From: David Foy (dfoy@marketactics.com)
Date: 06/03/03-05:10:54 AM Z
-----Original Message-----
From: Tor-Einar Jarnbjo [mailto:tor-einar@jarnbjo.de]
Sent: Tuesday, June 03, 2003 1:58 AM
To: alt-photo-process-l@skyway.usask.ca
Subject: RE: Age of Rodenstock lens
The camera's film plane and focus screen plane were probably
the same.
The lens dates from the time when many, if not most,
photographic emulsions were still only blue-sensitive, and
only the more advanced films were orthochromatic or (gasp!)
panchromatic. Blue light focuses at a different plane than
visible light. Photographers in those days were in the habit
of focusing on the ground glass, and then moving the back
forward by some amount to ensure focus on the film plane. In
this case, the lens-maker took the guesswork out of
refocusing.
Blue sensitive films were in widespread use, particularly in
technical and process photography, well into the 1950's.
David Foy
--------------------------------------------------
Frugal Photographer
http://www.frugalphotographer.com
> The illustrated lens appears to have a two position
> adjustment of some sort. Perhaps someone with better
German
> than mine (just a few words) could make out what the words
> near the arrows next to the movable screw say.
The positions are marked "Mattscheibe" (focusing screen) und
"Bildaufnahme"
(exposure). By setting the lens in one of the positions, you
move the entire
lens a few mm back and forth. My first guess was that this
is to correct for
the slightly different positions of the focusing screen and
the film plane
in the camera. On the other hand is my impression, that the
focusing screen
(at least in my camera) is sligtly further away from the
lens than the film
plane, but putting the lens in the focusing position moves
the lens closer
to the focusing screen.
Tor
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