What Do Those Words & Numbers Mean?

There are two numbers used to describe binoculars. All W&P marine binoculars are 7 x 50, but you can also find 7 x 28, 12 x 50 and more. The first of these numbers refers to the amount of magnification and the second number refers to the objective lens diameter (the big lens).

Magnification: Weems & Plath binoculars are 7 power magnification, which is the best for use on a moving boat. Greater magnification makes it very difficult to hold and view any object. For land use, increased magnification of 10 power is fine and with stabilized binoculars, such as Fujinon and Canon, you can go up to 12 and 14 power (even if you are using them on a boat) because of their internal stabilization feature.Binoculars Magnification Power

BN50 Lens Diameter

Objective Lens Diameter: This is the diameter of the large lens which is measured in millimeters. The objective lens of all W&P marine binoculars is 50 mm in diameter. The greater the objective lens diameter, the better it is for situations when the light level is low because the larger lens gathers more light. This is important if you are out on your boat at night, dawn, or dusk.

Exit-Pupil-DiameterExit Pupil: (Diagram from CosmicPursuits.com) This refers to the circle of light you see in the lens when you hold the binoculars away at arm’s length. It is a number that indicates how bright an object will appear when viewed in low-light situations. The objective lens diameter divided by the magnification power determines the exit pupil dimension. For 7 x 50 binoculars the exit pupil is approximately 7 mm which is the same diameter as the eye when dilated, so the ratio of 7 x 50 is the most efficient design.



Prism Styles: Roof and Porro (Diagram from Maisonea.com)



  • Roof Prisms are stacked pieces of glass that make the binoculars compact and tubular shaped.
  • Porro Prisms require more space for the path of light that reflects through the binoculars, but they offer more light transmission to your eye than roof prisms.

Binocular Lens CoatingLens Coatings: A coating on the lens helps to maintain optimal brightness, sharpness, and image contrast. A thin layer of minerals or chemicals is applied to all the glass surfaces. Weems & Plath binoculars have Fully Multi-Coated (FMC) surfaces, meaning all of the glass surfaces have multiple coatings – the finest possible level of optics. The coatings cut down on internal reflection so your binoculars perform perfectly in all frequencies of light.

Focus: All binoculars need to be focused to compensate for the differences between the right and left eye, and to make adjustments for the distances to the objects you are viewing. There are two types of focusing methods and W&P binoculars offer both types: the Weems & Plath BN20C, BN40, and BN50 are individual eye focus binoculars where as the BN10 binoculars are center focus.Binocular Focus Types

Binocular Eye ReliefEye Relief: This is the minimum distance between a binoculars’ ocular lens and your eye which allows you to see the entire field of view. Eye relief is important if you wear glasses because your glasses keep you from getting close enough to the eyepiece to see the full field. To shorten the distance to see the full field of view, twist down or roll back the eyepiece.

To get the most from your binoculars, check out this video that teaches you how to fine tune your binoculars for personal use:

Posted by blognewsweemsplath

At Weems & Plath, we take great pride in manufacturing time-tested nautical instruments for safe and enjoyable boating. Since 1928 we have successfully navigated the waters of changing times by keeping our customers at the forefront of thought. Our longevity is due to our commitment to you as we provide top quality products with exceptional warranties to back them up.

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