[Leica] Bubbles, coating and all that

[Erwin Puts <imxputs@knoware.nl>]

My brother (Hubert, still remember him) urges me to go mainstream as it is
infinitely more relaxed than trying to unearth the real story about
anything Leica-ish.
Still in my heart I am a Startrek believer: exploration without prejudice
is the mission of any Vulcan.
So here it is:
On bubbles in glass:
Bubbles are unavoidable in the manufacture of glass, but they come in
classes: we have very small bubbles and larger ones, we have a low
concentration or a higher concentration per volume unit of glass.
The Schott catalogue identifies four classes of the incidence of bubbles,
from zero to three. The zero class has an extremely low amount of bubbles
per volume unit and class three has a higher amount.
The designer then can choose beforehand what type of optical glass he needs
and how high an amount of  bubbles he can  handle in his design.
Generally bubbles are optically absolutely uninteresting from a performance
view. Cosmetically it is a different matter. We might see bubbles and feel
insecure. Still the effect of the presence of bubbles on performance is to
be measured in promilles.
Bubbles can be annoying and performance degrading: this will happen when
the bubbes are in glass that is very close to the focal plane. Any
respectible designer knows about this and will ensure that he selects glass
that will not degrade the image beyond the tolerated optical aberrations
that is.
So do not worry. Bubbles are optically harmless when the desiger is a
responsable girl or guy, but may be unsettling emotionally. But remember
Cindy Crawford and the small beauty spot on her face?

On coating.
Some light is always refleced from any surface. We know about that and we
can control it. If we apply a very thin film on a glass surface we can
reduce reflections. The "thickness" of this thin film is generally 1/4 of
the relevant wavelength, very small that is. If we use one layer of coating
to a 1/4 wavelength of light in the part of the spectrum of light in which
suppression has been chosen (mostly the green-yellow area) we talk about a
single layer coating. In an optical system with many elements  it is not
advisable to apply this coating to all surfaces if we want to have neutral
transmission. Therefore some coating is yellowish in tint to compensate.
High refraction glass also demands yellow coating to compensate the
inherent blue absorbing characteristic of dense glass.
As the single layer can only correct for one wavelength, we need more
layers if we are to compensate for more wavelenghts. So multicoating can
stack up to nine layers of single thin film coatings.   This we call
multi-layer coating.
The coating must be adjusted to the angle of incidence of the predominant
light falling onto the glass. A double layer coating used for the front
glass of wide angle lenses ( light incidence at large angles) will b green
tinged.
The colour of individual lens surfaces is a carefully designed composite of
refractive angles of incidence and many other reflective characteristics of
the glass employed.
Most manufacturers use multi-layer coating by various names. But be aware
taht a bad design will never become a good one becuase of MLC.
Multicoating is the same as multi-layer coating. BTW.


Erwin