In recent years, some lens designers have updated their lenses with new lens coatings. For example, Pentax’s new limited lenses feature an existing optical design with only a few improvements, including better lens coating.
When testing such lenses, reviewers generally agree that new lens coatings significantly improve image quality (especially contrast and flare), but never quite explain How? ‘Or’ What lens coatings work. This is the purpose of this article.
The origins of lens coatings
Historically, coatings were introduced to reduce light loss in optical systems. Indeed, each time the light passes from one optical medium to another, a fraction of the energy is lost due to a phenomenon of reflection. This phenomenon of reflection occurs naturally on any type of surface between two materials, be it the surface of a river, glass or a mirror. The only difference is the amount of reflected light. For glass, it is commonly accepted that 96% of light is transmitted and 4% reflected.
The equation behind these numbers is:
R is the reflected energy, not1 the refractive index of the first medium (1.0 for air) and not2 the refractive index of the second medium (the glass, in our case). The refractive index of glass is generally between 1.4 and 1.8. The 4% value derives from a typical refractive index of 1.5.
This loss of energy may seem minor at first glance. However, it accumulates for each lens surface within a given lens system. A prime lens typically contains 7 to 12 elements (meaning about 15 to 20 lens surfaces, since each lens element has both an air/glass interface and a glass/air interface) whereas a a modern zoom lens has more than 20 elements (meaning about 40 lens surfaces).
This typical prime lens would only let in half the light, while the zoom lens would transmit less than 20% of the incoming light.
The first lens coating dates back to English mathematician and scientist Lord Rayleigh (John William Strutt, 3rd Baron Rayleigh). To his surprise, he discovered in 1886 that old tarnished glass transmits more light than new, untarnished glass. Lord Rayleigh discovered that two successive air/tarnish and tarnish/glass interfaces transmit more light than a single air/glass interface. Several patents followed this discovery and the lens coating gradually improved.
For photographers, a major improvement came in the 1930s. In 1935, Zeiss engineer Alexander Smakula patented the first coating using multiple layers of chemicals. This design, as we will explain later, has dramatically improved the performance of lens coatings and led to unprecedented levels of optical performance.
How effective are lens coatings in improving light transmission?
A lens coating typically brings the transmission from about 96% to over 99.7%. This means that a typical prime lens can now transmit 95% of light (down from 50%) and our zoom can transmit 88% (down from 20%).
Obviously, the lens coating brings a big improvement to low-light photography. The improvement is all the more striking as the number of optical lenses used in photographic lenses tends to increase in modern designs. While in the early days of photography it was common to use a doublet lens, today it is common to exceed 15 lens elements in computer-designed lenses. Therefore, light transmission is an increasingly important issue for lens designers.
Low contrast and stray light issues
The use of a coating on the lenses has other advantages. The energy that is not transmitted is reflected several times in the lens and ends up being added to the final image. At best, dark areas are illuminated with diffused light, reducing dynamic range and contrast. At worst, a strong light source in the scene also produces bright spots inside the image, called flares.
In 2016, lens manufacturer Zeiss conducted an interesting experiment to demonstrate the importance of lens coatings. The manufacturer produced two exact copies of the same lens, a Distagon 21mm f/2.8, one with optical coatings and one without.
Here are some of the images obtained by the two lenses in the same state. Overall, image quality is significantly reduced for all photos taken with the uncoated lens.
The Physics of Lens Coating Designs
The design of a coating can be based on various physical principles. The list includes index-based methods, GRIN materials, polarization, diffraction theory, and even metamaterials…
The simplest form of anti-reflective coating, historically, brings us back to the transmission equation. It appears that the total transmission can be improved by adding a medium with a lower refractive index (eg, 1.3) than that of glass (eg, 1.5).
With the simple coating proposed above, the light transmission can be improved from 96% to 97.8%. However, this type of single layer coating is still far from 0% reflection.
To improve coating performance, lens designers tend to use diffraction theory instead. By using the wave nature of light, one can choose a thin layer of material to perfectly cancel the reflection. A layer 1/4 wavelength thick means that the wave reflected off the glass will travel an additional 1/2 wavelength (1/4 wavelength entering and 1/4 outgoing wavelength) compared to the wave reflected on the glass. AR coating. Thus, the two waves are shifted by opposite phases and their sum is zero.
There are a few caveats to this ideal case. First, light usually comes in a spectrum instead of a single wavelength (a single wavelength doesn’t really exist in nature, you can find some in artificial laser sources). For visible light, the wavelengths range from 400 nm (blue light) to 800 nm (red light). This means that the thickness needed to eliminate reflections varies greatly with color. It can also mean that not all colors are transmitted equally, which actually means that the lens coating will introduce a color cast.
Second, our calculation assumes that the light rays are perpendicular to the glass surface. In practical cases, however, they can fall on the lens at a wide angle. As soon as an angle is introduced, the optical path inside the anti-reflective coating increases which results in lower transmission.
In order to solve these problems, the best solution is to add several layers of coating. A common structure alternates a 1/4 wavelength coating with a 1/2 wavelength coating. It is common to have lenses with typically 7 layers of coating.
How are lens coatings mass produced?
The wavelength of visible light is about 500 nm and lens coatings are typically thin films from 100 nm to 250 nm. To put that into perspective, an average human hair is about a thousand times thicker.
The layer is also meant to be uniform throughout the glass, so the layer thickness only varies by a few percent. This step cannot be performed until the glass is cut and polished to its final shape, as the polishing process would otherwise remove the coating.
The modern industrial process uses vapor deposition technologies. This is usually done in a vacuum chamber with chemicals to evaporate.
Here is a short video of a machine designed for this purpose:
You can see on the top of the system a set of lenses ready to be processed. These lenses will be rotated throughout the coating process to even out the anti-reflective coating layer.
The science of lens coatings is nearly a century old. However, the subject is still the subject of active research. The much-discussed meta-material technologies making headlines these days could provide possible improvements over existing lens coatings.
With the increasing complexity of lens designs, any advancement in lens processing is the best because it also improves light transmission and image contrast.
Picture credits: Header photo from Depositphotos