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[OM] Lens Coating Data

Subject: [OM] Lens Coating Data
From: drchrisbarrett <drchrisbarrett@xxxxxxxxxxxxxxxxxxxx>
Date: Sun, 13 May 2001 21:34:01 +0100
Dear Paul,

Sorry I'm late. Here's mine:

Nose
Markings
Colours                    S/N

Black        OLYMPUS OM-SYSTEM  H.ZUIKO AUTO-W 1:2.8 f=24mm
160xxx                                  Blue/purple           160xxx
"LENS MADE
IN JAPAN" on lens mount
Black        OLYMPUS OM-SYSTEM  ZUIKO MC AUTO-W 1:2.8 F=28mm
139xxx                             Green/Blue           139xxx  "Japan"
on lens
mount
Black        OLYMPUS OM-SYSTEM  G.ZUIKO AUTO-W 1:2.8 f=35mm
230xxx                            Blue/Purple           230xxx   "LENS
MADE
IN JAPAN" on lens mount
Black        OLYMPUS OM SYSTEM  ZUIKO MC AUTO-S  1:1.8  f= 50mm
312xxx   Japan               Green/Blue           312xxxx on lens mount
Black        OLYMPUS OM SYSTEM  ZUIKO AUTO-S   50mm  1:1.8 made in
Japan                             Green/Blue           379xxxx on lens
mount
Black        OLYMPUS OM-SYSTEM  F-ZUIKO AUTO-S 1:1.8 f=50mm   149xxxx
Japan                         Blue/purple           312xxx "LENS MADE
IN JAPAN" on lens mount
Silver        OLYMPUS OM SYSTEM  ZUIKO AUTO-MACRO   1:3.5    f=50mm
101xxx                        Blue/Purp/Green   101xxx   "LENS MADE
IN JAPAN" on lens mount
Black        OLYMPUS OM SYSTEM  ZUIKO AUTO-T  100mm  1:2.8   192xxx
Japan                          green/blue            192xxx   "LENS MADE

IN JAPAN" on lens mount
Black        OLYMPUS OM SYSTEM  ZUIKO MC AUTO-T  1:4 f=200mm 219xxx
Japan                     Green/blue           219xxx   "LENS MADE
IN JAPAN" on lens mount
Black        OLYMPUS OM SYSTEM  ZUIKO AUTO-ZOOM   1:4  F=70-150mm
370xxx                      Blue/Purple           370xxx "LENS MADE
IN JAPAN" on lens mount
n/a            OLYMPUS TELECONVERTER 2X-A 120xxx
......................................Japan
Green/blue            120xxx


On a previous occasion when we did this exercise, my 50mm macro confused
me.If I compare it with my 24 and 35mm, which are definately SC, then I
would say it is MC. However, since I bought lenses which are definately
MC (28,100,200mm), and compared it with them I conclude it is SC.

I think that the colour is about the best guide you can get, but it has
to be used with care.

The multi coating is in fact a type of filter,called an interference
filter. More accurately in the case of a lens it is being used as an
impedance matching device. It matches the impedance of free space to the
impedance of the lens surface (substitute refractive index if you like
into this statement). A single coating is just that, a single thickness
on the lens surface which is a quarter of a wavelength thick at the
centre wavelength at which the lens will be used (usually green). It
will be most effective at this wavelength with poorer performance at
higher and lower wavelenghts. A multi-coating is a stack of at least two
layers, with thicknesses and refractive indices aimed to give good
anti-reflection performance over a broader wavelength range. However
designing good multi-coatings is quite a task, and depends upon:

1) the designer being able to use a good mathematical model
2) the availability of coating materials and their refractive indices
3) compatibility of the coating materials with the lens materials
4) the ability of the coating process to accurately put down layers of
the correct thickness and refractive indices
5) the designer being able to accurately check the design against the
actual reflectivity performance, and change the coating formulation

In short there are multi-coatings and multi-coatings.

These could range from a double coating which is poorly designed and
only applied to the first element (the one the customer sees), and worse
than a single coating, up to a multi layer which is well designed and
controls reflections from all surfaces within the lens.

I've looked at some lenses, on cameras and binoculars which are
multi-coated, with very pretty coloured irridescent reflections of red
and green of quite high intensity. And I've wondered if they are really
doing their job of controlling reflections, or whether they are a
marketing ploy to make the lens look more attractive to customers.

Just my 0.02 Euros.

Chris Barrett
Malvern UK



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