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Binoculars Basics

Beginners may do better starting out with a pair of binoculars instead of a telescope. The equipment cost is generally much smaller and it gives observers the opportunity to learn the sky and figure out what objects they are most interested in observing. Many astronomical objects can be observed with binoculars including the Moon, Star Clusters, Planets and Comets! When buying a pair of binoculars, consider the cost of the mounting as well, especially if you plan on purchasing large binoculars that can not be easily hand held.

First, it helps if you understand how binoculars work and how to figure out what instruments work best for observing celestial wonders.

Also enjoy Astra's Binocular Observing page!

Binocular Systems

Binoculars are optical instruments that are made up of two small telescopes mounted together to allow the user to magnify objects using both eyes together. There are basically two types of binocular systems, one that uses porro prisms and the other a type called roof prisms. In porro prism binoculars the objective or front lens is offset from the eyepiece. Porro prism binoculars offer greater depth perception and usually a wider field of view. Roof prism systems are more compact and generally weigh less. Roof prism has a smaller tube and usually costs more. The best instruments have fully coated optics.

Binoculars are usually defined by two numbers - the first number indicates the size of the eyepieces or smaller lenses that provide the magnification or power of the system, the second number is the size of the larger objective lenses in the front of the system. The diameter of the objective lens measured in millimeters will determine how much light the binocular can obtain for effective viewing. A larger objective allows more light to reach your eye. The weight of the binoculars goes up as the aperture increases so smaller lenses are more compact and portable. Common sizes for astronomical purposes are 7 × 35, 7 x 50, 10 x 50 or 8×40, 10 x 70, or 11 x 80.

Figuring Exit Pupil

Exit Pupil is the small circle that appears on the eyepiece of the binoculars when they are held at a short distance away from your eyes. The exit pupil relates to how well a binocular will perform in dim light. If you hold binoculars away from your eyes and up to the light, you will see a bright circle in the center of the eyepiece. The size of that circle of light is the system's exit pupil. It s determined by dividing diameter of the objective lens by the magnification power of the eyepiece.

Aperture ÷ Magnification

10 x 50 mm binoculars =
50 mm ÷ 10 =
5 mm exit pupil

A pair of 7×35 binocular will also have an exit pupil of 5mm. A 7 x 42 binocular would have an exit pupil of 6 millimeters. A very popular size for astronomy is 10 x70 as that will allow a 7mm exit pupil. Because the human eye can dilate up to 7mm, there is no need for a greater exit pupil. This varies from person to person and may change due to age. In any case, complete dilation is only possible in the dark as any stray light will cause the pupil to contract below its maximum.

Eye relief is another important consideration when choosing an optical system. Eye relief is the optimal distance from your eye to the eyepiece of the binocular. This is the focal point where the light passes through the eyepiece comes to a sharp focus. This becomes important if you wear glasses and must take them off. Most manufacturers make rubber eyecups that will fold back to accommodate individuals with astigmatism.

All binoculars have a fixed field of view, that is based on the optical system. This shows how much of the sky can be seen through the system. A wider field of view is better, of course but depends on the system and a wider field of view might affect the eye relief. The field of view is dependent on the size of the eyepiece, so higher magnification means smaller field of view.

Focusing the optical system for individual eye sight is accomplished by changing the eyepiece, usually by twisting the right eye piece. Many binoculars have an "instant" focus feature that will allow the lenses of both eyepieces to be changed, this is usually a "bar" that rocks between the two optical tubes. When using this system, it is good to check that the right eye is properly focused to ensure that images are sharp.


How to Choose Binoculars, Alan R. Hale, Not an observing guide, this book is filled with information on binoculars to help potential buyers

Astronomy with Binoculars, James Muirden, Arco Publishing, Inc., New York, 1984 - THE guidebook from my library, very good information and a great observing guide.

Binoculars on the Web

For a one-stop shop on binocular information try Binocular Sky - a site owned Steve Tonkin who wrote the book Binocular Astronomy sharing his long experience observing with binoculars

If you are considering making a purchase, check out his Binocular Evaluation page.

Binocular Chair - from Rod Nabholz's Home Built Astronomy Projects

The B & H Binocular Buying Guide - contains much more information than is covered here.

Light Grasp!

Diameter x 1.414 (Square root of 2) *
80 mm x 1.414 = 113.12

* Now here's a controversy! Do binoculars have more light grasp than monoculars? Some of us think they do. The reason being that the brain can add together the two images to get more light gathering power. I've seen arguments both ways on this and I'd really like to have a reference. If you know one, or have some pertinent argument please mail me!

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This page is copyright © 2020 Dawn Jenkins.

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