It's not just longer lenses but faster ones with bigger front elements. The
acceptable F-ratio for controlling dispersion without ED type glass goes up
(slower) the larger the front element regardless of the F-ratio. In simple
achromatic systems the acceptable F-ratio is 3 at F/15. But if the front
element goes up to 4 the ratio has to be F/20 or slower, while for 5 it
goes to F/27 or so.
So the focus dispersion problem is significantly higher for a 600/4 lens
than a 600/5.6 or a 600/8 lens because the front element is around 6
instead of 4 or 3. The steeper the light path the worse the problem
(faster the F-ratio).
Kent Gittings
Kent
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Fred
Sent: Wednesday, December 19, 2001 8:39 AM
To: [EMAIL PROTECTED]
Subject: Re: why do tele-lenses have ED glass
does anyone know why tele-lenses have ED glass, and why wide
angles do not?
First: As I understand it, low dispersion glass can be used to help
prevent color fringing (sometimes visible as a red or purple edge on
a viewed object). Since the different wavelengths (colors) of light
are refracted by ordinary glass at different angles, glass designed
to prevent this dispersion of light rays is cleverly called LD
(for Low Dispersion) glass or (if the marketing department has its
way - g) ED (for Extra low Dispersion) glass.
Then: Lenses with longer focal lengths are the ones that tend to
disperse the red and purple ends of the spectrum the most, if not
corrected (or minimized, actually). The weak point in my little
explanation here is that I do not know why this is more of a problem
for longer focal lengths than shorter. (Sorry.) I guess a little
knowledge is a dangerous thing...
So, my quick and dirty answer to your question, Frank, is that
telephotos naturally produce more color fringing than wide-angles,
so that they benefit more from the use of LD or ED glass, designed
to minimize fringing. Just don't ask me to explain why it is the
long lenses that produce the most color fringing - g.
Fred
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