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.

Boating - - FRONT PAGE - By Randy Vance

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.

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