Lens coating & terminology evolution

[xkaes]

I've read with interest this old thread about lens coating which included much about the history and evolution of lens coatings. It also uncovers how the terms used for "coatings" have evolved.

http://www.largeformatphotography.in...hlight=coating

The coatings of Japanese lenses (as opposed to Japanese cameras with German lenses) appears to have occurred after WWII. Supposedly, Minolta was the first Japanese lens manufacturer to produce coated lenses -- a single coat in 1946 (the Semi IIIa era). I assume it was a layer of magnesium fluoride, but that doesn't mean that all of the elements and all sides of all elements were coated. Fuji lenses, at least their large format ones, started to get a coating around 1954. Again, I assume it was a single layer of magnesium fluoride. In 1958, Minolta pioneered its "Achromatic coating" which was two layers of magnesium fluoride deposited in different thicknesses. I have no details if this means two layers of different thicknesses on the same glass surface or two different layers on two different surfaces. They were using the term "Achromatic coating" well into the 1970's on lenses that were obviously multi-coated with different minerals. Each of Fuji's and Minolta's lens coatings (and undoubtedly everyone else) changed with their new series of lenses (ex. Minolta's Rokkor, Auto-Rokkor, Rokkor-X, Yashica's ML, MC, DX, and Fuji's SW, SWD, W, NW, CM-W), but there is plenty of evidence that the coatings changed/improved even within the same series.

But is begs the question, "What consitutes multi-coating?" Minolta had lenses with two coats of magnesium fluoride. They didn't call it multi-coating, but it seems that it qualifies. On their enlarging lens spec sheets, they lists some some coatings as "amber", and others as "amber and magenta", but none as "multi coated". "amber and magenta" are obviously at least two coatings, but the "amber" lenses might have been more than one layer, too.

At some point, Minolta started to use the word "multi-coated". Fuji called their approach EBC (electorn beam coating). Tamron BBAR (Broad-Band Anti-Reflection) coating -- up to seven layers on glass-to-air surfaces. Most people would think that all lens elements get the same treatment/coating. To me, all this means is that SOME elements of all of these multi-(more than one)-coated lenses may have a lot of coatings, but others may have only one layer (or perhaps none?).

I've had the Fujinon f5.6 105mm EBC in the NW and CM-W. The NW version appeared multi-coated on the front and single-coated on the rear, while the CM-W version appears multi-coated on both. I'm not surprised since they are different optical designs. The same with my Fujinon f5.6 75mm EBC SWD -- it appears multi-coated on the front and single-coated on the rear. Some Carl Zeiss T* lenses have little multi-coating.

So it appears that "multi-coated" is apparently a very loose term. Sometimes lenses that have quite a lot of multi-coating are not labeled as multi-coated, but some that are labeled multi-coated have limited multi-coating.

Multi-coating -- More than one coating on any surface? More than one coating on more than one surface? More that one type of mineral?

[Ted R]

Some technical information here from Edmund Optics https://www.edmundoptics.com/resourc...-notes/optics/

[Bob Salomon]

There two different methods of applying a coating on a lens to reduce reflections. The method I use at work involves adding a clear coating that is uv cured, then placing it in a vacuum chamber. While it is in under vacuum it is bombarded with a spray of argon gas that creates micro abrasions in the clear coating I applied earlier. This is done in six different layers, some machines apply up to 14 layers. The color of the resulting lens is controlled by the mix of argon and nitrogen used. Every manufacturer likes a different color. Minolta/Sony likes green, Rodenstock likes red, Schneider likes purple/blue. The Kodak lenses I have seen are yellow.

The other method I have seen adds the coating under vacuum directly rather than by argon abrasion like my machine. Camera lens elements most likely use the second method as the elements are glass as opposed to plastics.

There are also steamed on coatings and not all camera lens elements are made of glass.

[huffmalw]

According to Jeffery Alexander Nikon and the sponsorship of Japan's optical industry by the Imperial Japanese Navy 1923-1945. Nikkor started coating its lenses in 1945-1946:
" According to the investigators in the U.S. Navy technical mission: In the evaporation method, cryolite is evaporated and deposited upon the glass surface, in vacuum. After treatment, the glass is baked at 150°C for one hour for durability" Nikkor / Nikon did start lens coating 1945-1946, but not clear if cryolite was in fact the method.
all the best, Larry Huffman
www.canonrangefinder.org my site had descriptions of all the Canon rangefinders and lenses - also of other M39 cameras

[huffmalw]

Larry Huffman
www.canonrangefinder.org

[Bernice Loui]

Some lens coating history:

https://mhcocm.wordpress.com/2011/12...ical-coatings/

The topic of lens coatings is complex, grew from humble beginnings. Know significance of lens coating is more important as the number of lens elements increases. This is one of the reasons why uncoated Dagors perform well as the number of glass to air surfaces is four limiting the potential of stray reflections. Limiting the number of lens elements was one of the ways pre WW-II lens designers aided optical performance of their lenses.

Once had an uncoated Zeiss Planar process lens, the flare-internal reflections were bad enough to pass it one after testing with a few sheets of film.


Bernice

[huffmalw]

Hi Bernice and thanks for the information and the link to optical coatings - most interesting.
Yes, I suppose limiting number of surfaces was important - also the photographer using a hood. The subject of lens coatings and multi-coated lenses is difficult to track even with the famous names.

thanks again, and all the best, larry

[Nodda Duma]

Here is a thread I posted over on Photrio which provides information from my lens design background. As a designer, I generate the specifications for coatings which go on the drawings sent to the optical shop. I’m also familiar with how they are designed, and for very complex coatings there is as much an art to their design as there is to designing optics. For consumer optics, coatings are fairly straightforward. Good coating designers are hard to find, so if you’re looking for a guaranteed high-paying career... There are a couple of good reference texts as well (mentioned in my post I think).

The post was focused more on coating durability (I was tackling that issue on a project at work at the time, so it was on my mind), but there is good info on coating design in general and some information on when different coatings were introduced.

https://www.photrio.com/forum/thread...bility.148890/

A Single-layer coating on refractive elements is 1/4 wave Magnesium Flouride. So there’s no significant difference from one manufacturer to the next vis single-layer coatings. Transmission peaks at > 99% at the design wavelength and rolls off towards red and blue by about 1%.

Multi-layer coatings vary due to the design and by requirements, but they all will transmit > 99% throughout most of the visible spectrum.

[reddesert]

The OP's question is a couple years old but seems to come up a lot. Nodda Duma has addressed this elsewhere, I think, and the Edmund Optics website linked above has some useful information about how coatings work. This is particularly useful on the difference between single and multiple layer coatings: https://www.edmundoptics.com/resourc...tion-coatings/

Basically a single layer coating is designed to minimize reflections at an air-glass surface by causing destructive interference (cancellation) between reflections off the air-coating and coating-glass surfaces. This requires 1. having a thickness that is about wavelength/4, and 2. having an index of refraction that is the geometric mean of the indexes of refraction of the air and glass. However, both the thickness required and the indexes of refraction depend on wavelength. So a single layer coating will work great at some wavelength (typically the middle of the optical band, eg green, 550nm), and OK at wavelengths away from it.

Multicoating refers to the use of multiple coating layers to address this limitation. The coating designer can use the freedom of several coating thicknesses, indexes of refraction, and possibly phase shift effects at layer interfaces, to make the coating cancel reflections over a broad band of wavelengths (thus "BBAR"). Coating design is something of a black art. It's still often difficult to make coatings that work well over more than about a factor of 2 in wavelength. (I'm basing this partly on work experience with optics and ordering custom coatings, but not in the consumer photography domain.) I think this may be one reason why some lenses that work well in optical have a reputation for hot-spots or veiling flare when used with near-infrared film or detectors.

Multicoating, as far as I know, is always used to refer to multiple layer coatings, not to whether multiple elements in a lens are coated. The hard part of coating, again AFAIK, is the coating design and achieving process consistency. Once that's done, the benefits of coating each air-glass surface are large. I would guess only an item manufactured at some transitional stage would have a mix of coating technologies. An exception is that some elements (some plastics and salts) may not tolerate some of the coating deposition methods. But other than plastics those are relative rarities that wouldn't be in, say, a generic 1960-80s lens design.