"Jean-David Beyer" wrote

But if you agree with Ansel Adams, and try to get a net density from Zone
I of your negatives to be 0.1

There is nothing sacred about that 0.1 OD number - that's just the
number AA found for his film, development technique, preferred
paper, enlarger and densitometer.

If one is trying to fit the film to the paper - AA's original
intent with the ZS - then there will a different number as one
isn't using the same paper, not to mention film, developers
and all the rest and one's criteria for an 'ideal' print will
be different.

It is equally possible the right number for one's own process
will be 0.05 or 0.20 - it is going to vary enormously with the
toe response of the film, the shoulder response of the paper
and how much shadow contrast - how far up the shoulder and
down the toe - one considers best.

As ZS speed testing invariably ends up with an EI that is roughly
1/2 of the ISO value it can be seen that AA liked more shadow
detail than the ISO standard assumes. With modern films, and their
capacity for overexposure, most photographers agree with AA and
rate their film at 1/2 the ISO speed - even if they have never
gone near the ZS.

At very low densities the readings from a bench densitometer
and an easel densitometer (AKA DA enlarging meter or one of the
old Eseco meters) will be identical.

You would need a calibrated step wedge to calibrate the enlarger "meter"

Er, I sell the meters to folks who make step tablets -
they use them for calibrating the tablets.

and it would drift around a lot.

Zilch. Nada. If you have the right meter...
You need a reasonably stable light source in the enlarger
- a standard incandescent source and a ferro voltage
regulator provides very steady light after everything
warms up.

My densitometer has a beam splitter in it that sends half the light direct
to a photodetector ...

I have been designing spectrophotometers for clinical
chemistry for 30 years now. They measure to 0.00003 OD
absolute with no drift.

... This compensates for changes in temperature

Put a wratten ND filter in the light path and you can
use the density readings for telling the temperature -
the filter's OD changes with temperature.

--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/da-main.htm
n o lindan at ix dot netcom dot com