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From: pwc@mitre-bedford.ARPA (Patrick W. Connors)
Newsgroups: rec.photo,sci.electronics
Subject: Re: Homebuilt transmission densitometer?
Message-ID: <24105@linus.UUCP>
Date: 10 Feb 88 15:18:50 GMT
References: <2659@dcatla.UUCP> <11237@shemp.UCLA.EDU>
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In article <11237@shemp.UCLA.EDU> korfhage@CS.UCLA.EDU (Willard Korfhage) writes:
>Recently I was wondering about using my color meter as a transmission
>densitomiter. I have the low end Beseler model, and it uses a photomultiplier,
>seems quite sensitive and has a very small metering aperature.
>The general idea, it seems to me, is to compare the unknown density with
>some known references until you find a match, or until you get close enough
>that you can make a good estimate. The question is how do you get your
>reference? A calibrated Kodak step wedge runs about $81 at the local store,
>and I didn't care to spend this much money on a whim. An uncalibrated wedge is
>less (about $45, as I recall), but then you need to find a densitomiter you
>can use to calibrate it. Even cheaper would be to make a series of negatives
>of slightly varying exposure and calibrate those. You do have to check for
>uneven density in the negatives, but you really need to calibrate only a
>very small spot on the negative.
>
>If anybody sees anything wrong with this scheme, I would like to know.
>It would not be as accurate as a real densitomiter, but it would seem like
>an inexpensive way to figure out 0.1 above film base + fog, or whatever,
>with accuracy good enough for a hobbyist. And it would not require finding
>a place to put another machine.
>
>   Willard Korfhage

   In the section on film testing procedures in _The Negative_, Ansel
Adams states that a spot meter may be used as a densitometer.
According to Adams, every 1/3 stop mark on the meter scale is
equivalent to .1 density units.  To find .1 above fb+f, set your
negative on a light table and take a reading of an unexposed developed
and fixed negative, and that is your fb+f density.  All you need to
find now is the exposure which reads .1 more.  All readings are
relative to the fb+f frame so no other calibration is neccessary.
I have tried this with my Pentax Spotmeter V and it works fine.

   There is a book called _Beyond the Zone System_ (I forget the
author), that gives a couple of other systems for controlling contrast
with an incident meter.  This book is very technical, but there is a
companion workbook that has plans for turning a spotmeter into a
densitometer.


  Pat Connors