How do Binoculars Work?

Essentially, binoculars are just two telescopes mounted side by side, one for each eye. To understand how binoculars work, 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, an upside-down-and-backwards image of the light bulb will form on the paper.

You can enlarge this image by examining it through another magnifying glass, as shown in the illustration. You may be surprised to find that if you slide the tracing paper away, the image will remain, only brighter and clearer. You have just made a working telescope.

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. Binoculars also have a third element, the erecting prisms.

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 has to flip the image, and that's what prisms do.

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.

All binoculars still have these three parts. An objective lens focuses an upside-down image. A set of prisms turns the image right side up. And an eyepiece magnifies it. Though modern eyepieces and objective lenses are each comprised of multiple elements, their basic functions remain unchanged.

Today you can buy binoculars that are made with roof prisms or Porro prisms. There are advantages to each.

Porro prism vs. roof prism Porro prism binoculars were standard until the 1960's, when the Zeiss and Leitz companies introduced roof prism binoculars, whose objective lenses were straight in line with the eyepieces. Roof prism binoculars were compact, light, and comfortable to hold. They made the offset, zig-zag shape of the Porro prism design look as old fashioned as propeller-driven aircraft.

Porro prism design has a jog in the light path through each barrel.	Roof prism design looks as if the light goes straight through.

Porro prism design has a jog in the light path through each barrel. Roof prism design looks as if the light goes straight through. Roof prism binoculars appeared simpler than Porro prism binoculars. But inside, they had a more complex light path and required much greater optical precision in manufacturing. As a result, they cost more to make. The Porro prism design was simpler and more light efficient, and its images showed better contrast. Nevertheless, the roof prism design's appeal was so great that manufacturers went all out to perfect it.

They succeeded. Today, roof prisms dominate the top-end birding binocular market. Porro prism binoculars are not obsolete, however. Dollar for dollar, a Porro prism design will give better performance for the money, especially in medium or low priced binoculars.

Better Porro prisms binoculars are made from a high density glass, BAK-4. If you hold binoculars away from your eyes and up to the light, you can see the circular exit pupils in the eyepieces. The less expensive BK-7 prisms will have squared-off, non-circular exit pupils.

--Michael and Diane Porter of www.birdwatching.com

 

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