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Binoculars
Author: MariSafe
Binoculars are a vital tool on board, filling dual roles for safety and navigation. They extend your vision, allowing an early "visual" on distant channel markers or landmarks. A good pair of glasses can cut through haze or poor light conditions so that you can find navigation aids invisible to the naked eye.
SPECIFICATIONS & FEATURES
Selecting binoculars may seem easy at first glance, but the range of prices, features, specifications and options often makes the selection a bit dicey if the goal is to end up with binoculars that suit your exact needs. Considerations include:
- Magnification
- Relative brightness
- Field of view
- Depth of field
- Focusing
- Water resistance
- Lens coatings
- Rubber armoring
- Stabilization and night vision options
- Compass and range finding options
Knowing a little about each one of these properties will assist in making the selection process easier and assure a better chance of choosing binoculars that will serve you well. The two numbers separated by an X that accompanies every pair of glasses represent the most basic of these properties.
Magnification, often referred to as power, is represented by the first of these two digits and indicates how many times an object viewed is enlarged. For example, with a pair of binoculars rated at 7 X 50, the seven indicates that magnification is seven times, thus an object 700 yards away will appear to be at 100 yards when viewed through these glasses. Put another way, the object will appear seven times closer than looking at it without the binoculars.
In fact, most authorities on binoculars feel that a magnification of seven is just right for use on a boat. While many sports fans, operagoers or birdwatchers use binoculars of eight, ten or even higher magnification, such power has serious drawbacks aboard. The most important of these is that as magnification is increased, motion is also enhanced, and this magnifies the wavering, shaking and vibration inherent in use on a boat. In addition, increased magnification is not always better at allowing light to pass, often results in a narrower vision of the object (field of view), and distorts depth of view—and all of these relationships can reduce the sharpness of the image.
Relative Brightness indicates the performance level of binoculars in dim light. The second digit, the"50" of the 7 X 50 example above, is very important in determining the amount of light that enters the glasses. This number represents the diameter of the front, or objective, lens measured in millimeters, so our 7 X 50 glasses would have an objective lens 50 millimeters, or just over 2 inches, across. The larger the diameter of the objective lens, the more light is let in for additional brightness and more relative brightness is always desirable as it permits more detail and color to be observed.
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After light enters the objective lens of the binoculars, it passes through several glass surfaces, or prisms, within each barrel. The light loses a small amount of energy with every surface that it strikes before arriving at the rear lens, which is known as the "ocular" lens. Thus, it is not only the size of the objective and ocular lenses that determine the brightness, but the quality and number of prisms that the light must pass through before reaching your eye. Lens coatings also affect light transmission and will be discussed shortly.
Field of View is the width of the area
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