addendum to Re: cyanotype question

Just thought I'd better add that I realize the emulsion may have a toe
and shoulder effect as well where the rate of density change is
increasing in the toe and decreasing in the shoulder. That would only
strengthen my interpretation of the observed effect being greater/faster
in the lower steps. My previous example assumed a straight line
response similar to the midtones of a typical characteristic curve.

Joe

>>> jsmigiel@kvcc.edu 09/03/04 2:12 PM >>>
Dave,

Can you cite the problem with my interpretation? I thought I was saying
the scale would shift upward at both the lower and upper end, just
moreso and faster at the lower end.

If we look at the steps, their corresponding densities and transmissions
this is what we have (approximately and I've rounded things a bit to
simplify...step 1 is actually 0.03 density and <100% transmission,
etc.):

Step 1, 0.00 density, 100% transmission
Step 3, 0.30 density, 50% transmission
Step 5, 0.60 density, 25% transmission
Step 7, 0.90 density, 12.5% transmission
Step 9, 1.20 density, 6.25% transmission
Step 11, 1.50 density, 3.1% transmission
Step 13, 1.80 density, 1.6% transmission

My thought was that if the 250 unit exposure was "correct" to get some
tone, say, max print density at step 1, then another identical
arithmetic increase unit exposure (250 units) would be sufficient to
produce the same tonal value (i.e., max d) at step 3 effectively
shifting the scale upward two steps at that point. However, up around
step 13, that arithmetic unit increase has an insignificant effect on
the resultant tone so that there is no perceived upward sliding of the
tonal scale at that point. Its going up a bit at that end, but not
enough to discriminate visually.

In the example, to get step 5 to max d would require 250 + 250 + 500
=1000 units.
Step 7 would need 2000 units (=250+250+500+1000) to achieve the same
tone, step 9, 4000 units, and step 11 would require 8,000 units. To
match at step 13 would require 16,000 units. As a result, the low end
is shifting much more rapidly than the high end as exposure is increased
(regardless of whether you talk of arithmetic or geometric exposure
units).

If I'm missing something here, please explain it in a bit more detail or
point me to a good reference so I can understand it better.

Thanks,

Joe

>>> fotodave@dsoemarko.us 09/03/04 12:31 PM >>>
Hi Joe,

Your interpretation of densitometry or log scale is not correct (no
offense
intended). If you double the exposure, the scale should shift up on both
lower and upper end. In hand-coated materials, sometimes it is not
exactly
like that because of many factors including mechanical factor
(repeatibility
issue) and visual factor (the low and high ends are toe and shoulder
region,
so when you shift, the change/difference might not be that visible).

But looking at your original data, I think the problem is something
else.
Could it be the warming-up factor of the exposure unit/light? You could
probably try warming the light up for about 5 minutes and repeat your
test
and see if there is any difference.

Dave S

----- Original Message -----
From: "Joe Smigiel" <jsmigiel@kvcc.edu>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Friday, September 03, 2004 12:01 PM
Subject: Re: cyanotype question

> Hmmm. Actually the effect is beginning to make sense to me based on
> what you've just said. The 2 step gain is on the low end while the
> higher zones are not increasing as much. The lower densities'
> transmissions are closer to 100% so an arithmatic unit increase in
> exposure would have a relatively greater effect there than on the high
> zones. In other words, if max density is reached on step 1 which has
a
> density of approximately 0.00 an additional 250 units would really
flood
> the adjacent steps with exposure effect. At the same time, step 13
> transmits only about 1.5% so it would require a huge arithmetic
increase
> to make a geometric increase of 2 steps (1 stop density) at that
point.
> Part of the exposure effect is also probably masked by the printed out
> image in both the low steps and higher ones, but more pronounced at
the
> lower end. Add to this a possible intermittency effect as well as a
> dark or drying effect. I don't think I would be able to isolate the
> variables enough to actully solve this dilemma.
>
> It really makes me see why the density range of the negative becomes
so
> important and why the negative density range needs to be tailored to a
> specific process and set of materials. I guess the easiest conclusion
> is that an additional unit of exposure isn't just going to shift the
> scale along the stepwedge evenly (as I assumed earlier) and the degree
> to which this is evident actually depends on where the unit print
> exposure matches the overall negative density scale.
>
> I think I'll give this a rest now and fine tune a couple prints. I'll
> post the images somewhere along with the stepwedges in a few days if
> anyone is interested.
>
> Joe
>
> >>> Ender100@aol.com 09/03/04 10:13 AM >>>
> Joe,
>
> That's what is sort of baffling.... two steps is equal to a full stop,
> or
> doubling the exposure. So you should go up 2 steps when you double
the
> time
> from 250 units to 500 units....and that is indeed what happened.
> However, you
> reported that you got two more steps merging when you went from 500 to
> 750
> units..... that should have only happened if you doubled the time from
> 500 units to
> 1000 units.
>
> Mark Nelson
> www.precisiondigitalnegatives.com
> www.markinelsonphoto.com
>
>
>
> In a message dated 9/2/04 10:19:17 PM, jsmigiel@kvcc.edu writes:
>
>
> >
> > Yes. You have it right as far as the mergers are concerned but I
> don't
> > follow you on the question about 2 additional steps merging at 750
> units. No
> > steps merge at 250. Three steps merge at 500 which indicates the
low
> end was
> > pushed up the scale 2 steps as an additional 250 units was given.
The
> same
> > thing happened as the exposure was further increased by 250 units to
> 750, i.e.,
> > the low end was pushed up the scale another 2 steps. This is also
> showing in
> > how many steps solarized initially. The highest solarized steps
were
> 6, 8,
> > and 10 for the 250, 500, and 750 unit exposures, respectively. The
> low steps
> > are acting in a consistent manner gaining 2 steps with each
additional
> 250
> > unit exposure. It is the high end that is behaving strangely
showing
> a
> > decrease in the rate that the tones print as exposure is increased.
> >
>
>
>
>
>
>
>
>
>
Received on Sun Sep 5 08:28:47 2004