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Re: [OM] Questions, questions...

Subject: Re: [OM] Questions, questions...
From: Kennedy <rkm@xxxxxxxxxxxx>
Date: Mon, 13 Jul 1998 20:03:35 +0100
In article , Ingemar Uvhagen <ingemar.uvhagen@xxxxxxxxxxxxxx> wrote
>I have finally found what ED glass and UD glass stands for.
>ED glass are "Extra Low Dispersion glass". (Taken from The Zuiko
>Interchangeable Lens Group at the Olympus America site.)
>UD glass are "Ultra Low Dispersion (UD) glass". (Taken from the C*n*n
>site in Japan.)
>Apparently the UD glass are used in super telephoto lenses, which demand
>high quality glass for their performance. I have yet now real idea of
>what kind of difference there is between ED and UD glass, except for
>what it sounds like, the even lower dispersion of the UD glass. There
>are also a glass called "Super UD glass" which is even better than the
>UD glass.
>The glass Zuiko used for their best lenses was UD, and these lenses are
>180/2, 250/2 and 350/2.8.
>So, what it is all about is that ED glass is concentrating the light
>better than "normal" glass, and that UD and Super UD is concentrating
>the light even better. This should result in better sharpness and also
>give the possibility to use big apertures without as much distortion as
>"normal" glass would give at the same maximum aperture.
>Well, that was the conclusion I made. I could be wrong as this is not
>explained anywhere I have looked, and I am not too familiar with the
>physics of photographic lenses.
>If anyone will correct me or ad something, please do. 

As usual with these acronyms it is no more than marketing hype!  :-(

You are interpreting "dispersion" as "scattering", which is exactly the
trap that the spin doctors at Olympus and Canon hoped you would fall
into - hence it makes the glass seem almost intuitively superior.

Dispersion is the change of the refractive index of the glass with
wavelength, so it is the ability of the glass to split white light into
its component colours or, equally importantly, the ability of the glass
to correct the chromatic aberrations of other elements in a particular
lens design.

Armed with this knowledge you can see that in some cases ED and UD glass
can actually be less effective at correcting lens aberrations than plain
old beer bottle glass!

Fortunately the designers at Olympus and Canon are a separate group from
the marketeers, and only use this type of glass where it is needed.
Needless to say, one day one of the spin doctors wanted to get some new
differentiater between Canon or Olympus and their competitors and took a
stroll down to the workshops to ask if there was anything special they
could say about their optics.  One of the engineers was heard to say
"this needs a low dispersion element to do the job right" and the
marketing man drew up the publicity without understanding what it meant.

Couldn't happen?  If it didn't I might have had a less stressful time at
work today!  :-(
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers         (replace 'nospam' with 'kennedym' when replying)

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