Understanding Binocular Lens Coatings
Often times many people have asked this questions “what makes binocular A different from binocular B of same magnification and objective lens size?”. For example, say the “10×42”.
Well, the difference is in the binocular lens coatings. Binoculars that have same objective lenses have same light gathering power. The difference, however, is how much of that light gathered by the objective lens gets to your eyes.
The difference in brightness between binoculars is the quality of their lenses and prisms coatings. If the quality of the glass and lenses are the same you won’t see any difference. Light is lost as it travels through the lenses to your eyes.
Bigger objective lenses can compensate for this lost light, but they result in heavier binoculars. In the 1940s, it was discovered that coating the glass with magnesium fluoride would let more light through. So this technology was used in helping reduce reflections to 1-1/2% per surface, instead of 5%. Today, more coatings are added.
Why do Binocular Lenses have Coatings?
Optical coatings are applied to reduce internal light loss and glare. When lenses are coated, they have a better light transmission resulting in greater image sharpness and contrast. Without optical coatings, each lens may lose up to 5 percent of the light passing through it and this can go to about 35 percent if the optics are poor quality.
When light travels through a lens, some light reflects off its surface and is lost thereby reducing the image quality. That is why manufacturers deposit thin chemical coatings on lens surfaces to reduce reflective loss and improve light transmission. When the glass is coated, they will appear less shiny. Due to the magnesium fluoride or calcium fluoride coatings the lens are made from, they may appear greenish, bluish, or brownish tinted in color.
Because light has different color wavelengths, the coatings will destructively interfere with certain colors, thereby eliminating or reducing their reflections. When this happens, you will have more lights transmitted to your eyes. That is when you get a brighter and sharper image.
Types of Lens Coatings
Coatings have different types. The more layers of coating a binocular has, the better its optical performance. Below are the different types of coatings that you will come across today among binoculars and scopes.
Coated(C): At least one of the major optical elements has a coating on at least one surface. It is usually a magnesium fluoride applied to at least one surface.
Fully Coated(FC): At least one thin anti-reflective coating on both sides of the objective lens system, both sides of the ocular lens system, and the long side of the prism.
Multi-coated(MC): In this coating type. multiple layers of coatings are applied to one or more of the lens surfaces. Even some of the best optics available have only a single coating on the outside lens surface. This is done under the theory that a single coating is harder and more durable and the light reflected from the outer surface does not affect image contrast.
Fully Broadband Multi-Coated(FMC): Predominately found in most binoculars and spotting scopes. Fully Broadband Multi-Coated decrease surface reflectivity, therefore, allows greater optical through-put across the entire visible light spectrum. Fully broadband multi-coated optics feature these types of coating on all optical surfaces throughout the entire instrument. Fully multi-coated lenses are typical of high-end optics. Although this level of the coating does not guarantee the best quality
Pase Correction Coating: Most binoculars today are roof prism type binoculars. Which means the eyepiece is in line with the objective lens. When light is moving through roof prism binoculars, it gets folded back to itself for a short distance. When this happens, the peaks of light waves that were lined up perfectly when they entered the binocular go out of phase and interference occurs which then reduce the brightness and sharpness of the image.
So to correct this, a phase correction coating which is a thin layer of dielectric material is applied to the to one face of the prism. This coating delays the light waves just enough for the peaks to come back into phase. These days, you can find fully multi-coated lenses and phase corrected prism binoculars at an affordable price.
More recently, advanced multi-layered coatings have reduced reflections to as little as 0.25% per surface. Today, in the best binoculars, 95% of the light gets transmitted to the eye.
How to Tell if a Lens is Coated
If you want to check for the presence of a coating on your binoculars, you can do this by simply looking at the reflections of artificial light in the objective lens. If that reflection appears green, yellow or purple, that means the lens has coatings but if the light appears clear, then there are no coatings.
Furthermore, these days some manufacturers are applying water repellant coating on the outside of the objective lenses and eyepieces. This coating causes water to bead on the surfaces instead of sheeting. This makes the lenses easier to clean because its hard for them to pick up dirt.
Josephine is Editor @ Binocularsinght.com, an Electrical Engineering Grad who loves optics. She is an Outdoor Enthusiast and a Writer who likes escaping into the mountains for hiking adventures and enjoys other outdoor activities as well.