All About SLR Lens Coatings
The manufacture and effect of coatings
By bdery in Articles and Tips on Feb 25, 2015
Coatings are an important and complex element of lens designs. They can improve light transmission, reduce flare and internal reflections, and generally improve the performances of a lens. No lens manufactured by a serious company goes without coatings nowadays.
Manufacturers never make it clear exactly how their coatings are designed. Furthermore, they use impressive names that all sound more high-tech than the previous one. Super multi coated, HD, Aerobright, Ghostless and Super Protect are a few of the brand names used by Pentax. Other manufacturers won’t be left behind; common coatings names include eBAND (Extended Bandwidth & Angular-Dependency), Super Multi-Layer, Nano AR, SubWavelength Structure, Super Spectra, Nano Surface and many others.
What’s in all of this? How are coatings made and what is it they do exactly? This article will try to shed some light on the matter.
Why bother with coatings?
Every time light travels from one medium to another (air to water, glass to air, one glass type to another, etc) a small part of the incoming light is reflected. This happens regardless of the materials and is inevitable. In most cases, about 4% of the light is reflected back.
This doesn’t seem like much. However, consider a simple lens design, similar to those used for many legacy 50mm lenses:
A classic 50mm lens design
This design is made of 6 glass elements in 5 groups. That means there are 11 optical surfaces for light to pass through. That’s more than 40% light loss: a huge amount. Thankfully, even a basic coating can bring that figure below 10% for such a design by treating each surface element separately. High quality coatings will lower light loss below 0.5%. this is a tremendous improvement, and the main reason to use optical coatings on lenses.
One needs also to factor in the multiple reflections inside a lens. This occurs because each time light gets reflected back, it again travels through the previous optical surfaces, and without coatings 4% of the reflected light is reflected back again! This is the main cause of flare.
What is a lens coating
A lens coating, in its simplest form, is a thin layer of material deposited on an optical surface. The choice of material and its thickness will influence the light passing though. For instance, a metallic coating (made of gold or silver, for instance) will act as a very effective mirror for wavelengths in the visible or infrared range. A non-metallic material will generally improve the transmission of certain colors but will not cover the full range that our eyes can see.
To improve the coating, the lens designer will use multiple layers of materials with different properties. By finely tuning this “multi coating” design, it is possible to obtain excellent light transmission over a large range of wavelengths. Photographic coatings, for instance, are designed to let through only the colors we see (the "visible" range, from 400 to 700nm), but other coatings used in science and engineering can improve transmission for UV or IR light. Coatings found on prescription glasses are made in the same way as those for photographic lenses.
The total thickness of a regular lens coating is a few micrometers at most, very hard to perceive with the naked eye. A good photographic coating will let visible light pass through with less than 0.1% reflection at all wavelengths, and for light coming both straight and at an angle. Coatings must be designed specifically for each types of glass so as to match their optical properties.
How to make a coating
Coatings are usually made by placing glass elements inside a vacuum chamber and evaporating the desired coating material so it deposits on the glass.
Vapor deposition process
Tight control over the process lets the manufacturer precisely determine the thickness of the layer. Different materials, evaporated in a predefined sequence, allow the creation of the multi coating.
About “nano” coatings
“Nano” may sound like a buzzword when talking about coatings, but it really isn’t. It basically signifies that, in recent years, the industry has improved its production processes. It is now possible to control with extreme precision the thickness of each layer. This permits more efficient designs with tighter tolerances and better performances and repeatability. Nano AR, Nano Surface, Nano Crystal, Aerobright are names used by different manufacturers that all refer to the same general principles.
About Pentax coatings
Pentax has long been regarded as having among the best coatings in the world. This was probably true when SMC was introduced, and the company still creates excellent products, however other manufacturers offer, for the most part, products of comparable quality. The various brandings used in the industry all refer to processes and techniques that are for the most part similar.
SMC was introduced many decades ago and remained the branding of Pentax lenses for a long period. The coatings themselves evolved over that time however, although the principles remained the same. Some lenses, like the Takumar bayonet line of inexpensive lenses, did not bear the SMC branding, but still wore basic coatings.
SMC Pentax-M 20mm F4 with SMC coating
HD and Aerobright were introduced recently. While there are engineering and optical differences between the two, for the most part they are the result of the same improvements in thickness control and material choices. By improving light transmission, especially when light rays arrive at an angle, flare is reduced and contrast is improved when compared to older coatings. Our Limiteds lenses comparison shows the effect that such a coating can have on optical performance.
HD Pentax-DA 21mm F3.2 Limited with HD and SP coatings
Super Protect (SP) is a type of coating used on the external surface of the lens. The choice of materials makes the surface harder than a regular coating, thus less prone to scratches and damage. It is also easier to clean because grease and dust do not adhere easily to the surface. This labeling does not relate to actual optical performance.