The Binocular Site
A Look into Binocular Lens Coatings
Among all the topics discussed in regard to binoculars and spotting scopes, one of the most common is lens coatings. What is applied to lenses (and prisms), how it is applied, and what it accomplishes makes up a large portion of the claims sports optic companies make about their products. As a result, it is of great interest to their customers. The subject of lens coatings can get complex very quickly, and as a result, become confusing. An understanding of lens coating basics will benefit anyone contemplating the purchase of a new spotting scope or binoculars. But first, a few things about light must be understood.
When thinking about how light behaves when it comes into contact with the lens of a binocular or spotting scope, the concepts of reflection, refraction, and transmission are important to keep in mind. When a wave of visible light encounters a glass lens, it may be partially reflected, partially refracted, and the remainder of it may pass through to the opposite side. In an optical instrument with multiple lenses, and perhaps even prisms included in it, it isn’t hard to understand how difficult it is for optical engineers to create the right balance of lenses and prisms.
Every image is composed of visible light waves, and must pass from the outer surface of the objective lens all the way through to the opposite outer surface of the eyepiece lens, and then into the observer’s eye. It must also be sufficiently magnified to be useful to the viewer, and at the same time remain sharp, clear, and bright. When you consider the challenge, it’s just half a step short of miraculous that it can even be done at all.
When Galileo created his first telescope, it was likely around 3x in magnification, and yielded an image that was most likely quite blurry. If modern engineers had only the quartz crystal available to that ancient astronomer for lenses, and no lens coating technology to enhance its qualities, we would likely not have progressed much further than Galileo’s design. Fortunately, we now have a dizzying array of types of glass and other materials out of which lenses can be fashioned. We also have a host of different types of coatings that can be applied to them in order to produce a seemingly infinite number of qualities and effects.
Simply put, each substance out of which a lens or prism can be made has certain qualities that produce a specific effect upon a visible wave of light when coming into contact with it. Coatings can then be applied to finished lenses or prisms in order to enhance, diminish, or slightly alter these effects. Why not simply choose a different type of lens material that provides the desired effect? Because while a particular type of lens material may have the precise light management properties desired for a specific effect, that material may lack another needed quality such as durability, it may be too expensive to use, or it may simply be too difficult to obtain in the quantities needed. Coatings can be applied to a type of lens material that, in combination with the coating, produces the exact effect upon the light desired by the optical engineer.
Metallic coatings are most common to the average person’s experience. In their simplest form, they are what make common household mirrors possible. In binoculars and spotting scopes, metallic coatings may applied to glass or other materials in order to increase the amount of visible light reflected from it. This process is often used on one or more faces of a roof prism assembly—the auxiliary prisms generally—in order to cause the light not to pass out through the face of the prism, but to change direction and travel on to another prism surface. The most common substances used for this are aluminum, and silver, which is preferred because it reflects visible light more efficiently.
Another type of lens coating often used in creating binoculars and spotting scopes is multicoating. The term multicoating, like many other technical terms encountered in marketing materials, can be tricky. While its benefits and capabilities have been explained correctly by some, others have misused or overstated them. Multicoating is, as the name implies, multiple layers of a coating applied to a lens surface. But unlike metallic prism coatings, the purpose for this type of coating is to reduce reflection when visible light comes into contact with the lens so that more of the light passes through it. While multiple coating layers are often beneficial, in some circumstances, a single layer coating may be better for the specific purpose the lens in question serves. As with most elements of optical design, the key is to use the right coating on the right lens, which in turn is made of the right material and placed in the right position in the optical system for the desired effect on the light to be achieved.
Multicoating commonly uses such compounds as magnesium fluoride, or calcium fluoride. A number of other more exotic, closely-guarded substances are also employed. In addition to the reduction of reflection in lenses, multicoating can be used to alter the way light is refracted as it passes through the lens. Simply put, these coatings, in combination with the substance of the lens itself, can help reduce the amount of separation of the visible light waves comprising the image to prevent distortion of the color, loss of clarity, or lack of sharpness in the final image perceived by the viewer.
The type of coating that has recently generated the most press coverage, but which still confuses many people, is phase coating. Applied to the roof prism in binoculars or spotting scopes, phase coating helps keep the image carried in the wave of visible light from being altered or rendered “out of phase” when it leaves the prism assembly. Roof prisms are somewhat tricky. Developed to allow a long optical path to be contained in a small space, roof prisms have the potential to disrupt an image composed of visible light by splitting it and recomposing it with the longer wavelengths out of phase to the shorter wavelengths. Phase coating helps prevent this by changing the speed of one portion of the split light waves so they are “in phase” with the rest when exiting the prism.
In addition to metallic coatings to improve reflectivity, and anti-reflective multicoatings to reduce it, an assortment of new coatings have been developed recently that can help reduce dust or water drops from accumulating on lenses. An example of the first of these is the proprietary Swaroclean coating found on Swarovski binoculars and spotting scopes. With this coating applied to the lenses, the surface energy is reduced, and the lenses are thus less adhesive to dust, water spots, and other residue, which helps keep them cleaner, and makes them easier to clean when necessary.
Regarding water on lens surfaces, Zeiss coats the lenses of many of its products with LotuTec, a proprietary water repellent coating that causes rain or mist to form small droplets on the lens surface rather than sheeting. Small droplets have less surface adhesion than water sheets, and can be more easily shaken free of the lens, leaving it dry and clean.
There is a lot more to lens coatings, but the basics are all you need to help you make the best decision about what you need, and what to look for when purchasing binoculars or spotting scopes.
This article was written for The Binocular Site by John E. Riutta. To learn more about John please see his full biography.
