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Binocular Basics

Binoculars are available in hundreds of specifications and with a multitude of different features. Armed with the right information about basic components, terminology and binocular types, you'll be sure to choose the right binocular for your intended applications, budget and personal preferences.

1. Objective Lenses: The main (objective) lenses of a binocular serve to collect light, thereby enabling the high-resolution observation of distant objects. In a quality binocular each objective lens typically is manufactured of two separate glass elements, the so-called crown and flint elements. The refractive specifications of these elements permit the objective lens to image objects free of false colors.

2. Prisms: Since the objective lenses form images that are both upside down and reversed left for right, prisms are required to invert the primary image.

Most commonly, binoculars utilize either Porro prisms or roof prisms for this purpose. Porro prisms give binoculars their familiar zigzag profile, while roof prisms permit a straight-line design (see diagram at right). Either type of prism, properly manufactured, yields excellent optical results.

3. Eyepieces: The ocular, or eyepiece, design included with a binocular has important performance implications. While the most basic function of an eyepiece is to magnify the image formed by the objective lens, in fact the eyepiece also largely determines the binocular's field of view, edge-of-field image resolution and other characteristics listed below. Eyepieces are manufactured from two to five glass elements.

Terminology

Now that you've been introduced to the basic components of a binocular, let's dive into the technical terminology and features you'll encounter while shopping for a binocular:

Binocular Specifications: Binoculars are classified as, for example, 7 x 35mm — read "seven by thirty-five millimeters." In this case the binocular is of 7-power ("7x") and includes objective lenses of 35mm (about 1.38") diameter. Other binoculars might range from a tiny 3 x 14mm to giant battleship binoculars that are 40 x 178mm.

The "WA" designation after a binocular specification, such as 7 x 35mm WA, refers to the Wide-Angle design of the binocular's eyepieces; wide-angle eyepieces can increase a binocular's visual field of view by as much as 60%.

Magnification: Magnification, or power, is perhaps the most misunderstood term of binocular optics.

While higher powers can be useful, power by itself does NOT increase the level of observable detail; image resolution is a function of objective lens diameter, not of binocular power. Higher powers result in images that are less bright and in a binocular that is more difficult to hold steady in the user's hands.

Powers of 7x or 8x are by far the most popular among regular binocular users. Binoculars with magnifications above about 12x are generally not recommended for use without a tripod.

Field of View: A binocular's field of view is measured in degrees of arc or as field-width (in feet) at 1000 yards distance.

Lens Coatings: An uncoated optical glass lens or prism reflects about 10% of the light incident on one of its surfaces, allowing only about 90% of the light to pass through.

Standard coatings of magnesium fluoride (MgF2) applied to the lens and prism surfaces reduce the level of reflected light to about 4%, and with substantially reduced ghost images of bright objects.

More sophisticated multicoatings of 7 to 15 layers further reduce reflected light and can result in total light transmission through a lens or prism of 99% or more.

Eye Relief: Binocular users who wear eyeglasses for near- or far-sightedness may remove their glasses while observing; the binocular can fully correct for these eye defects. Observers who suffer from astigmatism, however, may need to wear their glasses to maintain sharp imaging through the binocular. In this latter case choosing a binocular with longer eye relief will enable easier binocular observing with eyeglasses.

Binocular Types

Mini Binoculars generally include objective lenses not larger than about 26mm (1") in diameter, are of a straight-line roof prism design, and are foldable for compactness and ease of transport.

Mini binoculars are small, lightweight and highly versatile in their range of applications. For example, as a moderately priced gift, it is a rare person who will not enjoy, and find many uses for, a mini binocular.

Because of their relatively small objective lenses, however, mini binoculars are not intended for high-resolution birding or other nature applications.

Compact Binoculars utilize Porro prisms to invert the image and usually are styled to form-fit comfortably in the observer's hands; objective lenses are typically 26mm in diameter or less.

As their name implies, compact binoculars, while larger than mini binoculars, are relatively small and easy to carry.

Compact binoculars are extremely popular for sporting events, as a gift item, or as a general-purpose travel binocular because, again, for all but the most advanced applications, compacts provide a good trade-off between weight, performance and cost.

Standard Porro Prism Binoculars: Most binoculars referred to as general-purpose are standard Porro prism models. The typically larger objective lens apertures, 35mm or more, of these models enable bright, high-contrast images on the entire range of viewing subjects, from sporting events, to long-range animal observation in the wild, to high-resolution study of a bird's feather structure.

A moderately priced, high-quality, standard Porro prism model is a binocular for almost any observing application. Standard Porro prism binoculars are available in a wide range of specifications and price points.

Zoom Binoculars offer the convenience of zooming to higher or lower powers at the touch of a finger.

Standard Roof Prism Binoculars: provide professional-level binocular resolution and performance. Designed usually for advanced applications, standard roof prism binoculars are typically of 35mm objective lens aperture or larger; include sleek, straight-line roof prism styling; and incorporate the finest optical glasses, multicoatings, and multi-element eyepieces. The result is bright, extremely sharp, high-resolution images throughout the field of view, and with a level of image fidelity unobtainable in lesser binoculars. Although premium-grade standard roof prism binoculars are not inexpensive, they are usually treasured for a lifetime.