Re: [Leica] 2x extender R disappointment

["Henning J. Wulff" <henningw@archiphoto.com>]

Dominique wrote:

>I'm not an engineer in optics. I only remember simple experiments with a
>divergent lens behind a convergent lens (Barlow principle).I think i'm
>right but i'm going to put the question to God (i.e.Leica).
>You write :"adding optical systems such as a
>converter can actually improve certain aspects of optical performance, a=
nd
>actual performance losses are often a lot less than you surmise. A good
>converter often causes a loss of only 10 percent in lines per millimeter=
".
>When the french review Chasseur d'images(n=B0 116) tested the apo 2.8/28=
0
>with the extenders, the results were deceptive.At 2.8 without extender
>:very good (center), good (edge). With the apo 1.4-extender : good,
>fair;with the non apo 2x-extender :fair, poor.
>I do think that the resolution is mathematically cut in two with the
>2x-extender.Osterloh in Leica Angewandte leica technik is not clear :"Be=
i
>den im Prinzip ahnlich aufgebauten echten tele-objektiven erfolgt die
>Kompensation der Restfehler des negativen Hintergliedes durch
>entgegengesetzte Fehler, die der Optik-Rechner bewusst im positiven
>Vorderglied des Objektivs belasst.Das ist bei der Kombi Objektiv+extende=
r
>naturgemass nicht gegeben und die Bildqualitat wird dadurch mehr oder
>weniger negativ beeinflusst."(p.159)

Chasseur d'images tests tend to be quite decent, but subjective to a larg=
e
degree. This is not bad at all, per se, but will not provide a solution t=
o
the issue. Also, this is a 'one lens, one converter' comment, and each
system must be be anyalyzed separately. As I said earlier, the short fast
teles do not seem to work well in general with teleconverters. Some long
lens combinations are excellent.

I'm not an optical engineer either, but have 6 years of University Physic=
s,
including a number of courses in optics. I do understand some of this
stuff. The resolution is not halved by 2x extenders. Way too many other
factors come into play for such a simplistic analysis.

Osterloh states in the above quote that a normal telephoto is also
basically constructed as a 'normal' lens with a converter behind it, but
that the system is optimized. On the other hand, a self-contained lens wi=
th
an additional converter is the same functionally, but is not optimized.
Therefore, in the latter system the picture quality will be more or less
reduced. This is all just a restatement of my previous posts.

If you test a certain converter with a variety of lenses, you will see th=
at
the results are very good in some combinations, and very poor in others. =
I
don't have any Leica R stuff, but I have about 25 Nikon lenses and 3
converters. At one time or another, I have tried all of Nikon's converter=
s,
and many third parties'. Nikon's TC14b is a good converter with a number =
of
lenses, and outstanding with some. The TC201 and TC301 are good with some=
,
and useless with others. Vivitar's Macro TC is outstanding with one Nikon
lens (the 180/2.8) and decent with a number of others. No 2x converter is
at all useful with the 400/5.6 I have. As Osterloh says about the
construction of a regular telephoto, the rear dispersion unit cuonteracts
some of the aberrations of the front section; similarly, the converter ca=
n
counteract or more usually, worsen the aberrations of the main lens. To
what degree depends on the particular lens and converter.


   *            Henning J. Wulff
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