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From: briand@tekig4.TEK.COM (Brian Diehm)
Newsgroups: rec.photo,sci.electronics
Subject: Re: Homebuilt transmission densitometer?
Message-ID: <2493@tekig4.TEK.COM>
Date: 13 Feb 88 23:14:32 GMT
References: <2659@dcatla.UUCP> <11237@shemp.UCLA.EDU> <24105@linus.UUCP>
Reply-To: briand@tekig4.UUCP (Brian Diehm)
Distribution: na
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 52

>>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.
>
>   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.

This does indeed work, and work well.  However, I've found a few warnings from
my experience.

1)  Use a broad light source - the light table suggested above will work fine,
    as will a large globe light PROVIDED THE NEGATIVE AND METER ARE BOTH VERY
    CLOSE TO THE LIGHT.  If you don't fill the frame of the spotmeter with
    the light source, you will get specular density rather than diffuse
    density.  Because silver particles actually block photons in bunches with
    the grain, the specular and diffuse densities of B&W materials vary widely;
    with color materials the silver is washed out and the remaining dyes
    scatter the light rather than block it.  This is also why B&W negatives
    will shield your eyes while viewing a partial eclipse but color slides
    won't.  And why diffuse enlarging light (cold light) makes a difference in
    B&W printing but matters less when printing color.  Anyway, diffuse density
    is what most agrees with densitometers, in my experience.

2)  The fluorscent source of many light tables isn't corrected in any way for
    color balance; if yours isn't then you may have some spectral response
    mismatch between your meter and your source.  Theoretically this doesn't
    matter if you are comparing densities.  However, you might want to be
    cautious.  I have used tungsten with good matching to real densitometers.

The relationship between 1/3 stop and 0.1 density is very close to absolute;
you needn't take Adams' word for it.  Basically, it is because the base 10 log
of 1/3 is very nearly 0.1 - to about 4 places.  If you're getting into this,
it is a useful exercise to work out this relationship for yourself; you will
get a good understanding of the sensitometry and mathematical relationships.

Finally, one-hour labs all have process densitometers for color QC.  I
have gotten gray-scale density readings from these labs simply by taking in
my 4x5 negatives during low-demand hours; invariably they are willing to give
me a few readings for a smile.

-- 
-Brian Diehm     (SDA - Standard Disclaimers Apply)
Tektronix, Inc.
briand@tekig4.TEK.COM   or  {decvax,cae780,uw-beaver}!tektronix!tekig4!briand