Reciprocity Failure Correction - Gadget Gainer's Formula from '05 - EFKE PL100M?

Hi all,

In my quest for the proper reciprocity failure compensations for EFKE PL100M I've come up nearly empty handed. I did however come across this excellent thread from nearly 7 years ago that seems incredibly useful for any film.

The formula is Log(tc) = log(tc,1) + 1.62 log(tm), where tc,1 is the correction at 1 second indicated time.

Originally Posted by gainer

Due to the fact that the factor 1.62 works for these different films of different manufacturers, it is my opinion that it will work for any current emulsion to acceptable accuracy. That is to say that I expect it to be within the spread among readings of indicated exposure made by a number of proficient photographers of the same scene. If this is the case, all one needs to know is the reciprocity correction to one indicated exposure to find the correction for any other indicated exposure.

Given the emboldened statement, I'm wondering if anyone can come up with a reasonable number to apply to Efke PL100M?

Originally Posted by gainer

The factor 1.62 is accurate for all the films tested which were 400TX (0.169), TMY (0.061), TMX (0.069), HP5+ (0.101) and 100 Delta (0.046). The numbers in parentheses are the values of tc,1.

Or, if all that is required to come up with the right coefficient for a given film is based off 1 or 2 tests then I'd be more than glad to do it myself. The thing is, I need a bit of help understanding the formula and how to apply it; like where does that coefficient come from exactly?

Originally Posted by gainer

To convert that equation to one that can be solved quickly on a TI-30 pocket calculator...

tr = tc,1*(tm^1.62) + tm

tm = exposure time indicated by the meter
tc = amount to be added to indicated time
tc,1 = correction to be applied to a 1 second tm
tr = exposure time adjusted for reciprocity failure
__________________________________________

I've basically reposted the information here because it seems like a useful tool, and one that is perhaps forgotten(?)

I don't know, I've never understood why a given reciprocity formula should work for all films. I prefer specific testing. For example, I tried using Howard Bond's rather exhaustive TMAX and Delta tests and found them pretty much spot on for my purposes.

Last edited by Michael R 1974; 09-08-2011 at 08:35 PM. Click to view previous post history.

oops I went right to the equation without reading the old thread. Sorry about that.

But to answer your question about the coeficients, they come from experimental testing. In this case, the coeficients were based on the "brute force" tests Bond conducted - ie based on the experimental measurements/results, the coefficients are the additional amounts of exposure needed at 1 second (metered) to adjust for reciprocity departure. Howard Bond's testing was extremely labour intensive, exposing and developing hundreds of sheets of film to determine the reciprocity adjustments in small exposure increments out to 4 minutes (metered). But if you were going to do this for just a 1 second exposure to determine the coefficient, and then apply Gainer's formula to determine the adjustments for other metered exposure times, then yes presumably it would not take very much testing work for a given film.

Last edited by Michael R 1974; 09-08-2011 at 08:42 PM. Click to view previous post history.

This is great! Lee, it's nice to see you are still a whiz with this formula, just as you were 6 years ago! I had hesitated to follow the Bergger 200 times because in my search I saw somewhere a dissenting comment regarding those times. But as they say, 'good enough for jazz'.

Michael, indeed that seems to be the beauty of this formula; that one only needs the coefficient.

Now, I'm having a bit of trouble with the math here, to be honest... putting metered times into Lee's formula is getting me values that don't match the above Bergger times given by Ted. Are there some parantheses, or some order of operations I'm missing? And the time spit out should be the time, not the time to add, right?

I'd have to look at what you're doing more directly to comment on order of operations, but if you know the concept and apply it, you may be getting 'correct' numbers without realizing it. I'll attach a screenshot of a spreadsheet with metered times, Bergger data from the Azo forum, calculated exposure times with the Gainer formula and the coefficient I derived, and then the percentage difference between the Gainer and Bergger times. If you look at the adjustments given in the Bergger data for metered times below 30 seconds, you'll find that it's not a very smooth set of numbers, the 'ragged' stuff I referred to before.

However, even though there are differences between the proposed Bergger data and the derived Gainer formula, you can look at the last column and see that the Gainer formula differs by -0.5 stops at 4 seconds metered time, most of the time it agrees with the Bergger data to better than +/- 0.3 stops. I'd guess that the original work to generate the Bergger 'data' wasn't done with tolerances as tight as the supplied numbers might suggest.

Thanks Lee, that confirms that I was doing the math correctly, but indeed, I was thrown off by the discrepancies. I'll be using the data you generated and I'll report back if it works well or not.