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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