[rollei_list] Re: Back-Focus and Retro-Focus
- From: Marc James Small <marcsmall@xxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx,rollei_list@xxxxxxxxxxxxx
- Date: Mon, 07 Jul 2008 18:00:08 -0400
At 04:52 PM 7/7/2008, Eric Goldstein wrote:
>Marc -
>
>It depends upon the design of the SLR. If you will recall the history
>of the early 35 mm SLRs, they featured longer "normal" lenses
>(55-58mm) so that there was sufficient clearance for the mirror. Then
>came the retrofocus versions.
>
>As Richard Knoppow pointed out, a very simple lens can be designed
>with enough back focus to clear an SLR mirror, but lenses with limited
>corrections are not what we are discussing here...
Eric
YOU are the one who has consistently raised
retrofocus lenses as being necessary for normal
lenses. You and Carlos have been off on a fast
run through broken country. Perhaps the two of
you mis-spoke but it seemed to this innocent soul
that the both of you were stating that normal
(80mm) lenses for medium-format cameras had to be
of a retrofocus design. Yes, you said
this. Yes, Carlos let this slide. Such is just
not the case. Might I suggest that you review the bidding?
I would strongly urge you to reread my earlier
posting. KW and Steenborgen (Exakta) produced
the first miniature-format SLR's in the
1930's. The problem was in reducing mirror size
while maintaining image brightness in the
viewfinder. This took some engineering
tip-toeing. In 1938, KW, after its forced sale
to the German-American Nazi, Noble, contracted
with Zeiss to develop a better lens mount than
the original M40 TM used on the original
Praktiflex miniature-format camera. One stated
goal was to allow the use of a smaller
mirror. In 1939, CZJ developed the original
version of the 2/5.8cm Biotar lens in M42 as
being the best compromise under the
circumstance. CZJ suggested a couple of methods
to allow shortening the mirror to allow the use
of a 5cm lens as was then the norm on Leitz and
Zeiss Ikon rangefinders. The War
intervened. Postwar work allowed the large
mirror and dim image of the Contax S to morph
into the bright image and smaller mirror of the
later Praktica and Praktina cameras with their 50mm normal lenses.
Richard Knoppow has been hopping into these
discussions and you've just not been reading his
postings. Retrofocus has nothing to do with the
design of any of these normal lenses as the only
limitation on SLR normal lenses is the size of
the mirror and, by 1952, they knew how to do this
to allow 50mm lenses to be the normal lenses for
miniature-format cameras and 80mm lenses to be
the normal ones for medium-format lenses.
Marc
msmall@xxxxxxxxxxxx
Cha robh bàs fir gun ghràs fir!
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