BoatUS Foundation Findings Report #2: Binoculars

Foundation Findings Report # 21 - March 1995
Binoculars - Oh Say Can You See?

COMING SOON! - We will be publishing an updated version of our binoculars test. The following is a reprint of our first test, conducted in 1995. Stay tuned to learn more about some of the latest technologies available in binoculars.

Most recreational boaters recognize that a good set of binoculars can be both an important piece of safety gear and fun to have aboard. They help boaters identify reference points on the water and shore, allowing them to better find their way. Binoculars also can provide a close-up view of wildlife, other boats, and objects of interest along the shore. They even help fishermen locate fish feeding areas from a distance by spotting birds gathered over them.

But few of us know what to look for or how to ensure that we get the most out of our investment when buying a pair of binoculars.

To help boaters decide which binocular characteristics may be most important in the marine environment, the BoatUS Foundation for Boating Safety and Clean Water tested an array of binoculars with various features at a wide range of prices and under differing conditions. We will discuss the results of this evaluation and look at some tests you can do to help decide which pair is best for you.

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BoatUS Foundation for Boating Safety and Clean Water team test member reads off the letters and symbols as she perceives them on a distant eye chart while another tem member records her observations.

What Binoculars Do:

Binoculars enhance vision by enlarging images - making them appear closer than they are - and by allowing us to see better under reduced-light-conditions - at dawn or dusk, during the night, or on cloudy days - by gathering more light than is possible with the naked eye.

Many believe that the more binoculars cost the better they will work. Some also think they must have the most expensive pair to get the best performance, regardless of how or where they will be used. But our tests show that, while several higher-priced units did score will, there is not necessarily a direct benefit-to-cost ratio.

Binocular Basics:

Knowing what the numbers on binoculars' exteriors mean can help you decide which pair is best for you. They indicate basic features like magnification, lens size, and - in some cases - field of view. For instance, the "7" in 7 x 50 means that an object 700 feet away would appear to be only 100 feet away.

The"50" in 7 x 50 indicates the diameter in millimeters of the front, or objective lenses. All other things being equal, larger objective lenses will gather more light, called relative brightness, making objects easier to see. But the most critical measurement for relative brightness is the size of the rear lenses' exit pupils, where your eyes see the object's reflection.

Our eye pupils vary in size from approximately 2 millimeters to 7 millimeters, depending on how bright or dark it is. Therefore, binoculars with 7-millimeter exit pupils can provide the maximum relative brightness usable by the human eye under reduced-light conditions.

To figure a binoculars' exit pupil diameter, divide the objective lens diameter by the magnification. For instance, 50/7 = 7.14, an exit pupil diameter that reflects the most usable relative brightness in dim light, when eye pupils may dilate to their maximum 7 millimeters. On the other hand, 10 x 50 binoculars would provide only a 5 millimeter exit pupil.

A third number, displayed on many binoculars, indicates the width of the field of view, in feet or meters, at a range of 1,000 yards. In some instances, the number indicates the number of degrees in the viewing angle. By using a mathematical formula that multiplies the angular degrees by 52.4, we can calculate the width of the field of view in feet.

Therefore, with a 7.1-degree angle of view, we find that the field of view is 372.04 feet by multiplying 7.1 x 52.4. If there is a 7-degree, 30-minute angle of view, we have the equation 7.5 x 52.4 = 393 feet, since 30 minutes translates to one-half degree.

Evaluating Performance:

Four BoatUS Foundation staff members evaluated nine sets of binoculars in tests designed to simulate real-world situations under various natural-light conditions. Our four subjects represented a good cross-section of visual acuity, including one with 20/20 vision, one who is only slightly nearsighted, one with an astigmatism in one eye, and one who wears bifocals to correct both nearsightedness and farsightedness.

We tested eight pairs of new binoculars - seven 7 x 50s and one 8 x 30 - and on older set of 10 x 50s.

Testing at Night:

In a night test, we attempted to match some of the conditions that would prevail on a broad body of water in darkness by evaluating binoculars in a park virtually devoid of light except for a waxing half-moon on a clear night. From distances that varied between 200 feet and 125 feet we focused in on 10-inch reflective letters and numbers of a type used by the U.S. Coast Guard on navigational buoys.

Starting at 200 feet, with the moon behind us so we could get the best possible view and the characters would get maximum exposure to the available light, we peered through each set of binoculars and recorded what we saw. We then moved forward 25 feet at a time to each of the next three stations.

The superior optics of the more expensive binoculars proved their worth during the nighttime tests by scoring at the top. The generally accepted premise that 50-millimeter lenses collect more available light in low-light situations seems to have been borne out by the dead-last finish of the smaller, 8 x 30 binoculars.

On The Water:

To evaluate how wave motion and other open-water conditions might affect the use of binoculars, we conducted a daylight test from the deck of a 30-foot boat in choppy water. Again, we viewed the mounted, reflective numbers and letters and recorded our observations, beginning at about 1,600 feet and moving as close as 1,000 feet.

Even with the bright sunshine, it was difficult to focus on the easel-mounted figures as we tried to filter out reflected light from the waves and objects in our line of sight while compensating for the wave action. Also, "sailing" back and forth at anchor forced us to refocus many times to pick our target out from the background clutter through a maze of shrouds, forestay, and pulpit.

In Our Own Backyard:

We conducted several other tests under more controlled conditions in a flat area behind BoatUS headquarters in Alexandria, Virginia. We viewed a sign with differently sized lines of eye-chart-style letters and symbols at a fixed distance and under different light conditions - daylight, twilight, and night.

The speed with which visual conditions deteriorated during the twilight tests made us aware that it is best to take necessary sightings as long before sundown as possible because navigational markers tend to fade quickly at dusk. The superior light-gathering ability of the better quality binoculars became evident during these tests.

The results of our various tests are tabulated in the chart below.

What Are Your Needs:

Most authorities agree that 7 x 50 binoculars are best suited for nautical use because they're relatively light weight and there is a good balance between magnification and light gathering capability.

If you will be using your binoculars only in bright sunlight when your pupils are contracted, magnification may be more important than light gathering capability and you may be able to get by with a relatively inexpensive pair. Conversely, if there is a chance you might need them in limited light, the size of the lenses and quality of the optical system could become very important.

Additional Features:

When you go to the store, take time to check out some of the binocular features that may prove important for you. Does it come with a carrying case and/or neck strap? Are the lens covers attached for security or removable? Is the exterior undamaged and are the lenses unscratched and clean?

A look at the manufacturer's literature will tell you to what degree the binoculars are shock resistant and if they are waterproof. Are there any guarantees? The literature may also tell you about interior lens quality and coating, which affect light transmission and image clarity.

Lens and prism coatings permit more light to pass through binoculars are shock resistant and if they are waterproof. Are there any guarantees? The literature may also tell you about interior lens quality and coatings, which affect light transmission and image clarity.

Lens and prism coatings permit more light to pass through binoculars. They do this by reducing the amount of light reflected by or bounced off lens and prism surfaces to increase relative light efficiency - a measure of the amount of light exiting the rear lens relative to the amount of light striking the front lens. Of the four lens coating levels, "fully multicoated" is the best, with only about a 5% light loss between the objective and ocular (rear) lens. Other coating specifications include "multicoated," 11% light loss; "fully coated," 17% light loss; and "single coated," about 35% light loss. Uncoated lenses can cause as much as a 47% loss of light. But the exact relative light efficiency can only be provided by the manufacturer.

Waterproof binoculars are internally purged, replacing oxygen with nitrogen, and O-ring sealed. Most Japanese manufacturers require that, to be considered waterproof, they must withstand submersion in approximately 16 feet of water for up to five minutes without damage. Truly waterproof binoculars are impervious to saltwater and dust. And the internal purging also prevents oxidation and internal fogging.

What About Eyeglasses?

Generally speaking, people without astigmatism who normally wear eyeglasses should remove them when using adjustable binoculars. But they mustn't expect these instruments to offer a "magic bullet" that gives them telescopic vision and improves their ability to see so that it is on a par with people whose sight is unimpaired. The person with the best regular vision will probably always see a little better when using binoculars under most sets of circumstances.

Keep your glasses on when using nonadjustable binoculars and fold down the rubber eyepieces to maximize vision. Since no binoculars compensate for astigmatism, users with that problem should wear eyeglasses to get the best possible view, even when using adjustable ones.

If you ever do wear eyeglasses while using binoculars and you suffer from a combination of myopia and farsightedness, be sure to wear your reading glasses because you will be looking at the image inside the binoculars, not at a distant object. Wearing eyeglasses may also result in peripheral light loss.

Doing Your Own Tests:

To examine binoculars' interior features, hold it to your eyes and pivot the two barrels so only a single circle is seen. Next, focus on an object in the store that is as far away as possible. Never look through a window at an object as it can distort the image, nullifying the test.

For most people, there is a vision variance between their two eyes, so adjustable lenses should be focused separately. The best way to focus on an image is to keep both eyes open as you cover one barrel's objective lens with your hand while adjusting the other.

With center focus binoculars, first use the center control to adjust the left barrel until the image appears in sharp focus. Next, cover the left barrel with your hand and rotate the right eyepiece until the image appearing in that lens is sharply focused. Now you can look simultaneously through both eyepieces and use the center control to focus on objects at different distances. You should also adjust binoculars with individually focused lenses one barrel at a time.

Next, use the binoculars you have selected to focus on an object in the store. If they offer good, basic image clarity, proceed with the tests. If not, you may want to consider another pair. This is also a good time to check that you feel comfortable with the weight and size of the binoculars as you hold and adjust them.

To select binoculars that will provide maximum performance with minimum eye fatigue, try the following checks:

Lens Prism Types - In most cases, you will be testing binoculars with Porro prisms, widely used because they are relatively inexpensive and create a view as good as roof or Dach prisms, which are used mainly in compact binoculars. Porro prisms also provide better viewing depth with a more three-dimensional effect. Of the two common Porro types, BAK-4 high index prisms are superior in quality to BK-7's, because they transmit virtually all of the image-light from the objective lenses.

BK7 low index prisms have a higher refraction index which results in interior reflection, reduction of image quality, and some image blackout - all of which reduce the image light from the objective lenses, particularly in low-light conditions.

To check the binoculars you are examining for prism type, hold them at about arm's length and point them at a good light source while looking at the eyepiece lens. The BAK-4 prisms will produce a bright circular image, while the BK7's project an easily discernible gray blackout area in the shape of a wide diamond around the inside edge of the exit pupil.

Barrel Alignment - Having the two binocular barrel lenses out of alignment can result in severe eye strain and even double vision, caused by your eyes trying to compensate for this focusing imperfection. To test for this problem, focus with both lenses on an object in the store. Rest your eyes for 30 seconds. Then bring the binoculars up to your eyes with your hand covering one of the front lenses. Now drop your hand. If the image is out of focus, then slips back into focus, it may bean the binocular barrels are misaligned and your eyes are trying to compensate.

Horizontal Linear Alignment - Another check for correct barrel alignment can be done by focusing on a horizontal line with binoculars that are properly adjusted so you see only one image with both eyes. Then move the binoculars away from your eyes until you see separate horizontal lines in the two eyepieces. Now open the binoculars up until the two barrels are side by side. The two lines should fall on the same plane, producing proper axial symmetry for strain-free viewing.

Internal Blackening - Another factor that affects light transmission is internal blackening, which cuts down reflections inside the binoculars. Tilt the binoculars at about 45-degrees as you look inside the objective lens. If you see any metal parts or anything other than a dull, blackened surface, it can cause reflected light to bounce around inside causing a distracting glare that interferes with image quality.

Conclusion:

Binoculars are a very personal piece of boating gear, so you should select a pair that feels right to you. Keep the following in mind as you shop:

In our tests, the higher quality, more expensive binoculars generally tended to score better, particularly in low-light conditions.

Binoculars will not restore 20/20 vision to those with visual impairments, but they will greatly enhance everyone's ability to see distant objects,so - if you have vision problems - look for a pair that is easily adjustable.

Each person must find the binoculars that are best for them, in terms of handling comfort, weight, size, price, and intended use; and

Taking time to properly focus and adjust binoculars to your particular needs and characteristics will produce the best image, therefore this important step should not be overlooked or hurried.

Finally, choosing even the least expensive pair of binoculars is preferable to carrying none at all aboard your boat.