I want to share some of the more technical aspects regarding lens repairs and servicing of Leica lenses, focusing on anti-reflection (AR) coatings. In a sense, this is 'elephant in the room' territory!
I'll keep this simple...
When light hits an uncoated lens, or any other optical component, about 4% of that light is reflected from the front surface. The remaining 96% of that original light continues through the glass until it hits a glass/air interface, where 4% of that remaining light is reflected back from that surface.
The key takeaway is that uncoated glass only allows about 93-94% transmission. Lenses are usually made up of several elements, so light losses can be significant. This issue held back more complex lens designs until a solution could be found. Lost light due to internal reflections typically bounces around within the lens, reducing image contrast. Lens flare from bright light sources can also be problematic but it's the scattered light within the lens that is the real problem.
With a few exceptions, pre-war Leitz lenses were not coated. If you own a coated example, it may be because a previous owner arranged to have the lens coated, either at Leitz or by firms offering these services, such as De Oude Delft.
You may also be aware of multi-AR coatings, such as Zeiss T* or 'Super-Multi-Coated Takumar.' These are more efficient than a single coating, but the first coating makes the biggest impact, with subsequent layers producing diminishing returns. This article mainly discusses single coatings. A simple AR coating on both sides of a lens can increase light transmission from about 93% to 98%, resulting in a more contrasty image because internal light losses are controlled.
Zeiss perfected lens coatings in the 1930s. The invention by Olexander Smakula involved placing a lens into a vacuum chamber, where a material such as magnesium fluoride would be heated to its melting point. A controlled amount of the resulting vapor would condense on the lens surface, resulting in a very durable AR coating. This process is called physical vapor deposition (PVD). Zeiss had a protected patent on this process, which wasn't available to Leitz until the early 1960s. However, the external lens surfaces, both inside and out, appear to have been coated using PVD.
The internal coatings of post-war Leitz lenses until the early 1960s were applied using a different process altogether. One alternative method, soft coating, involved creating a solution of colloidal particles of the required material and applying it by dipping or spin coating. Whether applied by PVD or the wet method, the thickness of the coating is critical. With PVD, thickness is a function of exposure time, but with dip coating, the rate at which the substrate is withdrawn from the solution determines coating thickness. With spin coating, a lens is placed on a rotating turntable. A few drops of solution are dropped onto the center of the rotating lens, and the turntable is ramped up to a particular speed, determining coating thickness.
The advantages of the wet method of AR coating are mainly cost and simplicity. Early 20th-century vacuum systems were expensive and quite crude compared to their modern counterparts. Today, we can incorporate accurate surface thickness monitors, but early operators often relied on a combination of experience and observing the color of light reflected from the lens surface, often a deep blue color.
AR coatings are actually transparent. The color you may see is due to thin-film interference. Just like oil films on water, different thicknesses result in different reflected wavelengths.
Both methods produce high-performance AR coatings, but the wet method has several disadvantages:
Wet AR coatings are not as effective as PVD coatings. Adhesion to the substrate is poor, and the AR layer can be porous with a low hardness rating. PVD coatings bond to the substrate more successfully, resulting in a denser, homogenous layer with hardness that may equal or even exceed the substrate, offering protective qualities.
The porous nature of wet coatings makes them hydrophilic. Internal moisture caused by condensation can create ideal environments for fungus and bacteria to thrive. Enzymatic activity from biological processes can interact with magnesium fluoride, releasing small amounts of hydrofluoric acid that can etch the glass.
Wet coatings have a high surface roughness and are fragile and difficult to clean. Polar solvents such as water or alcohol can dissolve the coatings, and being very soft, they are easily scratched even by the gentlest cleaning techniques.
Wet coatings are unsuitable for cleaning with polymers such as First Contact. The polymer will not release from the rough coating and will instead pull the coating from the glass.
Wet coatings may become hazy over time, possibly due to oxidation or the absorption of atmospheric dirt that can't easily be removed.
Good PVD coatings can be tough (hard to remove even if you want to) and resistant to most chemicals and solvents. They are less prone to lens fungus and bacteria. When we service such lenses, we can safely use products such as First Contact Polymer, resulting in an atomically clean surface.
Early 50mm f2.8 Elmars are particularly prone to bacterial infections (oleophilic bacteria) on the inner side of the front element. Internal lubricants eventually find their way to the aperture blades, which are in extremely close proximity to the front element. Unlike fungus, which can survive on minimal amounts of organic material, lens bacteria thrive on hydrocarbons and a little moisture. Hence, the rear of the front element is often affected. When warm, oil fractions may condense on the lens surface, resulting in a bullseye pattern of infection. Infections may look like regular haze. Under magnification, the haze may consist of thousands of tiny dots – each one a bacterial colony. Unfortunately, the coating and usually the glass surface may be permanently etched by the leaching of alkalis from the glass surface. Believe it or not, a repairer who has stocks of oil and greases, passed down to him, sometime in the year dot, may be infected with oleophilic bacteria. This is a way a lens might be accidentally inoculated with nasty bugs. My advice: avoid any f2.8 Elmar with a hazy front element! Bad ones can't be saved.
So, if you choose us to service your lens with soft internal coatings, it may not be possible to fully restore the lens's clarity. In certain cases, we may find that one particular lens surface is badly affected. In such cases, removing that coating and carefully cleaning others usually results in an overall improvement in lens contrast. In other cases, depending on the construction of the lens element in question, it may be possible to re-coat that unit, though this isn't always feasible.
Newer lenses with hard coatings are not without their problems but generally respond to cleaning more positively.
You may be interested to know that I did experiment with spin coating a few years back. I managed to produce a liquid coating that replicated the blue Leitz coating very well and when tested on a device that measures visible light transmission through glass, performed beyond my expectations. The solution had to be made in small batches and required a hydrolysis reaction of 15 hours at 35 degrees C for each batch. While I figured out the process, it just wasn't practical for a small repair shop like us and I don't know what the long term stability would be. Maybe one day I'll have another go.
One final point. I talk about internal coating of early Leitz lenses. The outermost coatings appear to be robust as if they were PVD coated. My understanding is that Leitz had to wait until about 1960, when the Smakula patent expired before they could use PVD, so how is it that the outer elements of earlier lenses look like they were PVD coated? Was this a special soft coating treatment in order to make them hard? I don't think so. Did they have limited permission to coat external elements only? I don't know. If anyone has any interesting information to share about this, I would love to know so that I can share with others.
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