[pure-silver] Re: DS-10/HP5 curves and questions

----- Original Message ----- 
From: "Craig Schroeder" <craig@xxxxxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Sunday, June 12, 2005 6:54 PM
Subject: [pure-silver] Re: DS-10/HP5 curves and questions


>
> Richard Knoppow wrote: How are you making the test 
> exposures for your
> sensitometric curves?
> I've been taking individual frames of a Kodak gray card 
> and also have a
> Stouffer reflective zone scale that is just the right size 
> and mounted
> acrossone end of the card.  I measure the card on-center 
> in each frame on a
> Macbeth TD-902 (calibrated).  I also have a german Wallner 
> projection
> densitometer that is part of my Labex/Wallner modular 
> darkroom
> meter/timer/analyzer system.  These agree within .02 
> whenever I've checked
> and then I can also review the zone chart when it's 
> projected.  I've got a
> shutter tester and know that the shutter speeds I'm using 
> in combination
> withmy typical &#402;stop/shutter speed combos are 
> accurate.  I'm getting
> less confident about my little jimmy-rigged system 
> recently after this
> episode and a couple of others.  I think I was better off 
> without so much
> data!  Do you see potential issues in what I'm doing?
>
> I just bought a Bowens Illumitran slide copier rig and 
> have heard that their
> light source is very consistent and repeatable.  I thought 
> I'd try
> calibrating to a projection step wedge and see if I can 
> prove it to be
> consistent.  I could then just include one (or a few, if 
> needed) frames in a
> roll to verify or test combinations.  Kind of a poor-man's 
> sensitometer.  Do
> you think this might work?
>
   Its certainly worth trying. From what I read about lab 
sensitometry the quality of the sensitometer is critical. 
There seems to be reasonable amount of literature on 
sensitometry but I am not sure where to direct you. In any 
case I am not suggesting you must duplicate one of the 
elaborate sensitometers but only that it might be helpful to 
understand the problems that led to the standard forms. As 
an example it turns out that the results of exposures made 
on constant intensity- variable exposure time sensitometers 
is often different from constant exposure time/variable 
intensity exposures. Probably this is not significant if 
both exposure and intensities are within the reciprocity 
failure limits of the film.
   Another problem is determining film speed in order to 
establish a starting place on the curve. The ISO method, 
which is based on the DIN method uses what amounts to a 
fixed contrast index with a definite Dmin and Dmax. The 
measured speed is valid only for this contrast index.
   Many years ago Loyd A. Jones, of Kodak Research 
Laboratories, came up with a method of measuring film speed 
based on actual printing results. Essentially he determined 
that the minimum exposure that would result in an 
"excellent" print would put the minimum density for shadow 
detail on the toe at a point where the gamma was 1/3rd of he 
overall average contrast. He called this the minmimum 
gradient method. Greater exposure would also result in good 
tone rendition but, especially at the time, thin negatives 
were preferable to denser ones because they had finer grain 
and better resolution. A modification of this method was 
used by the ASA from 1943 to 1958 but was replaced by the 
modified DIN method because it was too difficult to make the 
measurement.
   The ISO/DIN method was adopted after finding that if the 
minimum exposure was placed a constant ratio above the point 
where the density was log 0.1 above gross fog and support 
density the tone rendition as equal to the minimum gradient 
method. Now, when determining effective speed it should be 
kept in mind that the minimum exposure for good shadow 
detail has to be greater than the so called speed point of 
the ISO method, i.e., logD 0.1.
   What I think may be happening in your measurements is 
that perhaps you are measureing the toe from too low a 
density.
   Kodak published curves are interesting. They appear to be 
actual curves from a sample of production film rather than a 
smoothed average. The curves are NOT smooth! I don't know 
the source of the irregularity. I might be able to get an 
explanation from my contact at Kodak but the existence of 
these from a reliable source suggests to me that it might 
not be unusual to find them in home made curves or an 
indication of error.
   The standard sensitometer used in the film industry for a 
couple of decades was the Eastman Type IIb, a rather 
elaborate machine with a barrel shutter. The appelation IIb 
is not a model number but rather a classification as a 
constant intensity- variable exposure type, with, I think, a 
diffuse light source.
   I don't know if any of this is helpful, perhaps not. 
Mostly its stream of conciousness.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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