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:
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, 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 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.
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.
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.
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.