Re: [Leica] 21 or 24 mm M lens: advice sought

[Mark Rabiner <mrabiner@concentric.net>]

"Henning J. Wulff" wrote:
> 
> >snip
> >> Please note that all Leica M lenses below 35mm are now retrofocus lenses,
> >> although only to a slight degree in comparison to SLR lenses. This is to
> >> allow the M5 and M6 metering. On the positive side, this makes it easier to
> >> design a lens that has even illumination over the field, but on the
> >> negative, it makes it harder to control distortion. As a consequence, the
> >> new 21/2.8 ASPH has more distortion than the 21/3.4. These differences are
> >> slight, but they do exist, and might possibly make some difference in your
> >> shooting.
> >>
> >>    *            Henning J. Wulff
> >snip
> >
snip
> >It's my impression that retrofocus somehow means: telephoto turned
> >backwards. If this might be the case how can something be slightly retrofocus?
> >As to the distortions issue I am recalling Erwin's articles which I'm
> >pretty sure are convincingly to the contrary, but I'll go check now.
> >thanks again,
> >Mark Rabiner
> 
> The terms 'retrofocus' and 'telephoto' are bandied about rather loosely,
> but generally are used to describe optical systems which respectively have
> their elements farther away from the film plane and closer to the film
> plane than what might be called 'standard' construction lenses. In
> telephoto lenses this means also that the entrance pupil is larger than the
> exit pupil of the lens. snip
> Linear distortion is due to a variation in magnification over the field
> which renders straight lines in the object as curved lines in the image.
> 
> A symmetrical system (which pretty much describes the SA) has intrinsically
> little or no distortion. A completely symmetrical system has none. A
> retrofocus lens, for a number of reasons, generally has a distortion patter
> which shows slight barrel distortion at around 8 to 10mm off axis on a 35mm
> frame, and then to compensate shows some pincushion distortion at around
> 18mm off axis. snip
> Architectural photography is my job, and I depend on lenses with this sort
> of coverage. Believe me, a 21/3.4 SA has less distortion than a 21/2.8
> ASPH. At the moment I still have all 3 21's, and I have determined their
> capacities for myself over the last year. Depending on your priorities,
> either the 21/3.4 or the 21 ASPH is best, but all three are extremely good,
> and much better than nearly all SLR lenses.
> 
>    *            Henning J. Wulff
>   /|\      Wulff Photography & Design
>  /###\   mailto:henningw@archiphoto.com
>  |[ ]|     http://www.archiphoto.com

I have printed your considered reponse and reread it over a few times,
these are issues I have desired clarification on for quite a while. I am
appreciative to the max.
It is a coincidence that the reason I found out about the LUG in the
first place a few Months back was my calling Leica and talking to an
interesting tech rep about these specific issues of 21 verses 24 and
retrofocus, how they might compare to a Biogon ('blad Superwide) which
he instructed me on how to pronounce Biogon correctly in the German
vein. This is a coincidence because I did not start this thread here and
now in the first place.
In rereading Erwin's pages on the 21s he does not specifically address
the distortion issue with the ASPH as he does with the other 21's as a
word "distortion". He says all aspects of the lens are vastly better. He
says: "Astigmatism and curvature of field are almost fully corrected"
which are distortion issues I should think. But he doesn't specially say
better corrected than the SA. People should read this themselves to make
sure my comprehension agrees with theirs.
I had asked the tech rep at Leica if the new 21 or the SA were "true"
wide angle lenses like the Biogon and view camera lenses and thus
possessing their intrinsic lack of distortion which you describe here so
excellently. His answers were relational and I was confused and not
perhaps absorbing important points. Your present response clarifies some
of that now.
I had found it interesting that the 38 mm Biogon on the 'blad Superwide
has the same or close angle as the 21 mm on a 35mm. 
I'm not saying this to be a troll but when the time comes with the money
at hand I could easily fall for a used superwide. It's always been a
dream. People forget it's a viewfinder camera like the M, not an SlR and
very Leicalike if you ask me. Maybe this deserves the proper title of
the real original Texas Leica!
Thanks again Mr. Wulff
Mark Rabiner