Marine Binoculars

How they are measured

Binoculars are described using two numbers such as 7 x 50 or 8 x 25. The first number identifies the magnification or power. The second is the diameter of the objective lens (the lens at the front) in millimeters.

Magnification power describes how much closer objects appear when you view them. In a 7 x 50 binocular, the image is magnified seven times. Increased magnification reduces the brightness of the image, so as magnification increases, binoculars require increasingly larger objective lenses to maintain brightness. The larger their size, the more light they can gather.

Features of marine binoculars:

Magnification

Many binoculars used for birdwatching or other land applications have too much magnification for use on rocking, pitching boats. The more an image is magnified, the harder it is to keep in view, so 7x power is the practical magnification limit for small boats (under 50’). Image stabilizing binoculars allow comfortable viewing with much greater magnification, up to 18x power, because they automatically compensate for movement.

Waterproof construction

The damp marine environment, combined with changes in temperature, causes interior lenses of non–waterproof binoculars to fog. Waterproof construction, with the interior o-ring sealed and filled or “charged” with dry nitrogen, combined with flotation in the strap, protects your binoculars if they fall overboard.

Light transmission efficiency

Cheap glasses may allow only half the light entering the objective lenses to reach your eyes. Our least expensive glasses pass about 75% of the light. Truly exceptional binoculars, such as top models from Steiner and Fujinon, pass more than 93% to 97% of light to your eyes, making all objects appear brighter. Quality optics also make the image sharper. Inexpensive glasses may produce astigmatic images that are fuzzy at the edges. Superior glasses are sharp from edge to edge and are less fatiguing to the eyes when used for extended periods.

Relative brightness

The relative brightness of a pair of binoculars is a function of the quality of the optics and the ratio of the objective lens diameter divided by the magnification, squared (50 ÷ 7)2 ³ 50. Thus 7 x 50 binoculars have a relative brightness of about 50, while 8 x 23 binoculars have a relative brightness of only 8.2. Objects will be visible in far less light with the 7 x 50 models.

Lens coatings

When light enters or leaves a piece of glass about 5% is reflected back. With as many as 16 air/glass surfaces inside your binoculars, there could be a lot of internal light bouncing around, reducing the brightness, sharpness and contrast of the image. Lenses are coated using one or more thin layers of chemicals (most commonly magnesium fluoride) reducing this internal reflection from 5% to 1% or less. But not all coatings are the same. If you look at the outside lens surfaces, quality lens coatings will appear as subtle tints of violet, blue or green. Heavily colored lenses in cheap glasses actually reduce the amount of light transmitted. Also, better binoculars include more layers, with more complex chemical combinations, on more surfaces, to achieve their amazing light transmission efficiency. The choices:

  • Coated: one or more surfaces coated with a single layer.
  • Fully coated: all air-to-glass surfaces are single-layer coated.
  • Multi coated: one or more surfaces coated with multiple layers.
  • Fully multi coated: all air-to-glass surfaces are coated with multiple layers

Field of view

Field of view describes the width of the image you see, measured in feet at the distance of 1000 yards. Binoculars offering 385’ field of view show the viewer a cone that is 385’ wide 1000 yards out. Higher-powered image stabilized binoculars offer narrower field of view (200–340’) than conventional units (up to 430’)

Prisms

Prisms, used to invert an upside-down image, are either Porro (with a dog-leg shape) or roof prisms. There is some disagreement as to which is best, but it’s generally believed that Porro prisms yield superior optical performance. They transmit more light, resulting in brighter images, and provide better depth perception, because their objective lenses are farther apart. However, some roof prisms with phase shift coating provide excellent performance. Prisms come in two glass types, BK-7 and BAK-4. BK-7 uses boro-silicate glass and BAK-4 use a denser, finer barium crown glass, which eliminates internal light scattering and produces sharper images than BK-7. The higher quality is reflected in the price of the binoculars. Roof prisms are lighter and more compact, but they are more complex and difficult to manufacture, and have more precise tolerances, so they are usually more expensive than Porro prisms.

Eye relief

Eye relief is the maximum distance from the eyepiece (ocular lens) at which your eye can still discern the full field of view. If you wear eyeglasses or sunglasses, this is important. Most quality binoculars have at least 1” or 25mm of eye relief.

Focus

Binoculars may have independent eyepiece focus to compensate for the differences between eyes and for different distances. In center focus binoculars, one eyepiece adjusts to accommodate the difference between your eyes. A central focus knob then adjusts both sides simultaneously for distance. For maximum convenience, we like center focus binoculars.

Bearing compasses

Built-in compasses, which appear superimposed near the image you see through the lens, let you take bearings from an object that would be invisible with a nonmagnified hand-bearing compass. Highly recommended for marine use.

Rangefinder reticule

If you know the height of an object like a hill or navigation marker (often printed on charts) and can measure the angle to its top using binoculars equipped with a rangefinder reticule, you can calculate your distance from that object.

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