How Binoculars Work

A demonstration you can do yourself

Essentially, binoculars are just two telescopes mounted side by side, one for each eye. So to understand binoculars, you need to understand how a telescope works.

Here's an easy demonstration that you can try yourself. All you need are two ordinary magnifying glasses and a piece of tracing paper. Do this once, and you will understand forever how binoculars work

Hold the tracing paper on the opposite side of the magnifying glass from a bright object, such as a light bulb. Move the paper back and forth. At a certain distance, a small, upside-down-and-backwards image of the light bulb will form on the paper.

The magnifying glass is acting like the objective lens of a telescope (at the far end from your eye). The light bulb is the object that will be observed through the telescope.

You can enlarge the image by looking at it through a second magnifying glass, shown at right in the illustration below.

Now the second magnifying glass, on the right, is acting like the eyepiece of the telescope (closer to your eye).

You may be surprised to find that if you slide the tracing paper away, the image will remain. In fact, it will appear brighter and clearer.

You have just made a working telescope! You could use the two magnifying glasses to magnify any distant object. And in fact, this is how the first telescope was invented. (Some children actually did it. It's a rather nice little story.)

To review: The magnifying glass nearest the object is called the objective lens; the one nearest your eye, the eyepiece. The objective lens and the eyepiece are two elements in all binoculars.

In the telescope we just built, everything is upside down and backwards. That would be OK for looking at stars. But for watching birds or following the action at a football game we require a right-side-up picture. A terrestrial telescope (used to look at objects on earth rather than in the heavens) has to flip the image, and that's what prisms do.

To make the image right side up, binoculars need a third element, the erecting prisms.

A prism is a solid piece of glass that functions as a mirror, but without a mirror's reflective backing. Light rays that have entered a prism cannot get out if they strike a surface at too great an angle. Instead, they reflect back, as if from a perfect mirror.

In the mid 19th century, an Italian named Porro designed a telescope with two prisms set at right angles to each other between the objective lens and the eyepiece. This arrangement not only erected and reversed the image, but also folded the light path, resulting in a shorter, more manageable instrument.

In 1894, the Zeiss Optical Works created the first "Hunting Glasses," incorporating the Porro prism design, and modern prismatic binoculars were born.

Since then, improvements in binoculars have all been refinements, but the essential element have not changed.

Copyright 2007 Michael and Diane Porter