When I went to calculate the adjusted time for 400TX at 90 seconds after I opened the message box, my chart scrolled off the screen. I just recalculated it, but I used 1.618 as the magic number whereas I had used 1.62 when I calculated the numbers for the chart. I doubt the 2 seconds or so will ever be missed out of 339. I hope I never have to do an indicated 90 second exposure. I get impatient with a 5 minute developing time.
where tc,1 is the correction at 1 second indicated time."
So, to find out what a films tc,1 is, you must conduct a test, OK, what is this test?
If you know the correction for ANY indicated time, you can calculate the film factor by reading the basic correction off the curve or from the chart and dividing that number into the amount you had to add to correct for reciprocal trade disagreement. Most film makers give you something, and the values I have seen will more likely give you some over exposure, so you should be able to zero in on a reasonable film factor after 1 or 2 trials.
yay! I can now populate a roughly accurate chart that has multiple films on it! then I can (hopefully) add in proper film factors after I (hopefully) do the appropriate testing.
I'll add more and more films to it over time, until I have the entire world included! mwaaahahahah...
I see where I made the mistake and I now do understand the chart
Thank you very much for your patience and time . I am starting to do a series of night exposures that I believe this chart will be invaluable for my project.
I've been at home with sick kids for the last few days while my wife is out of town. Working on this between other things has been much more interesting than cleaning house, and gave me something to ponder when I was cooking and cleaning. I've been aware of the Schwarzschild method for calculating reciprocity law failure for years, and was interested in how Gainer's method may relate to it, especially since Gainer's is based on readily available data that was carefully acquired. Every time I read a manufacturer's reciprocity data I get the feeling that I'm looking at a rough guess with Schwarzschild applied.
So I've been trying to find a way to convert Reeves' Schwarzschild exponents to Gainer film factors. I can find no direct mathematical conversion that's convincing. However, I did take the Schwarzschild equations and exponents from Reeves and the Gainer formula and factors posted here (I don't have the article at hand), and I took the four B&W films they share in common as a basis for finding out how closely the methods match. I found that I could apply the same corrections for all four films common to both tests and get calculated exposures from the Gainer formulas and from the Reeves test results (with conversions applied to yield Gainer film factors), that all agreed within about 1/6th of a stop. This agreement is across four films at exposure times of 1 sec, 128 seconds, and 900 seconds of metered exposure, and also across probable emulsion changes. I figured that was close enough to make the conversions useful as a starting point when applied to films Reeves tested that are not in the Bond data.
So I've attached the results in a three page .pdf file, with a Gainer basic correction chart (log axes with instructions for reading log axes), a film factor table with 13 B&W films (5 from Bond data and 8 from Reeves data), instructions, and a worked example. In case you want to know if this is immediately applicable to films you use, the films added from the Reeves data that aren't in Bond's data are: Pan F (not Plus), FP4+, Plus-X, Tech Pan (bye-bye), Ilford SFX, XP-2, T400CN, and TMZ.
Forgive me if I seem dense, but what is the correction preserving? Without even considering the developer, we could think in terms of density of development sites for a given light in a given time. Are we preserving zone V? How is this site density curve being changed? What I am shooting at is this; I have the niggling feeling that the curve is changed in a way that isn't fully correctable by changing development. If this is the case, then we can stretch tones around by deliberatly exposing so as to create RF, and hence RF can be used as a creative control with the right films. I'm seeking a correction function mapping an site density curve to a different curve in the face of RF. Do we already know this from Gainer's correction?
Forgive me if I seem dense, but what is the correction preserving? Without even considering the developer, we could think in terms of density of development sites for a given light in a given time. Are we preserving zone V? How is this site density curve being changed? What I am shooting at is this; I have the niggling feeling that the curve is changed in a way that isn't fully correctable by changing development. If this is the case, then we can stretch tones around by deliberatly exposing so as to create RF, and hence RF can be used as a creative control with the right films. I'm seeking a correction function mapping an site density curve to a different curve in the face of RF. Do we already know this from Gainer's correction?
This is always part of the problem with reciprocity failure. The Kodak recommendations for development adjustment earlier in this thread are a good place to start, but I don't have the backissue of the Bond article in Photo Techniques, so I can't say what he did. I can tell you that everything I've seen done in astrophotography appears to be aimed at preserving density on a gray card, or Zone V. I expect that Bond has tried to preserve the entire tonal scale to whatever degree is possible, but can't be sure until I see the article. Look at the Reeves websites in my last post to see the standard astrophotographers technique for determining the Schwarzschild exponent. As always, you'll probably have to test materials for your own workflow. Even then, Covington hints that the variation in film batches is enough to swamp your standard adjustment, especially at the typical 8 minute to hours long exposures in film astrophotography. Astrophotographers aren't typically concerned about preserving a specific dynamic range, they mostly want to go as faint as possible without getting the sky background washed out too much, and there are varying atmospheric conditions that determine that limit, often unpredictably. Astrophotographers bracket exposure times as much as possible.
I know this doesn't help with the moonlit landscape. I need to get the Bond and Gainer articles myself. You're not being dense, it's just that the testing for this is very tedious, and I've never seen anyone do an analysis as complete as the Bond article seems to be. I do recall having read some techniques used by cathedral photographers, including stand development, but that was years ago, and I don't have a ready reference. Read the Reeves web pages to get an idea of how to procede with your own testing using a Wratten #96 or a B+W 110 filter. (If you have and IR sensitive film, the Wratten #96 is not typically a 3.0 density in those wavelengths, so experienced people say to go with the B+W 110.)
There are a couple of books out there on "night photography" that might have useful information, but I haven't seen them.
Forgive me if I seem dense, but what is the correction preserving? Without even considering the developer, we could think in terms of density of development sites for a given light in a given time. Are we preserving zone V? How is this site density curve being changed? What I am shooting at is this; I have the niggling feeling that the curve is changed in a way that isn't fully correctable by changing development. If this is the case, then we can stretch tones around by deliberatly exposing so as to create RF, and hence RF can be used as a creative control with the right films. I'm seeking a correction function mapping an site density curve to a different curve in the face of RF. Do we already know this from Gainer's correction?
The failure of film to obey our elegant law of reciprocity has been known for a long time. The general effect is to change film speed with length of exposure. It is often thought of as having a threshold below which there is complete reciprocity. That is, we can exchange light for time. Less light needs more time in a reciprocal inverse manner. Actually, the failure is there from 0 time on up, but is negligible below a certain exposure time.
It is as if you put a slower film in your camera for longer exposures. The object of the corrections is first to preserve the shadow detail. Call it Zone I if you will. That is a matter of adjusting exposure time to make up for the loss of film speed. Once that is done, normal development may or may not preserve the desired contrast index. Ilford does not mention any change of development in their recommendations for reciprocity correction, but the change of exposure time they recommend for all their films is almost exactly what my table calculates for a film factor of 0.51 . It is fairly certain that by increasing the exposure time by the amount calculated from my table for those films Howard tested you will obtain a printable negative without change in development.
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Originally Posted by Bruce (Camclicker) 
