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How Do Binoculars Work?

Tell Me About Binoculars

You've decided to buy a pair of binoculars, but how do you choose which ones will be best for you? They come with all kinds of features and each model uses lenses and prisms in slightly different ways. Choosing carefully can mean the difference between a sharp, clear image and one that looks like a watercolor caught in the rain.

Galilean (remember that name?) binoculars are the simplest to understand. They each use four lenses, two are convex and two concave, and these lenses automatically produce an upright image, but the view is very narrow and only low magnification is possible. This is the type of design you will find on opera and theater glasses. In modern binoculars, prisms are used together with lenses. The prisms offset the lenses which, although they have a better field of vision and higher magnification, present the image upside down.

What Are Porro and Roof Prisms?

The two types of prisms used in binoculars are Porro and Roof prisms. The Porro prism binoculars are created by placing prisms in a double "Z"-shape. This takes more area which means the binoculars are wider, the lenses are bigger and consequently, there is better light collection. They may need recalibrating after time to make sure they're doing the job properly. Roof prism binoculars are more popular than the Porro prism because they're more durable, lighter, and smaller. Roof prism binoculars are built using shallow, small prisms and by doing so, the end product is smaller - convenient, and easy to carry. They are also capable of handling more abuse and rough handling without the need to recalibrate them. On the flip-side, these prisms need special coatings in order to use the light effectively. Coatings include phase coating, aluminum coating, silver coating, and dielectric coating.

What is This - Math Class?

Have you wondered what the numbers are on the binoculars? You'll see numbers like 7X50 or 17X25. These numbers represent the magnification and objective lens diameter. The first number refers to the amount of magnification and the second to the diameter of the objective lens. The second measurement is in millimeters. So, that means that in a 7X50 combination, the image is magnified seven times and the objective lenses have a diameter of 50 millimeters. The quality of the image is affected by these numbers, so pay attention to them.

The higher the magnification, objects farther away can be seen, but the field of view (FOV) will be narrow. So, you'll be able to see things far away, but when it comes to closer viewing, you'll be restricted and you'll need to refocus a lot if the object is moving. The lower magnification is fine for most situations. The higher magnification is suitable for star gazing.

They Work in a Similar Way to Your Eyes

Just like the pupil in your eye, the lens diameter allows light to enter and reach your eye. Bigger lenses are great for night viewing because they let in the most light. If you are hunting at dusk or dawn (which is when game move), or going through dark forests, the larger lenses are best. Field of view is the area that is visible to you when you use your binoculars. The number given in this case tells you how big an area you can see at 1,000 yards distance. So, an FOV of 372 ft. means you will see 372 ft. 1,000 yards away from you.

Just a couple more things. The eye relief is the maximum distance you must hold the eyepiece away from you. If you wear glasses, this is important information. Be sure to test the eye relief in the store before you leave with the binoculars. Finally, the exit pupil is a virtual opening at the center of each lens through which light travels through the binoculars. It operates on a system very much like the pupil in your eye - so the exit pupil should be as close a match to your natural pupil as possible. When you use binoculars at night, for instance for astronomy, a bigger exit pupil (about 7mm) is perfect. For day use, 2.5mm is more than adequate.