From: Tor-Einar Jarnbjo (Tor-Einar@Jarnbjo.de)
Date: 06/03/03-05:59:05 AM Z
Hi David,
>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.
What you probably mean is that the focus plane of visible light depends
on the wave length (blue light is also visible :-).
>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.
I never thought of that, but you are probably right. I've made a
few test exposures and at least with the smaller apertures, I can't
really see any difference at all between the two positions, but then
with modern film material, I suppose I can leave the lens in one
position anyway.
>Blue sensitive films were in widespread use, particularly in
>technical and process photography, well into the 1950's.
Aren't they still? BTW, Ilford makes an Orthochromatic film Ortho+,
which I find very nice if I want to get this "old look" on portraits
(darker skin tones).
Tor
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