[pure-silver] Re: hp5+ at 125 development times
- From: Jean-David Beyer <jeandavid8@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sat, 10 Oct 2009 18:11:13 -0400
Richard Knoppow wrote:
I am always curious where these "real" ratings come
from. In fact film speeds are determined by a well
controlled method detailed in an international standard. I
think, however, Ilford rates film at a lower contrast than
the one obtained by following the ISO standard so, if
measured by the exact ISO method the film speed should be
slightly higher than the box speed.
I do not know where other people get their "speed" ratings from, but I get
mine by testing. First I test my paper to see how much exposure it requires
to get maximum black. I then print a step wedge with 0.15 density steps on
that paper with the exposure required to get the maximum black through the
"clear" step of the wedge. Then I notice what step gets me a Zone V on the
paper. The paper and developer I use requires about 0.9 net density for
this. This is about one step higher than Ansel Adams worked for, but he used
papers with a lower maximum density. If I print a wedge with 0.75 net
density on my paper at Zone V, the exposure is too low to get maximum black.
So to get film to come out like this requires about a stop more exposure
than usual. So I give that. To get normal contrast, I develop as specified
by Kodak (in the case of the TMax films). That works fine with a cold light
enlarger.
Now doing all this makes TMY film come out with an exposure index for me at
about 200, and TMX film at 50 to 64. That is all there is to it for me. Now
it happens that my Zone VI spot meter is pretty well calibrated, and I test
the shutters every year or so with my Calumet shutter speed tester.
I have detailed the speed measurement method in detail
in this group several times, once quite recently, so will
not do it again here. _Effective_ film speed is dependant on
many factors such as the type of developer used and the
contrast index desired. Also, while its true that the ISO
speed is supposed to give the minimum exposure needed for
good shadow detail many photographers find that a somewhat
greater exposure give them better tone rendition. This may
well be due to exposure and processing conditions that do
not match those in the standard.
Overexposing, if you want to call it that, modern films is not the problem
it was in the old days when the D/H curve of the film shouldered off at
around Zone VIII or Zone IX as Panatomic-X did. And since I mainly shoot
4x5, the grain is not the problem it might be were I shooting 35mm.
The fact is that nearly all pictorial B&W films have
enormous overexposure latitude, as much as twelve stops for
some film. That means the film can be overexposed by that
much and still give good tone rendition. Certainly a couple
of stops is well within tolerance for good tone rendition.
--
.~. Jean-David Beyer Registered Linux User 85642.
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