Lens coatings are good. If a single coating is good, then multiple coatings must be better.
This concludes the extent of my knowledge about lens coatings. I suspect I am not alone as a good number of digital and film photographers and collectors probably have about the same understanding as I about those mysterious bluish purpley green, sometimes yellow and orange, possibly radioactive reflections we see inside of our beloved pieces of glass.
In the years after World War II, lens coatings became more and more common. Usually associated with color photography, various methods developed by American, German, and Japanese companies to coat their lenses also helped to reduce flare, haze, and even make the lenses more resistant to scratches.
This week’s Keppler’s Vault brings you two articles about lens coatings, the first from March 1960 and the second about multi-coated lenses from June 1975. A bonus third article is a short one from December 1958 which discusses the use of radioactive glass in lens making which was another method employed to help improve contrast in mid century lenses.
The first article starts off similar to what I said above in that photographers take lens coatings for granted, knowing that we like them, but maybe don’t quite understand how they do what they do.
If you are like me and have a rudimentary understanding of lens coatings, the first two pages of the first article are definitely worth checking out. Things start to get technical by the end of the second page, but a simple primer of why an uncoated 4-element Tessar lacks contrast is explained in very simple terms. A little more history of how the earliest lens coatings occurred naturally is fascinating to think that in the 1800s, the more you used a lens, the better it got is cool to think about as well.
The rest of the first article discusses how lenses are coated and the challenges involved in making perfect corrections across the visible spectrum of light. Red, blue, and green light have different wavelengths that react differently when passing through or reflecting off glass elements. The lens coatings from the era in which this article was written were not able to perfectly correct these differences, but still, the improvements from uncoated lenses are still worth it.
At the very end of the article is a section called “To coat or not to coat” wherein the question is asked if it’s worth it to have previously uncoated lenses sent in to be coated. This is pretty amazing to me to think that there was a time when photographers would consider sending in their pre-war Elmars and Biotars to have them coated. I am unsure how big of a market this was, but I have to imagine at least a decent number of photographers had this done.
The second article fast forwards a decade and a half to 1975 to a time when a single lens coating wasn’t enough and lenses were receiving “multi-coatings”. As I pondered at the start of this article, more is better, right?
With the release of the Asahi Takumar SMC 50mm f/1.4 lens in 1972, which was first multi-coated SLR lens, multi-coating became THE buzzword in the photographic community. The idea that a single lens coating has limits, that adding more layers could improve was probably an easy sell to someone looking to upgrade. Multi-coated lenses promised further reductions in flare with improved contrast, and better light transmission making images shot with these lenses even better.
Modern’s article has quite a number of test images shot with uncoated, single coated, and multi-coated lenses, showing off how things like ghosts and flare are controlled with each. On the fifth page, they show how lens coatings affect light transmission. As light passes through each glass element of a lens, some of the light is lost. In a 5-element uncoated lens, only 63% of the light that entered the lens, exits it. With the same lens single coated, the transmission rate improves to 84%, and with multi-coating, improves further to 91%.
The more elements in a lens, the greater improvement there is. Using a zoom lens with 15-elements, the transmission rate from an uncoated to multi-coated lens jumps from an abysmal 28% to 83% proving how valuable lens coatings were in the widespread use of zooms.
As was the case with the 1960 article above, the 1975 article continues to preach the benefits of coated vs uncoated lenses, but suggests that although multi-coating is better than single coating, in real world use, it’s less noticeable. The examples of flare in badly overexposed and slightly underexposed images show significant improvements from uncoated to single coated, but not so much to multi.
I don’t want to steal too much of the thunder from the article as I definitely think it’s worth reading from start to finish, but the article does an excellent job of walking that line of offering technical information about lens coatings, without losing the interest of the casual reader. Although the earlier article offers a bit more history and a great explanation of why lens coatings are needed, this second article shows a lot more examples to further prove the point.
Finally, I found a short bonus article from December 1958 that’s not specifically about lens coatings, but talks about the widespread use of rare-earth lenses from the era that were found to help improve light transmission similarly to lens coatings. As collectors know today, many lenses from the mid 20th century used glass that contained thorium or lanthanum that is slightly radioactive, some of which can be detected with simple consumer grade Geiger Counters. I’ve heard of stories of collectors getting packages containing these lenses rejected by the postal service for excessive radiation.
Many of the best pre-war lenses were made of something called “Schott glass” which is named after Otto Schott, who in 1881 began research into ways of adding these materials to glass in an effort to improve their performance.
Today, the thought of a radioactive camera lens sounds scary and to many, brings back flashbacks of Chernobyl and Three Mile Island, when in reality, the amount of radiation contained in these lenses is far below trace amounts that we encounter every day in our normal lives.
There’s a bit of history here about how rare-earth lenses became common and how the technology was advanced throughout the years.
All scans used with permission by Marc Bergman, 2021.