# Thread: Portra 160 and 400 Reciprocity Failure

1. ## Portra 160 and 400 Reciprocity Failure

I was talking to Peter Gomena last month about his experiences with Portra sheet film in pinhole cameras and he said he'd found some good data on Portra's reciprocity failure but that he hadn't had much chance to test it himself. Surprisingly, the reciprocity failure for 160 and 400 seem to follow the exact same curve. I put the data into Excel and cleaned it up a bit (for example, 8 seconds metered exposure time certainly needs more correction than 4 seconds, contrary to the data I got from Peter) and found a best-fit curve. Myself and dukeku have been using this as a guide for nighttime exposures over the last month and so far it seems to be pretty much dead on.

The important part is this:

y = 0.5167 * ln(x) - 0.2
x = metered exposure time in seconds
y = stops of correction

Now, this doesn't mean you have to carry a calculator with you, since you could easily just carry a table with the data points on it. But if you're looking to do extremely long exposures - whether at night or with pinholes or both - that equation should work for arbitrarily long exposure times, even out past the end of the chart above.

It seems really hard to find solid data on Portra reciprocity failure out there on the internet, so I hope this helps many of you!

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

That's great, thank you!

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

That is definetely good info, thanks!

Did you find anything about the increase in contrast?

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

Great chart! I look forward to this data being added to the Reciprocity app. Until now, I have simply used the Ektar numbers in the app, cross references with a chart that I found a while back and that has yielded good results. Probably not absolutely perfect but since Portra is so forgiving, I have not had any issues. I'm not shooting extremely long exposures though....no more than 1 or 2 minutes.

I do have a question, as I am mathematically challenged. I understand the chart and how to calculate reciprocity based on the numbers you provide (eg. 4, .321928095), but in the formula what is ln?

I have tried plugging in various numbers for ln but haven't gotten anything correct. If you could briefly explain it I would greatly appreciate it, thanks!

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

Originally Posted by Eric Bryan

I do have a question, as I am mathematically challenged. I understand the chart and how to calculate reciprocity based on the numbers you provide (eg. 4, .321928095), but in the formula what is ln?

I have tried plugging in various numbers for ln but haven't gotten anything correct. If you could briefly explain it I would greatly appreciate it, thanks!
ln is not a variable. It stands for natural logarithm. So, y=0.5167 times the natural log of x - 0.2.

See the definition of natural logs for more... http://en.wikipedia.org/wiki/Natural_logarithm

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

ln stands for natural logarithm. I won't bore you with the details, but it is basically the logarithm of x to the base e (a mathematical constant).
If you are a windows user you can just calculate the number for ln(x) by putting in x into your calculator and then hitting the 'ln' button in expert(?) mode (see attachment). Probably easiest to do that up front and just store that number in the memory and then put in the whole sum to get y. Other calculators have probably similar buttons.

Sorry, I can't explain it any clearer, I took most of my basic algebra in Dutch.

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

Any ten buck math calculator will have a log function.

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

Thank for the time & effort to create the chart. This is very useful for the type of shooting I do.

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

Thank you everyone for your explanation! I understand the formula now

I have been using Portra more and more for my landscape work, so this proves to be VERY handy.

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

Okay, I could use this, thanks!
However, I do not get the same numbers as is shown in the chart.
Yes, I am mathematically challenged.

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, confused...

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