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Binoculars: What Are All the New Features and What Do They Mean to Me?

July 26, 2019

Back in the 1990s, binoculars were very simple to understand. There were Porro prism binoculars (below top) and roof prism binoculars (below bottom).

Figure 1. Porro Prism

Figure 2. Roof Prism

The general rule was at the same price, you got a better image with the Porros, and as the price went up, so did the quality. The problem was that roof prism binoculars were much more durable and once waterproofed, they tended to stay that way. So, binocular manufacturers began dealing with the issues.

Phase-Corrected Coatings
The first solution was the addition of phase-corrected coatings on the prisms. In a roof prism, when light hits the first diagonal face, it is split into two beams which both get circularly polarized. The lengths of the paths of the two beams are different by less than a wavelength, so when the two beams recombine, their polarization axes are out of phase. You’ve probably seen the effect of polaroid sunglasses or a polarizing filter used on a camera, which results in a reduction of brightness. A phase-correction coating is used on the shorter of the two paths to equalize the distance, so when the beams recombine, they reinforce rather than interfere. In the late ‘90s, phase correction was found on only the more expensive binoculars. Now, every binocular over $100 has it.

Extra-Low Dispersion Glass
The next major upgrade was extra-low dispersion (ED) glass used to reduce chromatic aberration (CA). If you’ve ever seen a band of color along a high contrast interface, you’ve experienced chromatic aberration. CA is caused by the edges of an objective lens acting like a triangular prism, splitting the light into its component colors. When the light emerges from the eyepieces, the colors focus at distances from the lenses and your eyes and brain try to eliminate the problem by constantly refocusing. If you get eye fatigue using your binoculars for extended periods of time, this is why. ED glass reduces the spread of the prism effect. It is made with a variety of materials in a wide range of costs and success at fixing the problem. Glass or crystal containing fluorite compounds do the best job.

Normal lens – Chromatic aberration & blurry eagle

Fluorite lens – No chromatic aberration & sharp eagle

So-called High-Definition (HD) Binoculars
What about binoculars called “HD”? The term “HD” is a marketing term. It says nothing about the quality of the glass, as no glass has the term in its name. Some manufacturers use “HD” for products with ED glass, while others use it for improved versions of lower quality binoculars. Whenever you see this term, be sure to read the description carefully, and if you’re still not sure, call for the answer.

Newer Coatings
Finally, there are a wealth of new coatings, both internal and external. The most important internal coating is another one applied to the prisms. Made of aluminum (good), silver (better), or dielectric material (best), these coatings improve the amount of light reflected through the prisms. The most important external coatings have a different name from every manufacturer, but all do essentially the same thing. They resist scratching and repel water, dirt, and oil, helping to keep your lenses clean and water-free, even in the rain.

Each of these features raise the price of a binocular, and top of the line binoculars have them all. If you have a problem with your current binocular, look for a binocular which addresses that issue. Read descriptions to discover which binoculars fit your needs and your budget. If you still have questions, we’re here to help.

[Steve Sosensky]

Editor’s Note: I first birded with Steve in the mid-1990s, most likely at Malibu Lagoon, when he lived locally. Eventually he decided to make a career of his hobby and moved to southern Orange County to work with the Optics4Birding team. Several members of SMBAS have trekked to southern Orange County and purchased optics from Steve. All were very satisfied. When Steve offered to write some articles on birding equipment for our blog, I knew I’d never find anyone more knowledgeable and jumped at the chance. Here’s the first of what I hope will be a very interesting and helpful series.

Optics4Birding brings a variety optics for tryout and sales to Sea & Sage Audubon’s Pancake Breakfast on Saturday Oct. 26. The breakfast also features used bird books at low prices. This is a good opportunity for you to check out the latest in binoculars or scopes and ask questions. If you want breakfast, make an advance reservation as seats sell out.
[Chuck Almdale]

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