BOAT BINOS

Getting a good look through lenses on a boat is a different thing than on land. We review a few binoculars that you can choose to use on your boat.

How do you know whether premium binoculars are worth the money? Optically, that’s a tough thing to determine. Sometimes the clearest elements of the choice come from brand equity, warranty and mechanical preferences. But we devised some tests to give you an experienced layman’s review. POWER Optimum magnification for boats is considered to be 7x, or a magnification of seven. More powerful binoculars make it harder to focus on a moving target from a moving platform — your boat.

OBJECTIVE LENS

This is the front or larger of the lenses. A 50mm objective lets in lots of light, valuable at dusk and dawn. In bright light, the distinctions between optic quality didn’t change dramatically from pricey to value-priced, but we could detect the difference on our eye-chart test.

EXIT PUPIL

A 7x binocular with a 50mm objective lens will have a 7.14 mm (50 divided by 7 equals 7.14) exit pupil. Theoretically, the larger the exit pupil, the brighter the image. But inferior optics could diminish the impact of a large exit pupil.

FIELD OF VIEW

A wide field of view is preferable when viewing moving tar- gets from moving platforms. Field of view is measured as X feet at a range of Y. Typically, you’ll see FOVs of 300 to 400 feet at 1,000 yards.

EYE RELIEF

Eye relief is a measurement of how far from the eye the ocular can be held and still give a full image through the binoculars. This is especially important for users who wear corrective glasses.

FOCUS

Internally focused binoculars are more intuitive to use. The center knob adjusts the left eye, then adjust the focus ring on the right ocular (eye lens piece). Refocusing for different distances is done only with the knob. Binoculars for sports tend to have an ocular focus for each barrel, and once focused for, say, 100 yards, anything farther will be in focus.

COATINGS

Anti-reflective coatings come in various chemical makeups. Some binoculars only coat the outer sides of the outer lenses. Fully multicoated binoculars have all lenses and prisms coated for optimum light transmission with minimal loss to reflection.

PRISMS

Binocular lenses invert the image. Prisms are used to turn the image upright for the eye. Porro prisms use two prisms to turn the image 180 degrees. These larger prisms are optically ideal but require a wide shouldered binocular frame to house them. Roof prisms employ more and smaller prisms to invert the image, and these prisms are more compact, giving a smaller, linear look to the binocular barrels. The best prisms are of BAK4 glass; lesser prisms are of BK7.

TESTING

We used Snellen eye charts at 25 yards, one in dim light and one in brighter light. Outdoors we used one Snellen eye chart at 100 yards in bright light and noted how many lines on the chart we could read.