[rollei_list] Re: Lens coatings and veiling flare.
- From: Carlos Manuel Freaza <cmfreaza@xxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Thu, 9 Jul 2009 20:11:41 -0700 (PDT)
From that point of view, I don't think improvements could be perceptible too
much 'cause the principles of Physics don't change, if we consider that the air
refractive index is 1.0, and f.e. crown type glass refractive index is about
1.6, a single layer refractive index must be a compromise about both indexes,
f.e. 1.3 to cancel reflections from glass and coating each other, the single
layer refractive index will always be a compromise between the air refractive
index and the glass refractive index to cause AR effect. The material most
commonly used for this purpose is Magnesium Fluoride with a refractive index of
about 1.37, it also offers other desirable properties like durability.
BTW, since a single coating layer works 100% for one given wavelength only, you
need to choose that wavelength for your single coating layer considering the
visible spectrum from 400nm (violet) to 700nm (red), most manufacturers chose
about the center, 550 nm, within the green-yellow colors, the practical problem
is that you need 1/4 of magnesium fluoride wavelength thickness to eliminate a
given wavelength reflection.A coating of 137.5nm thickness (550 divided by 4)
would virtually eliminate any reflections of this color -green- from the
surface of the lens. It would reduce reflections of other wavelengths also, but
the efficiency of its effect would become progressively lower as the wavelength
deviates from 550. The net result is that the overall goal of suppressing
reflections is achieved, but it is not achieved equally for all colors. We see
this when we look at the reflections from the coated glass surface: there is no
green light reflected at all,
but there are reflections of the other colors. The overall effect is that of a
reflection in the complement of the targeted color. that is the reason most
single layer coatings look blue-violet.
Every lens has at least two reflective surfaces, the front and the back. In
lenses used for practical photography, there are generally at least six, and
there can be 18 or more in complex wide angles or zooms. Each surface presents
an opportunity to adjust the color effect of the coating: with, say, six
surfaces to coat, the designer can specify six slightly different thicknesses
of coating, each corresponding to a different point in the spectrum. This is
why, in some lenses, you will see a number of reflections in different colors:
each coating layer is the same Magnesium Fluoride material, each is a single
layer, but each is a slightly different thickness to correspond to a different
color within the visible spectrum to optimize balance. Not all designers did
this – some felt it was better to optimize the entire lens in the center of the
spectrum for best overall efficiency, and their lenses tend to have a uniform
blue-violet color on every surface.
These principles (taken in part from a Rik Oleson page) don't change, they
exist from the beginning, however you could be closer or farther regarding the
ideal results according your coating process quality and design, BTW talking
about an old lens, its condition has a lot to do.
Carlos
--- El jue 9-jul-09, Eric Goldstein <egoldste@xxxxxxxxx> escribió:
> De: Eric Goldstein <egoldste@xxxxxxxxx>
> Asunto: [rollei_list] Re: Lens coatings and veiling flare.
> Para: rollei_list@xxxxxxxxxxxxx
> Fecha: jueves, 9 de julio de 2009, 10:05 pm
> This is precisely the question we are
> trying to get smart about... we
> know that modern coatings are more durable but is there an
> improvement
> in flare reduction and bandwidth with modern single
> coatings when
> compared with the coatings of the mid to late 60s and 60s?
>
>
> Eric Goldstein
>
> --
>
> On Thu, Jul 9, 2009 at 7:56 PM, Carlos Manuel
> Freaza<cmfreaza@xxxxxxxxxxxx>
> wrote:
>
> > BTW, as Marc explained yesterday, there were
> improvements about the single layer lens coating along the
> years, f.e. materials selection and mixing, vacuum chamber
> quality and temperature degree and temperature control among
> several other factors. Improvements tended to increase
> lenses coatings durability and hardness and then you can
> expect that a newer coating is harder than an older one, of
> course.
> >
> > Carlos
> ---
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