How Binoculars Work

By: Perception
If you understand how telescopes work, then you understand how binoculars work. Binoculars are, after all, nothing more than two telescopes attached to each other.

Binoculars are essentially a series of mirrors and prisms made of glass that take incoming light and provide the viewer with a magnified image. Each half of the binocular has three major parts.

The lenses at the front of the binoculars, the largest lenses, are called the Objective Lenses. The Objective Lens is where all the light comes into the binoculars. The Objective Lens focuses the light coming into the binoculars, and projects the image it receives as an upside-down image into the binoculars. In this way, the Objective Lens is a refracting lens. The larger the Objective Lens, the brighter the final image will be since it can take in more light. The trade off, however, is in convenience, as a larger Objective Lens will result in larger and heavier binoculars. Of course, higher quality lenses can be brighter than cheaper Objective Lenses of any size. Indeed, to ensure that the Objective Lens projects the sharpest image possible, it is important to use a high quality lens. With lower quality lenses, not all the light will be refracted at the same angle, causing the final image to be distorted in shape and color.

The image is turned the right way up by the second major part, the prism. In the common Porro design, the image is turned the right way up by two prisms set at right angles to each other. This is what accounts for the traditional binocular shape. For more money, you can get binoculars with a roof prism setting that allows for a more streamlined design. No matter what layout the prisms use, the final result is always the same. The prisms then project the corrected image down the binoculars towards the last major component.

The third major part of the binoculars is the eyepiece lenses. These lenses fulfill the binoculars' main purpose of magnifying images. The eyepiece lenses then project the magnified, right way up image to the viewer's eyes. It is important to consider this last step, from the eyepiece lenses to the viewer's eyes. Every pair of binoculars has a different eye relief, or optimum distance between the viewer's eyes and the eyepiece lenses. In particular, if you wear glasses, you should look for binoculars that have a long eye relief. So, now that you know how binoculars work, get out there and try some!
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