Reciprocity Formula - Tri-X 320

Flauvius · Sep 30, 2007

Hello:

I would appreciate someone posting the mathematical formula to determine reciprocity corrections for TRI-X 320.

Regards,

Robert

tommy5c · Sep 30, 2007

page three on the data sheet shows the exposure compensation data.

http://www.kodak.com/global/en/professional/support/techPubs/f4017/f4017.pdf

it's the best info that i've found on Tri-x Reciprocity.

Rich Ullsmith · Oct 3, 2007

These data sheets put those numbers out with an air of certainty, and they very well may be spot on, but. . .if your metering is not absolutely perfect, those numbers are less than meaningless.

Maris · Oct 3, 2007

Try this:

Take the measured time and multiply it by 7.5 then

raise this product to the power 1.42 then

divide the result by 10.

This should be very close to the correct exposure time.

If tonal matching over a range of reciprocity influenced exposures is required then development compensation is called for.

I don't compensate development because I like the reciprocity induced contrast increase (John Sexton's "Quiet Light" theory) for dimly lit landscapes.

Flauvius · Oct 4, 2007

What is the basis Maris' Formula

Maris:

How did you arrive at the "numbers" you propose for use with your Tri-X 320 reciprocity formula?

Flauvius

Pasto · Oct 4, 2007

Tri-x 320 reciprocity funtion

Based on the data available from the Kodak data sheet, I plotted the best function for the following data.

Calculated Corrected
1 2
3 10
5 20
10 50
35 250
60 550
85 950
100 1200

I used a statistical analysis program called SPSS to fit the best function, which turned out to be a quadratic as follows:

Corrected = -6.302 + 5.004(Calculated) + 0.072(Calculated^2)

The r-square for the above function is 1.0, indicating a perfect fit over the range of data used. Technically, plugging in calculated exposures of greater than 100 seconds is very risky. Within the range 1 to 100 seconds, this function should work very well if Kodak supplied the right data!

Let me know how this works for you. Good luck!

Louie

Lee L · Oct 4, 2007

The data in the current Kodak tech info for TXP, and the formula derived from it by Dr. Louie are both essentially perfect matches for a data sheet and generic Kodak B&W film reciprocity information dated in the mid 1970's. While TXP is an older style film, I believe it's been updated, as have all other Kodak films since that time, most of them with changed reciprocity characteristics requiring less compensation. My personal take, based on a bit of my own shooting and on reading the results of testing by others, is that Kodak hasn't really tested reciprocity carefully and published updated information for all their emulsions, and that TXP is likely to fall into that group of films.

I'd test for any critical applications. If it's not critical, the Kodak suggestions are OK for a reasonable estimate, but you might find yourself a stop or more off the mark.

Lee

JBrunner · Oct 4, 2007

Reciprocity is a funny bear, and personal taste in density and contrast isn't considered in the published charts. I use the published data as a way to judge the general failure rate of the film, then test. I don't think there is a single film I've shot to failure that has given me the results I sought at the published compensations. YMMV.

Pasto · Oct 4, 2007

As suggested by others (Lee and JBrunner), the formula is perfect if the data is perfect, which is unlikely! Best advice is to test the film yourself.

pgomena · Oct 4, 2007

Tested results for 1980s Tri-X 320, distributed to students at Brooks Institute of Photography:
1" 1.5"
2" 4.5"
4" 13"
8" 39"
16" 1'56"
32" 5'44"
60" 15'21"
2' 45'28"
4' 134'40"
10' 565'

These times have worked for me. Of course, the film has changed since then, so your results may vary.

Peter Gomena

Rich Ullsmith · Oct 4, 2007

Yeah, unless you have a boatload of Tri-x you will be working with at long exposures, Tmax 100 is a better film for this, as it requires no correction up to several minutes. In a sense it's faster; 16s indicated will need 16s, instead of an hour plus 56m. Yikes.

Lee L · Oct 4, 2007

Rich Ullsmith said:
Yeah, unless you have a boatload of Tri-x you will be working with at long exposures, Tmax 100 is a better film for this, as it requires no correction up to several minutes. In a sense it's faster; 16s indicated will need 16s, instead of an hour plus 56m. Yikes.

16" 1'56"

means 16 seconds to 1 minute 56 seconds, just under 2 minutes.

Howard Bond had a good set of complete and careful tests for several films in the July/Aug 2003 issue of Photo Techniques magazine, including TMX and Tri-X (not TXP 320). He found the manufacturers suggestions to be very different from his results. I'd go with the Brooks tests that Peter posted as a starting point for TXP.

Lee

Pasto · Oct 4, 2007

In case anyone is interested, I fit a function to Peter's data from the Brooks institute and it looks like this:

Corrected = -148.163 + 17.665(Calculated) + 0.065(Calculated^2)

I find these functions very useful as I have a program for my Palm that uses them to calculate my exposure in the field.

Louie

Maris · Oct 4, 2007

I reckon the general converging function for non-integral order chemical reactions such as film/reciprocity response is like:

Tc = Ka+Kb(Tm)^1+Kc(Tm)^2+Kd(Tm)^3+.....

where:
Tc is corrected time
Ka, Kb, Kc, Kd, ...are diminishing system characteristic coefficients
Tm is the measured time and
^n is the exponent.

The function goes on forever but successive terms get smaller and smaller and cease to be worth chasing.

Flauvius, I used to have a mathematical background so I eyeballed Kodak's numbers, conjectured a few coefficients, tested the outcome, and collapsed the long formula into a short one.

Dr.Louie's approach is far saner.

Flauvius · Oct 5, 2007

Recirprocity Influences & Principles

The proposed empirical formulas and manufacturers' reciprocity charts fail to address the influence of film developers, personal development techniques, the actinic qualities of the light in which film is exposed, and goals of extended exposure as functions of reciprocity. As such, I submit that we need "target" film densities to determine the reciprocity characteristics of any film we use and process.

In this regard, does anyone know what are principles and goals Kodak used to determine their reciprocity chart for Tri-X 320? While TMX whose minimal need for exposure correction is well suited for grainless applications, Tri-X 320's grain structure is well suited for many out-of-door scenes which required extended exposures. As such, I am attempting to develop a principle based exposure system for my use with Tri-X 320 in the bush.

Flauvius

Lee L · Oct 5, 2007

Flauvius said:
The proposed empirical formulas and manufacturers' reciprocity charts fail to address the influence of film developers, personal development techniques, the actinic qualities of the light in which film is exposed, and goals of extended exposure as functions of reciprocity. As such, I submit that we need "target" film densities to determine the reciprocity characteristics of any film we use and process.

In this regard, does anyone know what are principles and goals Kodak used to determine their reciprocity chart for Tri-X 320?

Flauvius

I've never seen Kodak methodology or goals published. One reason the Bond article I referenced is interesting is because he establishes both to a greater degree than most. He uses a Stouffer step wedge and blue photofloods with specific target densities. However, he doesn't mention developer or development times. His article was briefly available from the PhotoTechniques web site as a .pdf, but it's gone now.

I expect that the manufacturers don't see how they could reasonably replicate the variety of developers, shooting conditions, or techniques used by any particular photographer when working in the realm where reciprocity failure occurs, so they don't invest the time or effort to attempt it.

One method used by astrophotographers to establish a Schwarzschild exponent to describe a given film is outlined briefly here: http://www.robertreeves.com/filmtest.htm This method is one used by Michael Covington, Reeves, and others. It is covered more completely in their respective books on astrophotography, both still in print. Astrophotographers limit their exposure length, usually to 45 minutes or less, because of problems with scattered light in the atmosphere causing sky glow that obscures the background sky.

Lee

mawz · Oct 8, 2007

Rich Ullsmith said:
Yeah, unless you have a boatload of Tri-x you will be working with at long exposures, Tmax 100 is a better film for this, as it requires no correction up to several minutes. In a sense it's faster; 16s indicated will need 16s, instead of an hour plus 56m. Yikes.

Not according to Kodak, which recommends exposure adjustments for TMax 100 at 1s and longer exposures. It however has distinctly better reciprocity than Tri-X, at 100 seconds metered TMax 100 only needs 200 seconds of exposure.

Fuji Acros is the B&W Emulsion that doesn't require exposure adjustments for reciprocity until 120 seconds (or longer).

Reciprocity Formula - Tri-X 320

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