# Thread: Portra 160 and 400 Reciprocity Failure

1. ## Re: Portra 160 and 400 Reciprocity Failure

Wow, I still don't understand why mine aren't working, I'm using the same thing.

ln(4) = 1.3862943611198906188344642429164 on my calculator
x 0.5167 = 0.71629829639064748275176767431488 - 0.2006 = 0.51569829639064748275176767431488

ln(4) = 1.3862943611198906188344642429164 on my calculator, still
- .2006 = 1.1856943611198906188344642429164 x 0.5167 = 0.61264827639064748275176767431488

You can also type ln(4) into Google search = 1.38629436112

I don't get a .3something, STILL confused...  Reply With Quote

2. ## Re: Portra 160 and 400 Reciprocity Failure

Did you try the numbers from the PDF I posted?  Reply With Quote

3. ## Re: Portra 160 and 400 Reciprocity Failure Originally Posted by buggz Wow, I still don't understand why mine aren't working, I'm using the same thing.

ln(4) = 1.3862943611198906188344642429164 on my calculator
x 0.5167 = 0.71629829639064748275176767431488 - 0.2006 = 0.51569829639064748275176767431488

ln(4) = 1.3862943611198906188344642429164 on my calculator, still
- .2006 = 1.1856943611198906188344642429164 x 0.5167 = 0.61264827639064748275176767431488

You can also type ln(4) into Google search = 1.38629436112

I don't get a .3something, STILL confused...
Ok, I see it. The 0.3something is the measured (from what I gather) data. The fitted function is an estimate. Higher degree of error in the shorter exposures.  Reply With Quote

4. ## Re: Portra 160 and 400 Reciprocity Failure

Beat me to it. The equation is "happier" at longer exposure times  Reply With Quote

5. ## Re: Portra 160 and 400 Reciprocity Failure

I believe that the values on the chart are the raw data. The calculation that you're using calculates what a model would predict. One would never expect that the calculated values would equal the raw data. But given the effectiveness of the model, we would hope that the calculated value would be good enough for use in the field. (So, pack a calculator with your gear.)

As to "the" model, we have a choice of three. The model I provided is a standard least-squares fit, which is typically what is used in practice. I've verified the results I originally obtained from a TI calculator in SYSTAT, which is a well known statistics package.

I'm not sure what method was used by Excel or used to get the results in the original post. But looking at how accurately each predicts the raw data (numbers provided on the chart), any of the three is usable. The least-squares fit (mine) appers to do better at a metered reading of around 4 seconds and will do acceptably well at higher values. The other two models appear to do better for higher metered readings.

Given that the differences between them are on the order of a tenth stop, they appear to all work acceptably well. Forced to select, I would probably use the least-squares fit, because of it's improvied performance at the low end of the data. Originally Posted by buggz Wow, I still don't understand why mine aren't working, I'm using the same thing.

ln(4) = 1.3862943611198906188344642429164 on my calculator
x 0.5167 = 0.71629829639064748275176767431488 - 0.2006 = 0.51569829639064748275176767431488

ln(4) = 1.3862943611198906188344642429164 on my calculator, still
- .2006 = 1.1856943611198906188344642429164 x 0.5167 = 0.61264827639064748275176767431488

You can also type ln(4) into Google search = 1.38629436112

I don't get a .3something, STILL confused...  Reply With Quote

6. ## Re: Portra 160 and 400 Reciprocity Failure

The more I read this thread the more I love ILFORD's simple charts... RR  Reply With Quote

7. ## Re: Portra 160 and 400 Reciprocity Failure

On the stops of increase...

I get whole stop increases. It is a doubling, right? So, 1 second + 1 stop = 2 seconds. A 2 stop increase would be 4 seconds, right? Another doubling?

Same applies for other starting times, like 6 seconds. 1 stop is 12, 2 stops is 24, 3 stops is 48 seconds. Is that right?

I know this is pretty basic, but it has always confused me.  Reply With Quote

8. ## Re: Portra 160 and 400 Reciprocity Failure

Assuming I did the math right, here is the time vs. adjusted time chart that RR is looking for. If it is not correct, let me know what is wrong and I will...correct it.

The data is based on the fit that I did, which has an R^2 of 0.9954106.

EDIT: I think I calculated my R^2 wrong. Still not sure I calculated it right, but I think it is slightly lower at 0.9885. Given the coarseness of the graph, I doubt the difference is visible.  Reply With Quote

9. ## Re: Portra 160 and 400 Reciprocity Failure Originally Posted by rbultman Assuming I did the math right, here is the time vs. adjusted time chart that RR is looking for. If it is not correct, let me know what is wrong and I will...correct it.

The data is based on the fit that I did, which has an R^2 of 0.9954106.
Yippee! That is exactly what I hoped for. Thank you very much indeed.

(Kodak should have done this. Film is too expensive to waste on testing they should be doing anyway)

Thank you again

RR  Reply With Quote

10. ## Re: Portra 160 and 400 Reciprocity Failure

Well, gee, I hope the data was somewhere near accurate in the first place. I originally sent Isaac Sachs some untested data I found on the internet, and he plugged it into a graph and posted it here. It seems to work well enough. I certainly hope you math whizzes out there haven't spent too many hours working this out.  Reply With Quote

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