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[OM] Lens Coatings [WAS] 50mm (1.4) Lens Versions

Subject: [OM] Lens Coatings [WAS] 50mm (1.4) Lens Versions
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Thu, 11 Jan 2001 23:38:01 +0000
At 05:14 1/9/01, Mark Marr-Lyon wrote:
As one data point, I've got a silver nose 50/1.4, SN 184xxx, which aside
from having a nice crop of fungus and maybe a little element separation,
looks just like the 55/1.2's little brother - same yellow coating.  Did
the later SC versions look yellow too?

Mark Marr-Lyon

Yes, however you have to be cautions about coatings, their color appearance, and color reflectance.

The subject of single versus multi-coatings comes up from time to time. I've also discounted the importance of multi-coated lenses, especially for the simpler primes. So, for your reading enjoyment or to cure your insomnia, depending on your interest, here is a short tutorial on optical A-R (anti-reflective) coatings.

Uncoated glass with an index of refraction of about 1.50 reflects of about 40f incident visible light at an air-glass interface. Not too much of a problem for those who wear glasses. [I have an A-R coating applied to mine and it does make a difference, mostly at night.] Now consider a relatively simple prime lens for photographic use that has 5 groups. That's 10 air-glass interfaces, each of which only allows 960f the light striking it to pass through. By the time it reaches the film, if absorption in the glass and air itself is discounted, only 96%^10, or about 660f the light that was incident to the objective is left (within the lens' angle of view). This is a huge loss. What happens to it? Most of it bounces around inside the lens, much of it scattering. Some is absorbed by the lens barrel, but some also finds its way to the film resulting in loss of contrast or in the form of flare, the most notorious of which is aperture flare (an outline of the aperture on the film image).

A single, well-designed and carefully applied A-R coating can reduce this reflection to about 0.5 0mproving transmission (again, discounting air and glass absorption) to about 950f the light incident on the lens objective. This is an enormous improvement.

How does this work and why do we see different colors from A-R coatings? This involves the "wave theory" of light. An A-R coating has an index of refraction different from both air and the glass on which it is applied. It provides two reflective interfaces very, very close together where there was originally one. If the thickness of the A-R coating is chosen correctly, the light reflecting from the first air-coating interface will be 180 degrees out of phase with the light reflecting from the second coating-glass interface. Waves that are 180 degrees out of phase cancel. The energy canceled must go somewhere (conservation of energy)! Indeed it does; right past both surfaces thereby increasing transmission. The following ASCII Art diagram shows how this happens (turn off the cute proportional font and go to a fixed pitch one such as Courier).


           \   /   /  <-- Two reflections 180 deg. out of phase
    AIR     \ /   /
        ______v___/_____  Air-Coating Interface
    COATING   \ /
        ________v_______ Coating-Glass Interface
                \
    GLASS        \  <-- Light transmitted past both surfaces


[The astute among you will observe I've left out an internal reflection within the coating. It is so small at this point as to be insiginficant, less than 0.10f all the light originally incident to the lens, and that's for a single A-R coating.]

The reflectance of the surface depends on the index of refraction of the two materials. If the coating material is chosen correctly, the two reflections will be approximately the same magnitude (strength) resulting in near perfect cancellation, and near perfect transmission. Some of the materials used that are ideal for this are metal-Flouride compounds, one of the more common ones being Magnesium-Flouride (MgF2).

For a single A-R coating, the wavelength used to determine thickness is centered within the visible spectrum in the yellow region. This is the yellowish tint you see. Since it is centered, there is more reflectance of red and blue near the upper and lower ends of the visible spectrum causing a purplish reflection. It has nothing to do with the color of MgF2, but its thickness, what gets transmitted and what gets reflected.

For multiple A-R coatings, several different layers of several different materials are used. The thickness chosen for each is different to spread them across different wavelenths in the visible spectrum. The index of refraction of each is chosen to match each successive boundary to make the reflection approximately the same magnitude as the previous boundary (it gradually increases from that of air to that of the glass used). This is why you will see multiple colors. Apparently, for the materials used by Olympus in the MC Zuiko's, green is one of the colors more visible in what little is reflected.

With a single coating already improving visible light transmission from roughly 66% to 95 0n a single-coated 5 group lens, multi-coatings cannot make that much more improvement over single coatings, except under extreme flare risk conditions, such as the sun shining directly on the lens objective. All a multi-coating does is spread the improvement of transmission better across the spectrum.

The bandwidth (or how much of the spectrum is affected) by a well selected single-coating is determined by the index of refraction of the glass. The higher the index of refraction, the wider the band-width. Thus, if high index glass is used, there is greater transmission gain from a wider band-width covering more of the spectrum with just a single coating. This leaves less to be gained by using multiple coatings.

Thus, if you read this far, you now know how A-R coatings work, and why I discount how much more gain is had with multi-coated primes. The biggest gains for multi-coatings are for complex zoom lenses with upward of 15 or more groups!

-- John


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