Friends,
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. I would be more
than happy know, one cannot get too much knowledge... I remember
someone, don't know who, said that "knowledge is the only thing that is
not heavy to carry".
(Actually, I can't remember the right words...)
--
Regards/
Ingemar Uvhagen
Gislaved, Sweden
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