As a binocular expert, I do my best to educate. If you visit our website at OpticsPlanet.net, you will find a number of binocular articles that explain the basics. These include: Astronomical Binoculars, How to Understand Binoculars and Buy Binoculars, How to Choose a birding binocular, How to Choose a Hunting Binocular, and High Power Observation for Home and Office.

Since I am a big fan of the TV show, Myth Busters, I though it might be fun and useful to continue along this line by using my new blog to bust some popular binocular myths. Today, I'd like to look at the notion that field of view is determined by the size of the second binocular number (objective size). Many beginners believe that the bigger the second binocular number, the larger the field of view (amount of territory) that can be seen when they look through a binocular. Myth or fact?

Let's look at three binoculars and, to keep variables to a minimum, we will use the same series of binocular. We'll go right to the top with a premium series of binocular in the Leica Ultravid binocular. This is as good as it gets in a binocular, so if the Leica doesn't support the myth, no binocular can. The Ultravids we will look at are the Leica Ultravid 8x32 binocular, the Leica Ultraid 8x42 binocular and the Leica Ultravid 8x50 binocular. If the myth is true, the 8x50 should have the largest field of view, followed by the 8x42, with the 8x32 coming in last with the smallest field of view.

A quick check of the specs shows just the opposite is true. The 8x32 has the widest field of view at 400+ feet and the 8x50 coming in last with a field of view of only 350 ft. What's going on?

Field of view in a binocular or any optical instrument is determined by two things.

The first is the magnification (first binocular number). A basic rule of optics is that as magnification goes up, field of view goes down, all else equal. Going to a 10x or 12x Leica Ultravid, then, should reduce the field of view. This is supported by the specs. By the time we hit the 12x Ultravid, our field of view is down to 300 feet. In other words, the first binocular numner is more useful as an indicator of field of view than the second binocular number, but the story doesn't end, there.

The second factor that determines field of view is eyepiece design. There are many eyepiece designs used in binoculars, especially when you get to premium models. These can be very sophisticated, with many lens elements. There are design contraints imposed on eyepiece design by focal length of the optical system as well as physical dimensions of the eyepiece body. Things start to get pretty complex, here.

Okay, myth busted - field of view is not determined by the second binocular number. Bottom line is that you can get a general impression of field of view from the first binocular number, but for an exact number, you need to check the specs. No need to split hairs, though. For nearly any binocular application, a difference in field of view of even 30 feet between two models does not translate into anything significant at typical binocular ranges.