FotoDave@aol.com
Date: 10/28/01-11:23:54 PM Z
In a message dated 10/28/01 7:27:24 PM Eastern Standard Time,
sanking@clemson.edu writes:
<< If there is any specific reason why it would be better to use the
convention observed for silver papers, i.e, the speed point is 0.6 +
Dmin, instead of Dmax I would be very interested to understand the
logic. I have to say quite frankly that I don't understand the
rationale for the convention and perhaps am missing an important
consideration.
>>
Hi Sandy,
I think it is just a matter of convention. I do agree that it might be at
least natural to define speed with relative to Dmax. In fact, if I have an
emulsion for gum and I add a little pigment into it, I am interested in
knowing how much time I will need to get the Dmax since I do want to get Dmax
in my print. In a sense I am doing what you are also doing, that is, finding
the practical speed to get Dmax.
But by convention, some might be used to thinking the speed point as relative
to Dmin. The thinking is more or less like this: you define the speed point
by observing when you start to get some tone. On negative, you can get start
to get some tone but the curve is in the toe region, so changing the exposure
doesn't change the tone much until you get pass the toe, then you start to
gain "speed." I think there is some logical sense in that thinking too.
For paper, I don't remember exactly why the speed point is defined as 0.6 (or
0.7) + Dmin. It could be arbitrary, or if I remember correctly, it might be
related to variable-contrast paper because when you change the filtration, do
you change the contrast and the speed or just the contrast but not the speed.
I don't know whether the definition of speed point or the manufacturing
process comes first, but they decided that when you change the filtration,
the midtone would remain the same but both highlight and shadow would change.
The midtone is in the gray-card value, so it is about 0.6 or 0.7 + Dmin. I
seem to remember reading something like that, but I can't remember when or
where (probably in one of Kodak's paper's specification).
Other than that, perhaps one should avoid defining the speed point near the
toe or shoulder region because variation in exposure causes little change, so
it is hard to precisely define the point (like an exposure of 6 mins and 7
mins might cause small variation that it's hard to tell whether it is normal
variation or an actual change due to the exposure). But this is for
theoretical works. For our purpose, I think it is sufficient to define speed
point with first step lower than Dmax or first step above Dmin.
When I mentioned the usual way of defining speed point relative to Dmin, I
wasn't trying to say that was better. I think as long as you clearly define
your "pratical speed point" in your article, the reader should be able to
make use of the data. Maybe my main concern was whether some reader might
get confused if the definition is not what s/he is familiar with. Maybe the
other concern is there might be some ambiguity when we compare speeds over
internet. One might be using one convention and the other uses another. For
our practical uses and discussion in this list, I don't think many are
getting that technical in drawing all the curves and find the 0.6+Dmin point
and call it the speed, but I do think there might be some calling the speed
relative to Dmax while others might be to Dmin (and probably many are doing
visual comparison only), so this whole thing might be a good reminder for all
of us to clarify when necessary.
Dave S
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