Binoculars consist of two telescopes mounted side-by-side on a plastic or metal frame. It requires the use of both eyes to get a 3D image of the object being viewed, unlike in telescopes where the image is flat because it only uses one eye. Binoculars are magnification devices that are handheld, making them portable and easy to keep around.
How does the magnification in binoculars work?
Let’s start off with the numbers. If you see the numbers 7×35 or 8×40, those refer to the magnification power. The first number pertains to the power of the zoom or how close it can make the object, in our examples, it’s 7 or 8 times closer. The “35” and the “40” indicates the size of the objective lens expressed in millimeters. The bigger the objective lens, the more light it gathers. The more light the lens gathers, the brighter the image and the better the viewing experience becomes.
How does the lens gather light?
Binoculars use two convex lenses, with one positioned in front of the other. The lens in front will first collect light rays around the faraway object and will create a focused image behind the lens. The first lens is called objective because it’s the nearest lens to the object you’re viewing. The second one, the one at the back of the first lens, will then catch the image and magnify it, just like what a magnifying glass does. Double the lenses and voila! You now have binoculars.
However, the process doesn’t end there. Once the light rays pass through the convex lens, they cross over, making the image upside down. And here’s the bad news: The second lens isn’t the solution to this problem. But if it isn’t, what is? Well, the prisms solve this issue. Thank God for prisms! Inside the binoculars is a pair of prisms ready to flip the image through 90 degrees each, rotating the image through a total of 180 degrees. The first prism will rotate it first, then the second one will rotate the image again, so it won’t be upside down when it reaches your eyes. Roof prisms have a back-to-back arrangement and Porro prisms are arranged at 90 degrees.
Prisms are the reason why binoculars are hefty and a bit thick in the middle. Compact binoculars also known as field glasses don’t have prisms, they use lenses to rotate the image and therefore they are lighter, smaller, and more portable. However, the image they produce isn’t high-quality compared to binoculars that use prisms. So the next time you come across heavy binoculars, think twice before you complain. They usually give a better image quality than light ones.