Choosing Binoculars

One pair isn't necessarily as good as another. When you're choosing binoculars, make sure it's an informed choice.

| September/October 1984

  • choosing binoculars - man in profile looking through binoculars
    Some binoculars are better than others for night viewing.
  • choosing binoculars - diagram of prism binoculars
    If you want to make an informed decision when choosing binoculars, it helps to know their parts. This pair uses the porro prism system.
    Illustration by MOTHER EARTH NEWS Staff

  • choosing binoculars - man in profile looking through binoculars
  • choosing binoculars - diagram of prism binoculars

Good binoculars are an investment that can magnify your enjoyment of almost all outdoor experiences — and a good many indoor experiences. But there are more than a few points to consider before plunging into what can be a sizable purchase (top-line binocs go for several hundred dollars). Factors such as weight, size, magnification power, image brightness, field of view, focusing, lens construction and coating, and — most important — your own particular needs all deserve careful thought when choosing binoculars.

The Basics

Before we jump into the nitty-gritty of long-range optics, we need to get a few terms and definitions under our belts. The first of these is the most basic and frequently the least understood. I'm referring to the differences between binoculars and field glasses. Though the two terms are often used interchangeably, there is a distinction. Field glasses are the ancestors of binocs, and they're simply two Galilean-type telescopes — each with two in-line lenses hooked together with a common focusing apparatus — and fraught with several optical defects. Not least among the shortcomings of field glasses is a field of view (the area visible through the lenses at a given distance) that decreases dramatically as magnification increases. Consequently, field glasses are useful only for low-magnification applications, usually not to exceed a power of four (4X). But if you can get by with such moderate magnification, field glasses do have the singular advantage of being far less expensive than binoculars — more correctly called prism binoculars — to which we'll turn our attention in a moment.

But first we need to learn two lens-defining terms: Ocular lenses are the small ones on the eye end of binoculars (in fact, they're often called eye lenses, or eyepieces), while objective lenses are the big ones at the front, where light enters.

Binoculars are capable of optically precise magnification far beyond that of field glasses because they employ prisms as well as magnifying lenses. The most common prism arrangement used in binoculars is called the porro system, which utilizes two prisms in each barrel; the first magnifies and inverts the image, and the second turns what you see right side up again. In addition to increased power, the prism setup allows for wider separation of the objective lenses, thus increasing binocular vision.

Uh-oh, there's another term — binocular vision. In simpler words, this means two-eyed vision: seeing the same object through two eyes set a distance apart, thus allowing the brain to compute estimates of depth and distance from the two slightly different images. Well, the greater the separation of those two images, the greater the depth of field (the amount of image that can be sharply focused). OK so far? With field glasses, that separation is minimal, limited to the width of the two parallel telescopes, or about the normal distance between human eyes. But with porro prism binoculars, the reflecting prisms allow the front, or objective lenses, to be mounted farther apart, while the ocular (eyepiece) lenses remain a comfortable distance from each other.

Magnification and Lens Size

All binoculars have a pair of numbers stamped into one of the barrels and separated by an X, like so: 7x35, 8x35, 10x40, and so on. The first of those numbers (7, 8, 110) etc.) tells you the magnification power, while the second gives the diameter of the objective lenses in millimeters. For example, a pair of glasses marked 7x35 provides a magnification of seven and has objective lenses that each measure 35mm.

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