Home : Optics : Choosing : Bird Watcher's Digest: Optics: Lens Coatings

Optics Lens Coatings

You've been thinking about it for a long time. It's time to buy new binoculars. After much thought and careful consideration of all the features that go into good birding glasses, you decide to pull out the credit card and make your purchase. The model you've chosen has just the right combination of magnification and objective size to fit your needs. The eye relief is right, and the close focus is great. And talk about comfortable: They fit your hands like gloves and focus butter smooth. Not too heavy either. They're even waterproof in case you decide to do some birding in a hurricane. And, oh yeah, they have a great warranty and you are buying them from a good dealer. Looks like you thought of everything.

Well, almost. There is one more feature to check. To be honest, it probably won't affect your decision and make you switch to another model. It's a feature these days that most people take for granted: lens coatings.

Lens coatings used to be a big selling point. There was a time when only the finest binoculars had advanced lens coatings. But today, even some models that sell for less than $200 have excellent coatings—at least in terms of light transmission.

And light transmission is the main reason lenses are coated. An uncoated lens will reflect back or “lose” up to 4 percent of the light that strikes its surface. It doesn't sound like much. But because there are many lens surfaces for a light beam to traverse on its journey through binoculars, this means that binoculars that are not coated can lose as much as 50 percent of the light that enters them. In other words, binoculars with uncoated lenses will be only half as bright as they could be if they had good coatings. You could, of course, compensate for this light loss by going to a larger lens, but that increases weight and cost considerably. And it still doesn't eliminate all those reflections that produce glare and reduce image sharpness and resolution.

Sounds scary, doesn't it? Relax. The only binoculars these days that are uncoated are the little plastic jobs you buy to put in your five-year-old's Christmas stocking. All “real” binoculars are coated to at least some degree.

The most basic type of coating is a single layer of magnesium fluoride (MgF), which reduces light loss at each surface down to 1.5 percent. Binoculars with at least one lens surface coated with magnesium fluoride earn the title “coated.” If all lenses or air-glass surfaces are coated, the binoculars earn the title “fully coated.” Good fully coated binoculars will reduce light loss down to 15 percent, or will transmit up to 85 percent of the light that enters the binoculars to your eye. This is a definite improvement over uncoated models, but technology has moved well beyond this point. Only budget models stop here.

Multicoating made a big splash back in the 1980s. As the name suggests, a multicoated lens has up to 15 thin layers of chemical deposited on the lens. The chemical formula varies from brand to brand, but a good coating takes the light loss of 1.5 percent for single layer MgF down to an amazing 0.5 percent or less. This allows some fully multicoated models to transmit 95 percent or more of the light that enters the binoculars to your eyes. Binoculars that are advertised simply as multicoated do not have all lenses multicoated and will achieve somewhat less. The good news is that most binoculars of more than $100 today have at least some multicoating. Many less than $200 and most more than $200 are fully multicoated; these lenses have become the industry norm.

So how do you know what type of coatings your binoculars have? Forget color. Some multicoatings produce a green lens color, others a purple tint. The only reliable way to know what type of coating a model has is to check the specifications. For top-of-the-line models and brands, full multicoating is a given. But if you're trying to economize, this is a good feature to check.

Because multicoating is so common, then, light transmission is pretty much a nonissue. True, more expensive binoculars have more sophisticated coatings, but the overall quality of lens coatings is consistently good. Most manufacturers shop the job of coating lenses out to specialty houses, which survive by processing large quantities of lenses. Only the best optics houses do their own coating.

One place where quality in lens coatings does manifest itself is durability. Traditionally, even the finest lens coatings were somewhat fragile and easy to abuse by poor cleaning techniques. Top-quality binoculars now feature much harder coatings, making them more resistant to scratching and scuffing. But this new breed of coating is, at present, reserved for the expensive stuff. Cheaper coatings remain quite soft.

There is one final consideration when it comes to lens coatings: color rendition. Today's sophisticated coating technology allows for specialty multicoatings that can enhance certain color wavelengths by screening out less desirable ones. This produces a lens coating that can be tailor-made for specific applications.

You can, for instance, design a coating that screens out more of the wavelengths at the blue end of the light spectrum. This produces a lens that enhances color at the other end of the spectrum, namely red, orange, and yellows and combination colors such as brown and tan. This type of coating is useful for hunters trying to pick out brownish animals from a predominantly green background. It produces images that are softer or photographically “warm.”

Of course, you can design a coating that enhances the other end of the spectrum. This produces a lens that emphasizes blues and greens at the expense of reds and yellows. This is sometimes used to improve contrast between light and dark areas and produces images that give the impression of sharpness. Photographically speaking, such a coating would produce a “cool” image.

Which is best for birding, though? Well, because birds come in all the ranges of the color spectrum and are observed under every imaginable condition, it stands to reason that a coating that is color-neutral would be best. Such a coating would provide the most true-to-life color.

Fine. How do you determine this in binoculars?

Not easily. There is no industry norm for color rendition. It is rarely part of the advertised specs. In the most general way, optics from Asia tend to be cool, and optics that originate in Europe tend to be warm. But this is little help when it gets down to individual models. I've used warm and cool lenses and never lost any sleep over the issue. Both will do the job in an otherwise quality glass. My best advice is to avoid binoculars and coatings designed for specific uses like hunting or marine applications. These are most likely to have specialty coatings. Otherwise, select your binoculars on the basis of other, more important considerations.

What about those ruby red coatings on some lenses? Forget them for birding. These are typically low-cost coatings advertised as something special, but they are often nothing more than conventional lenses colored to attract customer attention, although some are in fact specialty coatings designed to reduce glare over water. Either way, they don't do anything that can't be done better with more sophisticated, albeit less spectacularly, colored coatings. That's why you don't find them on quality binoculars. Besides, birds are not color blind. Do you really want to be out there flashing two red beacons while you observe?