Re: [Leica] 2x extender R disappointment

[Dominique PELLISSIER <pelliss@droit-eco.u-nancy.fr>]

A 13:14 08/08/98 -0700, vous avez =E9crit :

>From: "Henning J. Wulff" <henningw@archiphoto.com>
>Date: Fri, 7 Aug 1998 16:43:11 -0700
>Subject: Re: [Leica]2x extender R disappointment
>
>Dominique wrote:
>
>>How do you explain with the laws of optics that a 2X-converter behind, =
say,
>>a 2/180, gives an optical system with an aperture of 1:4 ? If Osterloh =
was
>>right, we shouldn't have that result.
>
>The quote from Osterloh that you posted didn't touch upon this in any wa=
y.
>
>The reduction of aperture ratio is based purely on the fact that since y=
ou
>now have an optical system with a focal length that is twice as long as =
the
>original, and you have the same entrance pupil, the f number or aperture
>ratio is now twice as large; ie, you lose two stops. This has nothing to
>do, in any way, with the quality of the image that results.
>
>
>   *            Henning J. Wulff
>  /|\      Wulff Photography & Design
> /###\   mailto:henningw@archiphoto.com
> |[ ]|     http://www.archiphoto.com
>


Henning,=20

Maybe my understanding of the german text is not perfect. Osterloh says
that a set including a lens and an extender is like a true telephoto lens.
For instance : an Elmar 135.I don't want to talk nonsense but, if Osterlo=
h
is right, that means the front convergent group (focal length =3D 67.5 in
that case) of the Elmar has a nominal aperture of f:2 and that, putting
behind a divergent group (Hektor design), gives a complex system such as =
an
Elmar 4/135 ?
The problem is : if the lens is already a tele (apo 2/180), putting behin=
d
a divergent group, does not give a similar design.

Dominique