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What's the deal with Nikkor Ai/Ai-S coating colors?


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Picture is from someone on mflenses.com, not mine, but it's a great example of the title.

I do however have a small set of Nikkors that I use for stills and video use. I was cleaning them and noticed that some had a blue-purple sheen associated with the coating, while one of them had a greenish one.

I know that Nikon switched up the coating colors along the way of manufacturing many different lenses in their Nikkor line, but what really did the different colors mean and why did Nikon opt to switch it up?

Was it just a manufacturing/engineering reason or maybe inventory?

I'm sure the coatings, even if they were differently colored, were pretty much identical in function and design up until Nikon switched to their modern coatings.

I'm just suddenly very curious about this topic and thinking about the possibility of rehousing my Nikkor set in the future. My OCD wants to have identical colors so that the lens set looks really sexy together. But, I understand that this is a bit much, hahaha.

 

 

nikkor colors (2).jpg

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The color isn't really "color" in the sense that you're thinking of it.

It's not a transmission effect, like if it were a filter, instead, it's a dichroic effect caused by optical interference.

The coatings on these lenses are, at most, only few wavelengths thick. Their function is to manage the reflections caused by the sudden change of index of refraction as light enters and exists the glass, thus maximizing transmission at nominally perpendicular angles. The idea is the refractive jump is split into two steps between the air and coating and the coating and glass, keeping the steps individually low, which is less reflective than one large step.

When light gets into the coating from low raking angles, it can bounce between the air-coating interface and the coating-class interface in a phenomenon similar to fiber optics,  and interfere constructively or destructively with itself depending on the wavelengths involved and the coating thickness. This produces color depending on what wavelengths get absorbed and which escape.

This is something that should only happen to light rays entering the glass from outside the lens frustum, so it really doesn't affect imaging.

The coating varies from lens to lens because it is adjusted to the index of refraction of the underlying glass elements, which vary.

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1 hour ago, Steve Switaj said:

The color isn't really "color" in the sense that you're thinking of it.

It's not a transmission effect, like if it were a filter, instead, it's a dichroic effect caused by optical interference.

The coatings on these lenses are, at most, only few wavelengths thick. Their function is to manage the reflections caused by the sudden change of index of refraction as light enters and exists the glass, thus maximizing transmission at nominally perpendicular angles. The idea is the refractive jump is split into two steps between the air and coating and the coating and glass, keeping the steps individually low, which is less reflective than one large step.

When light gets into the coating from low raking angles, it can bounce between the air-coating interface and the coating-class interface in a phenomenon similar to fiber optics,  and interfere constructively or destructively with itself depending on the wavelengths involved and the coating thickness. This produces color depending on what wavelengths get absorbed and which escape.

This is something that should only happen to light rays entering the glass from outside the lens frustum, so it really doesn't affect imaging.

The coating varies from lens to lens because it is adjusted to the index of refraction of the underlying glass elements, which vary.

Wonderfully explained. I actually reached out and emailed Roland from photosynthesis.co.nz which is a wonderful resource for cataloging Nikon lenses and has a lot of info regarding older vintage lenses like serial numbers, tech specs, etc.

He said the following:

Hi Sam, 

Lens coatings are very thin layers applied to the glass. The layer is ¼ the wavelength of light thick, and works by destructive interference to reduce reflections. If you imagine light striking the lens. Some light is reflected off the air/coating surface but most passes through. Then some more light reflects from the coating/glass boundary. The light reflecting from the glass surface is now ½ wavelength out of phase with the light reflecting from the coating ( ¼ in plus ¼ out). Because the two reflected rays of light are ½ wavelength out of phase, the peaks of one coincide with the trough of the other, which cancel each other out – destructive interference, which reduces the amount of light reflected.

 

Because coatings cause destructive interference for a particular wavelength of light, reflections from other wavelengths (colors) are more obvious, which gives the coating its color.

 

Coatings only work well with one wavelength (color) of light. Multilayer coatings have layers of different thickness so each layer works with a different color to give better suppression of reflections across the visible spectrum.

 

Coatings work best if the light passes at right-angle through the glass. When it passes through at an angle the distance it travels between the air/coating layer and coating/glass layer becomes bigger so no longer causes destructive interference for that wavelength (it would work for a longer wavelength). That is why coatings are not so effective from light coming at extreme angles. This also explains why coating change color when you view them from a different angle.

 

Nikon apply different coatings to each lens to optimise the performance of the lens as a whole, which is why different lens surfaces often have reflections of different colors.

 

I hope that helps!

Roland

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