A Greener View

By Jeff Rugg

February 6, 2009 7 min read


What to factor into the perfect binocular purchase

Jeff Rugg

Creators News Service

Q: The choice of the right pair of binoculars seems overwhelming. How do I choose the best one for me?

A: This is a great question because many people give binoculars for gifts and they will often give the wrong kind. Binoculars make good gifts for any bird watcher or nature lover, but can also be useful to gardeners.

The construction of binoculars involves many compromises. Making the right purchase means finding which one best suits your needs. There are many items to consider, but none is necessarily more important than any other.

Let's look first at magnification because many beginners feel they want the most powerful binocular they can find. The first number in denoting a binocular (such as 7x35) is the magnification. A bigger number means that the objects viewed will look that many times closer. The compromise is that it will also magnify any movements caused by your own hands or even atmospheric heat waves by that same amount. Higher magnifications also tend to decrease the field of view and decrease how much light gets through to your eyes. A higher magnification than 7 or 8 often makes it harder to see very well.

The number after the x in the 7x35 designation is the diameter of the objective lens in millimeters. The objective lens is the one on the end of the binocular closest to the object. Just as the iris of our eye opens wider to gather more light in dimmer locations, a bigger objective lens gathers more light than a smaller one. A big lens that gathers more light is very useful, but is compromised by making the binocular bigger and usually heavier.

The diameter of the beam of light that comes out of the binocular to your eye is called the exit pupil. The bigger it is the more light reaches your eyes. The diameter of the exit pupil is determined by the size of the objective lens divided by the magnification. A 7x35 binocular has an exit pupil of 5 mm because 35 divided by 7 is 5. A 7x20 binocular has an exit pupil of 2.8 mm (20 divided by 7 is 2.8), which is about half the size of the exit pupil of the 7x35 binocular.

Smaller binoculars are easier to carry, but for the same magnification, the smaller objective lens gathers less light and results in a smaller exit pupil. Less light getting to your eye may not be a problem in well-lit locations, but can be a problem in the early morning, late evening or indoors at a concert. The pupil of our eye opens 2 to 3 mm during the day and as much as 5 to 7 mm during the evening, so larger objective lenses are useful in dimmer situations.

The farther the exit pupil focuses past the end of the eyepiece the easier it will be for eyeglass wearers to use the binocular. Long rubber eyecups indicate a binocular designed for eyeglass wearers. This is a distance called eye relief and is measured in millimeters. Look for at least 15 millimeters of eye relief if you wear glasses.

Most binoculars have several lenses and a pair of prisms for each side. Some light bounces off the surfaces of each piece of glass instead of going through it. To reduce this loss of light the glass can be made denser by adding minerals or coating with reflection-reducing chemicals. Denser glass is harder to shape and polish, but gives a good reflective surface. The more pieces of glass that are coated, or if multiple coatings are put on a piece of glass, the harder it is to make. A binocular with denser glass and many coatings gives better color quality and light transmission with the compromise being higher cost -- sometimes a much higher cost.

All binoculars of good quality have an adjustment for each eye that allows people who have eyes that do not see the same, near to far, to be able to quickly adjust the binocular for their use. Auto-focus binoculars that do not require you to focus usually also do not focus on things close up. They can be useful for cruises where things are not close by, but are not useful for much else. Binoculars that have image stabilizers are good for people with shaky hands, but do cost much more than regular binoculars.

Some models are water resistant or waterproof. Binoculars that do not have to be coddled are more useful for people who will use them outdoors. People using them for concerts and sporting events probably do not need this option. An extra benefit to waterproof binoculars is that they don't fog up when you go outside in the winter. When the inner pieces of glass in the binocular fog up, you cannot wipe them off.

Binoculars under $50 can be of OK quality and are good for kids or leaving in the car so there is always a pair handy. Binoculars in the $50-$150 range are good for most beginners. The next price range goes up to $300 or so and is good for serious bird watchers and sports people. The next price range is closer to $1,000 and is for people who really want to see good color renditions of birds in lower light conditions.

The best way to choose a binocular is to actually use it to see if it works for your eyes. Does the focusing knob turn smoothly and easily for your finger length? Does it come with a comfortable wide and cushioned strap or one of those tiny neck-breaking straps? Another good strap style is one that crisscrosses the back instead of the neck. Gift binoculars are best given if the receiver has tested them or are accompanied by a gift receipt.

Besides spotting things a long distance away, how can binoculars be useful to gardeners, you ask? I am sure most people know that if you look in the wrong end of a binocular, things will seem tiny. However, if you look at something only a half-inch or so away from the wrong end, it will look huge. It will be magnified about the same power as the binocular. So carrying a 7x35 binocular is also like having a seven-power magnifying glass. You can now look at bugs and flowers with a new view.

E-mail questions to Jeff Rugg, Kendall County unit educator, University of Illinois Extension at [email protected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

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