand submersion at a set depth for a specified time period, usually 16' for five minutes.

Compasses and Rangefinders

Several models have built-in compasses and/or rangefinders. They're particularly helpful when the compass is illuminated and can be used to pick up bearings at night. The compass should be accurate, easy to read, and swing freely even when the binoculars are not perfectly horizontal. Reticles, the scales superimposed over the image, allow you to figure range using objects of known height. Look for reticles with very fine lines and numerals for quick calculations. Be sure that the can see the compass easily in the field of view but that it doesn't impair your target.

Additional Features

If you wear glasses, look for soft, pliable rubber fold-down eyecups. They're more comfortable for all users, especially on a pitching deck. Check the eye relief, the distance from the eye to the surface of the ocular lens. Longer eye relief, between 16-20mm, allows eyeglass wearers to see the entire image and reduces eye strain.

If the model you buy doesn't have a neck strap, buy one and use it. The thicker ones are usually more comfortable for long wearing periods.

Lens caps will protect against moisture, scratches, and other damage to the lenses. Be aware that not all lens caps are attached.

At the end of the day, clean the lenses with special lens cleaning paper or cloth, and store them in the case.

Night Vision Systems

Night vision technology allows you to distinguish objects at night, even at distances of up to several hundred yards, in the absence of artificial light. Night vision systems amplify existing light and will not operate in complete darkness without the aid of infra-red enhancements. You only need a little ambient light-even starlight will work. Some models have infrared, which sends out a beam of light that is multiplied. This enables viewing in complete darkness and is particularly good for locating reflective buoys.

Night vision systems provide electronically enhanced viewing: you're not actually viewing the scene before you, but rather a video image of that scene as if viewed through a video camera. The system picks up the available light reflected from the objects being viewed, and converts the radiant energy of the light to electrical energy in the system. The electrical charge then becomes an accurate representation of the scene being viewed.

Night vision systems vary widely in performance, so it's important to recognize some basic performance criteria. The three most commonly cited specifications: Light amplification is the amount of light multiplied. Sensitivity is the light threshold at which the system will operate, the relative darkness if you will, under which it will perform. Resolution shows the amount of detail you will be able to distinguish through the system.

The development of night vision devices has occurred in steps, called Generations. Generation I viewers are the earlier, foreign night vision devices using Russian-made tubes. Generation II scopes are best used on nights with a quarter moon or more. Generation III viewers offer more advanced night vision. Generation III viewers also have a longer tube life. But this technology is still developing. For example, Forward Looking Infrared Imaging (FLIR) Systems represent a substantial development by one company.

Although this is a simplified explanation of night vision systems, it helps give you a working knowledge of these systems. If you do any nighttime boating, do yourself a favor-don't be left in the dark!

You may also want to take a look at our article on Night Vision Systems

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