Choosing Binoculars

May 24, 2004

Getting closer: Zoom in on the perfect binoculars

AMC Outdoors, May 2004

By Michael Lanza

There was a time when three types of people used binoculars outdoors: the ones looking for birds, the ones shooting at game, and the ones in military uniforms scouting terrain. Today the market has expanded considerably. With optical quality improving, and weight and cost steadily decreasing, it’s nothing to stick a pair in your pack — or pocket — for a day outside. Binoculars are ideal for watching wildlife from a safe and respectable distance, scouting an off-trail scramble or a technical climbing route up a peak, evaluating avalanche conditions, or scanning a shoreline for campsites from a kayak.

But what type of binoculars should you buy?  Answers to these common questions can help you decide.

How much should I spend?

Price ranges widely, from $30 to well into four digits. Clarity, or image sharpness, separates the wheat from the chaff in binoculars, and is where you’ll see the difference between expensive and inexpensive models. Optics — the quality of the lens glass, the mirror-like internal prisms, and the coatings — improve with price and serve to enhance the overall image sharpness. Price also increases when you add features like waterproofness, zoom functions, and lightweight materials. For most recreational uses, however, models in the $70–$130 range are more than adequate.

What size should I look for?

For backcountry travel, compact binoculars are the way to go. Often weighing in at less than a pound, they are small, convenient, and easy to carry. Larger, full-size binoculars are bulkier, heavier, and less ideal for hauling long distances. Their only advantage is the larger objective (front) lenses, which provide a brighter, sharper image than the smaller lenses of compacts — especially good for low-light viewing (dusk, dawn, or indoors). The objective lens size is indicated by the specification numbers.

What do those numbers mean, anyway?

Binoculars carry two specifications, like 8×25 or 7×40. The first is the magnification power, or number of times closer an object will appear through the binoculars than with the naked eye. The second measures the aperture, or diameter, of the objective lens in millimeters.

Most recreational binoculars’ magnification power ranges from seven to 10. Lower power gives a wider field of vision and makes finding objects, especially moving objects, easier. Higher power brings objects closer but narrows the view — too tightly for some closer-range viewing—and tends to make lenses heavier. Also, the higher the magnification, the more your hand movements are amplified. This can cause the image to wobble to a point where viewing is difficult. While generally not a problem up to 10x, the higher magnification of higher-end models requires a stand or tripod to stabilize the binoculars. Built-in image stabilization eliminates the problem, but adds more than $100 to the price.

The diameter of the objective lenses makes a noticeable difference in image brightness. Wider objective lenses create a larger “exit pupil,” which translates into better image brightness and makes it easier to find the image through the lenses. Though a definite advantage, keep in mind that the larger the lenses get, the bigger and heavier the binoculars will be as well. While objective lenses reach 50 mm and more in diameter, 25 mm is generally the maximum size that will fit in a compact model.

‘Exit pupil’? Where is it going?

Into your eyes! Image brightness is affected by the size of the exit pupil, or the diameter of the shaft of light that reaches your pupils from the binoculars. It is determined by dividing the aperture by the power. (For example, 7×35 binoculars have an exit pupil of 5 mm.) The average human pupil has a diameter that ranges from approximately 2.5 mm to 7 mm, depending on light conditions. The exit pupil matters less in bright sunshine when your pupils are constricted and the shaft of light easily spans the pupil size. However, for dim light, look for a minimum exit pupil of about 4 mm.

What does ‘field of vision’ mean?

The field of vision, or field of view, refers to how wide an area you’ll see at 1,000 yards. For example, 10×25 binocs see an area spanning 300 to 350 feet at 1,000 yards distance. Standard binoculars range from approximately 250 to 600 feet. Higher magnification equates with decreased field of view.

Why are binoculars different shapes?

Inside binoculars, prisms focus the image; they also affect the shape of the housing. There are four types:

  1. Porro prisms create the classic binoculars shape, with eyepieces closer together than the objective lenses. They are found in the most inexpensive models.
  2. Reverse porro prisms place the objective lenses closer together than the eyepieces, a design used to create a more compact shape.
  3. Roof prisms are characterized by eyepieces in line with the objective lens. Binoculars with roof prisms have two linear barrels, are durable, and are more compact than those with porro prisms, but are usually more expensive.
  4. BAK-4 prisms, found in the most-expensive models, are made with high-density BAK-4 glass and produce the sharpest image from edge to edge.

Will all binoculars fit my face?

No. For many binoculars users, this drives a purchasing decision. The distance between the eyes varies between individuals, which is why two people often adjust binocular barrels differently, and why some models will be easier for you to focus than others. Try looking through multiple models to determine which style fits you best.

What if I wear eyeglasses?

Eye relief — the distance a binocular can be held away from the eye and still present the full field of view — will be important to you. Look for more than 16 mm of eye relief, and models fitted with rubber eyecups that can be rolled down. Extended eye relief also reduces eyestrain. You can also take off your eyeglasses and use the binocs like a pair of giganto-glasses, correcting your vision with the focus mechanism.

Is a ‘zoom-in’ feature available?

Yes. Zoom binoculars let you vary the magnification — a 10-30×25 binocular, for example, switches between 10x and 30x. These are nice for birding and other activities, where finding the object is important (wider field of view with lower magnification) but an extreme close-up helps identify key characteristics. The trade-off can be reduced image brightness in the magnified image, and difficulty holding the close-up image steady by hand. A zoom function adds about $100 to the price tag.

What are coatings?

Coatings on lenses and prisms affect clarity by decreasing reflection and preventing distortion. They are applied in four ways; each step improves brightness and clarity, and increases cost. The four levels are:

  1. coated, a single layer on at least one lens.
  2. fully coated, a single layer on all air-to-glass surfaces.
  3. multi-coated, multiple layers on at least one lens.
  4. fully multi-coated, multiple layers on all air-to-glass surfaces.

Can I get night-vision binoculars?

Yes, but they vary hugely in cost, power, and features. Night-vision devices are either first, second, or third generation, which refers to the type of light-intensifier tube (the key component) used. First-generation models start around $400 and amplify ambient light several thousand times. Second-generation units, used primarily by law enforcement and other professionals, give greater light amplification and cost $500–$1,000 more. Third generation is the cutting edge of night-vision technology, and considerably more expensive — typically more than $3,000.

How durable are binoculars?

In general, they are very durable and will hold up well over time. For increased durability, look for lightweight models made from rugged polycarbonate/foam and aluminum. If you plan on venturing into an area with a range of temperature and/or humidity, including fog and rain, look for “marine” or “all-weather” models that are waterproof. Some are also “nitrogen-purged,” meaning the air inside has been replaced with nitrogen to prevent internal fogging.

I’ve got my binocs and am ready to head out! What’s the best way to carry them?

Animals generally won’t wait around while you fumble through your pack — keep your super-peepers readily accessible in a convenient pocket or around your neck for speedy viewing. And consider buying a comfortable neck strap at a camera store to replace the cheap nylon strap provided with most binoculars.

And finally, next time you zoom through the wilds, don’t forget to focus on the world around you.

Michael Lanza is author of The Ultimate Guide to Backcountry Travel, from AMC Books.

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