At first glance, all binoculars appear to have similar specifications. So how do you account for the wide range in prices, and how do you know which factors to consider when making a choice?

Magnification

The "7" in 7 x 50 indicates a magnification power of seven. An image 700 yards away will appear to be only 100 yards away and seven times larger than with the naked eye. The "50" is the diameter of the objective (front) lens in millimeters. Most boaters prefer 7 x 50 binoculars because the 7x magnification allows you to locate objects and keep them within your field of view, and the 50mm lens admits enough light to make objects visible under low light conditions. Using binoculars with greater magnification makes finding and holding objects difficult, especially on a pitching deck, because small movements of the binoculars translate into sweeping arcs in the field of view. Greater magnification also reduces the clarity and definition of the image. Larger objective lenses gather more light, but the trade-off is heavier, cumbersome binoculars.

Field of View

This is the width of a scene that can be viewed at a distance of 1,000 yards, expressed either in feet or degrees of arc. The greater the magnification, the smaller the field of view. If you increase the magnification from 7x to 8x, a buoy will appear larger and closer, but the smaller field of view will make it harder to scan the horizon to spot the buoy quickly. A wide field of view is better for following fast moving action or scanning for moving objects. Two types of prism systems are available. Roof prisms place the lenses in line for compactness. With porro prisms, the eyepieces are offset from the front lenses and generally provide better depth perception and contrast.

Relative Brightness

Relative brightness indicates how well the binoculars will perform in dim light, and is the square of the binocular's exit pupil diameter. Exit pupil diameter is the objective lens divided by the magnification. For 7 x 50s, that's 7.14. If the exit pupil diameter is as large or larger than your pupil (about 7mm in darkness), you'll see an image that is almost as bright as the image viewed with the unaided eye. That's why 50mm objective lenses are so popular with boaters. The larger the objective lens diameter, the larger the exit pupil diameter, and thus the greater the relative brightness. Compare the exit pupil diameters of 8 x 50s (6.25) and 7 x 35s (5).

Relative Light Efficiency

This is a measure of the amount of light reaching your eye through the ocular (rear) lens relative to the amount of light striking the front (objective) lens, expressed in percent. The greater the quality of the lens glass and the optical coatings applied, the better the light transmission, and the brighter the image. RLE is an empirical figure consumers can't measure; it must be provided by the manufacturer.

Coatings

Contrary to popular belief, optical coatings are not applied to protect the lens, but to reduce reflection and increase light transmission. Steiner and Jason use magnesium fluoride coatings to improve light transmission and reduce eye strain. Some binoculars are coated for UV protection and to provide a clearer view in bright sunlight. In coated lenses, only selected lens and prism surfaces are coated. Fully coated binoculars have coatings applied to all light transmitting lens and prism surfaces. Coatings should not be apparent when looking through the lens. You can check the quality of the coatings by looking through the glasses in bright light: glare, ghosting, flares, halos, or darkened images should not be evident.

Steiner's Firebirds have a unique brilliant red optical coating developed to cut off UV and infrared rays and protect eyes from damage caused by prolonged use in snow, reflective waters, or other bright light conditions. The coating actually improves contrast and definition in bright light, and the image isn't red.

Focus

Center eye focus is more convenient than individual eye focus, but it cannot be made waterproof. If you opt for center eye focus, choose a pair with one adjustable eye piece to compensate for the difference in visual acuity between your eyes. Fast-, auto- and perma-focus features allow binoculars to remain in focus, or to stay focused from a particular point - say, 40 feet - to infinity, once set. This allows you to pass the same binocular from crew member to crew member without having to refocus. Before you purchase a pair with one of these features, try them out. They are recommended for people with corrected vision, and if your eyes have different prescriptions, you may not be able to get a clear image.

Ergonomics

A binocular, like a camera, should feel comfortable in your hands and easy to manipulate. Three basic body styles are available. European, the most common, features a separate barrel housing the objective lens. American style uses a single solid body piece; it's sturdier, but generally more expensive. Compact bodies have a single, solid, lightweight body. Consider weight as well as size: most boaters think lighter is better because heavy binoculars are harder to steady and cause neck strain with prolonged use. Boaters with small hands may have difficulty adjusting large binoculars.

Armor

Rubber armor provides a sure grip when wet, and better skid-resistance when lying on deck. Because the rubber protects against shock and other onboard abuses, these models don't need cases or covers, and require less pampering. To be labeled waterproof, a binocular must be able to withstand submersion at a set depth for a specified time period, usually 16' for five minutes. Water-, weather-, spray- and fog-proof are not the same.

Nitrogen Purging

To prevent oxidation and fogging, some manufacturers replace the oxygen in the body with dry nitrogen. This reduces the possibility of interior corrosion in the binoculars.

Compasses and Rangefinders

Several models are available with built-in compasses and/or rangefinders. They are particularly helpful when the compass is illuminated and can be used to pick up bearings at night. The compass should be accurate to within 1°, easy to read, and should swing freely even when the binoculars are not perfectly horizontal. Reticles, the scales superimposed over the image, allow you to figure range using objects of known height. Look for reticles with very fine lines and numerals for quick calculations.

Additional Features

If you wear eyeglasses, look for soft, pliable rubber eyecups that can be folded out of the way. They're generally more comfortable for all users, especially on a pitching deck. Check the eye relief, the distance from the eye to the surface of the ocular lens. Longer eye relief, between 16-20mm, allows eyeglass wearers to see the entire image and reduces eye strain.

If the model you purchase doesn't have a neck strap, buy one and use it. The thicker ones are usually more comfortable for long wearing periods. Don't let a precious pair of binoculars go overboard for want of an inexpensive neck strap.

Lens caps will protect against moisture, scratches, and other damage to the lenses. Be aware that not all lens caps are attached.

At the end of the day, clean the lenses with special lens cleaning paper or cloth, and store them in the case.