View Full Version : hp 5 reciprocity
scott palmer
26-Oct-2004, 14:44
Does anyone have a reciprocity chart for HP 5? Is it close to Tri-X? Thanks.
tim atherton
26-Oct-2004, 15:22
HP5+ ?
http://www.ilford.com/html/us_english/pdf/HP5_Plus.pdf (http://www.ilford.com/html/us_english/pdf/HP5_Plus.pdf)
Walter Glover
26-Oct-2004, 16:28
Scott,
Some time back David Carper (now sadly ex-Ilford) posted a formula which will approximate the curve that Ilford has determined by testing for reciprocity. The formula is:
Time (calculated) = Time (measured) ^ 1.48
where the caret symbol (^) signifies the exponential power
OR
Calculated Time = POWER (Measured Time, 1.48)
An example for an Excel calulation of a measured time of 10 seconds is:
=POWER(10,1.48)
= 30.2
I am also able to perform the same calculation on my palm Pilot using a spread sheet. I just tap in the measured time and another field shows the calculated time. Very handy but unfortunately does not address the issue of altered contrast.
Andrew O'Neill
26-Oct-2004, 23:18
Hi Scott,
I did my own testing for HP5+ and reciprocity about 6 years ago and have been relying on it since. I know it's better than the old Tri-X, but not sure about the newer version. Never tried it.
My data for HP5+ is:
1 sec = 1/3 stop more exposure
10 sec = 2/3 (1.5x)
100 sec = 2 stops (4x)
256 sec = 3 stops (8x)
No development compensation is needed.
You can take the data and make a curve which will give you inbetween times. Time on the bottom and stops on the side. Ilford's compensation curve gives way too much exposure compensation. At least that's what I observed.
scott palmer
27-Oct-2004, 01:09
Hey Guys,
Thanks so much for getting back to me so quickly! Great information. Now I can go back to work with confidence. It looks like the curve for HP 5 is different than the one I've been using for Tri-X, so I'm glad I asked.
David Karp
27-Oct-2004, 09:25
Scott,
Steve Simmons gives a chart with his findings for Ilford films' reciprocity characteristics. I have tried and find that it is right on target for my work. It is found in his book "Using the View Camera."
Mark Mitchell
13-Dec-2004, 07:59
Does anyone have a similar formula for other Ilford mono films?
David Karp
13-Dec-2004, 10:02
I believe that Simmons' book lumps all Ilford films together. I think that Ilford gives the same information for adjusting exposure due to reciprocity for all their films as well.
Mark Mitchell
13-Dec-2004, 15:15
Sorry, I was talking about the Carper formula (above) rather than the Simmonds book -- although does Simmonds say that all Ilford films have the same characteristics? If so then the formula would be the same.
David Karp
13-Dec-2004, 17:13
Mark,
If I recall correctly, David Carper's formula applied to all Ilford b&w films, not just HP5+.
Mark Mitchell
14-Dec-2004, 00:09
Hi Dave and all
Thanks for this -- most useful. I'll now spend a happy hour programming the Psion....
Sidney Cammeresi
14-Dec-2004, 07:27
Programming? I have a master's degree in Computer Science, but for computations in the field, I use a slide rule.
Mark Mitchell
14-Dec-2004, 09:07
I have a master's in Eng Lit so it's a *****ing miracle I know what an exponential function is.... Delighted to hear about you and your rule, though. Go you.
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