Archived posting to the Leica Users Group, 2004/02/03
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At 4:39 PM -0700 2/3/04, John Collier wrote:
>From the Tamron site:
>
>><What is (Anomalous Dispersion)
>>
>>Anomalous Dispersion glass is a type of optical glass material that
>>delivers an abnormally larger partial dispersion ratio (amount of
>>dispersion at a given wavelength area within visible light range)
>>relative to a specific wavelength zone. Combined use of AD glass
>>with other optical glass material(s) of different dispersion
>>characteristics serves the purpose of optimizing the dispersion
>>character of an optical system at a specific wavelength. Thus, it
>>helps to compensate for on-axis chromatic aberration on telephoto
>>lenses, or for lateral chromatic aberration on wideangle lenses.
>
>I still don't know what it means though...
>
>John Collier
>
>On Feb 3, 2004, at 4:05 PM, Stan Yoder wrote:
>
>>Pardon my iggerance, but what is that (in lay terms, if poss.)?
>>It's mentioned in describing the new Vario-Elmarit R 28-90 in the
>>current "Viewfinder."
It's a kind of glass that, when used with regular glass, can
counteract the tendency for glass to make a rainbow from a white
light source when refracted. All transparent substances slow down
light, but the amount they slow it down depends on the frequency of
the light (or it's reciprocal, the wavelength). This causes the
rainbow and chromatic errors in lenses. If you have a type of glass
that doesn't obey the standard formula relating nominal refractive
index and the amount of dispersion (spread of rainbow) then you have
AD glass. If you normally would have one positive meniscus to focus
your light, and it gives a certain chromatic spread (dispersion),
then it is often possible to use a somewhat stronger positive
meniscus and a weak negative meniscus to give you the same focal
length, but if the dispersion of the weak meniscus is much greater
than its refractive index would lead one to believe from the standard
formulas, then you can 'collect' the chromatic smear while still not
offsetting the focussing power of the positive meniscus.
Early in the days of optical design, 'crown' (low dispersion) and
'flint' (high dispersion) glasses were used in combination because
they have different dispersion for the same refractive index, and can
produce achromats such as the 5.6 and 6.8 Telyts. These actually use
a type of glass which can be termed AD. AD glasses are extremes.
Generally refers to extremely low dispersion glass (and flourite).
For more info, look up 'Abbe number', a numeric index of dispersion.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com
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