The Benefits of Binocular Lens Coatings and Binocular Lenses Explained
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The Benefits of Binocular Lens Coatings and Binocular Lenses Explained

Posted by Team Bushnell on Dec 9th 2019

This article will address binoculars, specifically, binocular lens coatings and binocular lenses are examined.

Like rifle scopes, the specifications for binoculars identify the magnification and the objective lens size. For example, binoculars described as 20×50 have an objective lens 50 millimeters in diameter and a total magnification of 20×. These specifications, however, only scratch the surface. The quality of binoculars is defined by many other factors, including the materials used in the lenses, lens coatings, prisms, and prism coatings of the binoculars.

The Functions of Prism Coatings, Prisms, Binocular Lens Coatings, and Binocular Lenses Explained

All binoculars combine two magnifying optical devices to provide two-eyed, magnified vision. Binocular vision gives us depth perception – the ability to judge when one object is closer than another. Binoculars come in three different designs.

1. Types of Binoculars

Field glasses:Also known as opera glasses, field glasses function the same as a riflescope. An objective lens collects light reflected from the target and forms an inverted and reversed image. This image is flipped top to bottom and left to right by an erector lens or set. An ocular lens focuses the re-oriented image for your eye.

Roof prism binoculars:Roof prism binoculars look like field glasses because the ocular lens is aligned with the objective lens for each tube. In roof prism binoculars, the erector lens or lens set Is replaced by two aligned prisms called roof prisms. The roof prisms invert and reverse the image captured by the objective lens before the image is viewed through the ocular lens.

Porro prism binoculars: Porro prism binoculars look different from field glasses and roof prism binoculars because they have an objective lens that is laterally offset from the ocular lens. This is done for a purpose. The wider distance between the objective lenses provides improved binocular vision – and, consequently, improved depth perception – at long distances. Porro prisms are offset to redirect light from the objective lens to the ocular lens and invert and reverse the image.

2. Comparison of Prisms and Lenses

The broad purpose of prisms and lenses in microscopes, telescopes, riflescopes, or binoculars is to bend and redirect light that passes through them. One benefit of prisms over lenses is that prisms transmit nearly all the incident light. By contrast, lenses reflect and absorb some light that falls upon them. This means that images viewed through prism binoculars are usually brighter than images viewed through field glasses.

Moreover, prisms in binoculars are oriented to bounce light around as it passes from the objective lens to the ocular lens. An analogy for visualizing this bouncing is a queue of twenty people at the bank. A bank queue with no rope barriers will be twenty people long. A bank queue with a U-shaped rope barrier will cause the queue to double up with ten people on each side of the barrier. Adding another barrier further compacts the queue into an S-shape of about seven people per side. There are still twenty people lined up, but the queue has been redirected so the people take up less space.

A riflescope containing an objective lens with a 14-inch focal length must be at least 14 inches long because the light passes in a straight line from the objective lens to the ocular lens. Binoculars containing an objective lens with a 14-inch focal length, on the other hand, can be substantially less than 14 inches long because prisms redirect light in a zig-zag path rather than a straight line.

3. The Many Roles of Coatings

Light reflected from the objective lenses, ocular lenses, and any internal focusing lenses dim the image viewed through binoculars. Anti-reflective coatings reduce the light reflected from lenses, thereby increasing the light transmitted through the lens. The result is that binoculars with coated lenses produce brighter images than binoculars with uncoated lenses.

However, lens coatings can serve other purposes as well. Coatings can protect lenses against scratches by creating a surface that is too slick for dust, dirt, and other debris to adhere to. In fact, lens coatings can be so slick that condensation runs off the lenses rather than fogging them.

Coatings on both prisms and lenses can be used to correct chromatic aberration. As you may recall from school, prisms and lenses are very good at splitting white light to create a rainbow. The rainbow results from different wavelengths of light bending by different amounts as they pass through the glass. This inherent quality of prisms and lenses causes color fringing in optical devices such as binoculars. Multiple layers of coatings may be applied to prisms and lenses to counteract fringing by changing how the lens or prism interacts with each color (or frequency) of light.

Materials Used to Make Prisms and Binocular Lenses Explained

Optical glass is glass manufactured with a specific formula that is designed to increase light transmission and decrease reflection. Optical glass is often categorized into crown glass and flint glass, although crown and flint glass are often used together to correct chromatic aberration.

ED (or extra-low dispersion) glass is a specialty optical glass that is designed for high transmissivity and low dispersion. Because dispersion is the property that causes chromatic aberration, binocular lenses made from ED glass produce bright images with very little fringing. Many of our binoculars and scopes feature ED Prime Glass, our most advanced glass.

Like lenses, prisms are typically made from high-quality optical glass, although the formulas used to make prisms are different from lenses. BK7 glass is highly transmissive and is found in low-end to medium-end binoculars because it is relatively easy to manufacture. Bak4 glass has higher transmission and less dispersion than BK7 glass. Bak4 produces bright, clear, non-fringed images but is more difficult to manufacture so it is typically found in high-end binoculars.

Binocular Lens Coatings and Binocular Lenses Explained – and Understood

There are more binocular features and characteristics to be discovered. For example, size, weight, angle of view, and focusing system are all features that vary depending on the model. With an understanding of lenses and lens coatings, however, some of the most important decisions in selecting a pair of binoculars can be made using this information.

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