From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 06/03/03-09:18:55 AM Z
----- Original Message -----
From: "David Foy" <dfoy@marketactics.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Tuesday, June 03, 2003 4:10 AM
Subject: RE: Age of Rodenstock lens
>
>
> -----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
>
I think you are probably right. The Periskop type lens is
not chromatically corrected so there is probably a big
difference in focus between "visual" and "actinic" focus. It
would appear that this lens dates from the wet plate era.
While some knowledge of color sensitizing by means of dyes
did exist then it was a laboratory item only. Wet plate
coatings are sensitive to visible blue and near ultra-violet
light only.
--- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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