Binocular basics

A set of binoculars consists of two matched pairs of lenses and a prism, which is needed to make what you’re seeing appear right side up. The larger lenses, known as objective lenses, handle the magnification and light gathering in the binoculars, while the smaller lenses transmit the image to your eye.

When you’re buying binoculars, you’re paying for lenses. Creating sharp images through glass is a tricky process for manufacturers, and most binoculars include specialized coatings on their objective lenses to allow light to collect in the lens instead of reflecting off its surface.

Prisms can also add to the cost of binoculars. There are two basic types of binocular prisms: Porro and roof. Porro prisms are set at right angles. They typically cost less and offer a wider field of view. Roof prisms are mounted on top of each other. Binoculars with roof prisms will be larger than those with Porro prisms and offer a greater depth of field, or sharper images at greater distances.

It’s about the numbers

Every pair of binoculars is measured by a pair of numbers that tell you a lot about performance, such as 8x40. The number before the “x” tells you the binoculars’ magnification, or how much closer they make things appear.

This is the first area where use determines the best binoculars for you. If you’re using binoculars for sporting events or the theater where you’re sitting relatively close, look for a magnification of 8x or less. You can get larger magnifications, but as the objective lens size increases, so does the size and weight of the binoculars. This can make them very uncomfortable for extended use. Binoculars with 10x or 12x magnification need to be used with tripods for maximum stability.

The number after the “x” is the diameter of the objective lens in millimeters. Binoculars with larger lenses collect more light, allowing them to display more detail in shadows or low-light conditions. As lens size increase, so does the cost, weight, and size of the binoculars. Higher magnifications and larger lenses are best for bird watchers and star-gazers.

When you’re shopping for binoculars, remember that they just bring the world closer so you can see more detail. No set of binoculars will allow you to see things that you cannot see with the naked eye.

The importance of exit iris in binoculars

Dividing the size of binoculars’ objective lens by the magnification yields a third crucial number, the exit pupil. This number tells you how much light the binoculars transmit to your eye and this is where personal preference becomes important.

Your pupils get larger and smaller in response to different levels of light. They can be as small as 1mm in bright sunshine and as large as 8mm (though 6 or even 5mm is common for older adults) when it’s dark. Binoculars with an exit pupil of 6mm or 7mm work best in dark conditions and show more detail in shadows.

If you’ll be using the binoculars on a boat or after a few cups of coffee, you may want a larger exit pupil for bright daylight use. Binoculars shake when you hold them, and this movement reduces the sharpness of the image. A larger exit pupil compensates for any movement, keeping the image crisper.

Binoculars with electronic image stabilization are also available. These binoculars use battery-powered gyroscopes to reduce the effects of hand movement. Unless you’re an avid boater or need to use binoculars in a moving vehicle, you’ll probably find them too expensive and heavy for your needs.

Finding the right binocular fit

No two faces are exactly alike. It’s important to find binoculars that can adjust to the spacing of your eyes. Make sure that adjustable binoculars are convenient to use if you’ll be sharing them, and that the adjustment mechanism won’t wear out easily.

Eye relief is a measurement that tells you how far you can hold the binoculars from your eyes and still see the full field of view. If you wear eyeglasses or sunglasses, you’ll need binoculars with at least 15mm of eye relief to compensate for the lenses in your glasses.

Binoculars with larger amounts of eye relief are easier to use, but increased eye relief reduces the field of view at longer distances.

Fixed-view binoculars cannot be adjusted for longer or shorter distances. These are a good choice for sightseeing, sports, and theater, but won’t meet the needs of birders or backyard astronomers. For these uses, you’ll find binoculars with up to three focus adjustments. A center focus wheel adjusts both lenses simultaneously, expanding image sharpness across a range of distances.

More advanced binoculars include separate adjustments for the left and right lenses. These are idea for bird watching, fishing, and game tracking, because the binoculars will compensate for any differences in the way your eyes focus.

When shopping for adjustable binoculars, look for focus adjustments placed where you can reach them, and make sure the binoculars can hold their focus without constant readjustment.

Digital camera binoculars

Everything’s going digital, and binoculars are no exception. New combination digital camera binoculars let you take pictures of what you see. These binoculars are popular with bird watchers, but performance can be an issue.

If you choose binoculars with a built-in camera, shop for lenses first, as these will have the greatest impact on your experience. Choose a camera that can hold more pictures than you’re likely to take or that has a swappable memory card.

Finally, note the distance rating for the digital camera. In most binocular/camera pairings, the camera has a fixed depth of field, so what looks sharp at a short distance might end up as a blurry image.

Need to know more?

• Binocular Basics
  http://www.chuckhawks.com/binocular_basics.htm
• Selecting Binoculars for Birding
  http://www.binoculars.org/birding/Optics/SelectBinocs.html
• Binoculars: What to Look For
  http://www.outdoorlife.com/outdoor/gear/article/0,19912,635806,00.html