Nash Computer Technology (nashcom@btinternet.com)
Wed, 20 Jan 1999 14:08:41 +0000
I asked a question a few weeks ago regarding working out film speed and
contrast. I’ve done a little bit more work now, and I’d appreciate a few
more pointers. I apologise in advance if this eMAIL is too long.
I bought some Tri-X 5x4” film (rated at ISO 320), and also a Rollo Pyro kit
from Bosyick & Sullivan.
I set up a grey card, evenly lit it with flash (strobe), focused my camera
on infinity, then positioned it so that the grey card filled the frame. I
metered for Zone 5, reduced by 4 stops (to give a Zone1 reading), and made a
series of Zone 1 exposures at film speeds of ISO 250, 200, 160, 125 and 100.
I processed these sheets, and one additional unexposed sheet to give a FB+F
reference, in my Jobo CPE processor at 70degrees F, 21 degrees C for 6
minutes, following the Rollo Pyro instructions for a pre-wet in metaborate
and a second metaborate bath after development and first rinse.
The unexposed sheet had a very slight green tinge, and the other sheets had
more of a green tinge that increased with a decrease in film speed, as I
expected.
I took these to a lab yesterday, and obtained a density reading for each
one. The lab also measured colour (I’m not sure what the readings
represent)
ISO Density Red Green Blue
FB+F 0.09 0.05 0.07 0.13
250 0.17 0.13 0.15 0.30
200 0.21 0.18 0.20 0.37
160 0.29 0.25 0.28 0.51
125 0.33 0.28 0.31 0.56
However, my college teacher adjusted my aperture and shutter for each test,
and it turns out that he didn’t know how to read a Copal shutter! so I can’
t rely on these results – for example I had the same density readings for
ISO 160 and ISO100. I’ve just re-shot the test today doing it all myself,
but I haven’t processed it yet.
This is where my questions start…
1) How does the Pyro stain affect density, and what density should indicate
my ideal film speed (for Platinum and Ziatype printing)? For example, the
ISO 125 test gives a density of 0.24 above FB+F (0.33 – 0.09). I assume it’
s the difference between FB+F and the different tests that will indicate the
correct film speed.
2) In “The New Platinum Print” book says that it takes years of experience
to be able to read a Pyro neg. I understand that this is because the green
stain filters blocks UV light. What I’m wondering is if the tests I am
doing to read density are a waste of time if the contact printing process
will be altered by the “invisible” effect of the green stain.
3) Assuming I’m on the right track, and that I find my ideal film speed, I
will need to do some work on establishing development time for correct
highlights. With Rollo Pyro, would I have to make any time changes to
either of the metaborate baths, or would I simply adjust the actual
development time?
4) AS Platinum is capable of printing more highlight detail than silver
processes, is there an easy way to establish ideal development time. I’ve
been reading that for silver processes I should use my newly-found film
speed and expose Zone 8 tests and measure the density of these at different
development times. Would this work for platinum, or should I make tests at
a higher Zone level?
Once again, sorry for the length of this question, and for asking these
basic B&W questions. Maybe I should stick to using Velvia…
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