Portra 160 and 400 Reciprocity Failure

[neil poulsen]

I have a question: In using this chart, are you adjusting the aperture by the given number of stops?

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!

[rcmartins]

Looking for some reciprocity figures for Fuji 160NS I stumbled upon this fairly recent thread for Portra 160 and Portra 400. Apparently I missed this thread when it appeared. The original graphic is very useful, specially considering the lack of information that there is regarding negative color film. I would thus like to thank the OP for doing it.
The OP decided to make the graph expressed in EV increments. These increments can be carried out in time compensation or aperture. I would be careful though because the reciprocity failure has to do with the amount of light that falls on the film and thus should imply manipulation of time for a give chosen aperture. I don't think one should change the aperture because it changes the amount of light that hits the film and thus requires a different time altogether.
To compensate for time one just have to do Ts = Tm*2^(Compensation EV). Given the expression the OP showed this means that:

Ts = Tm*2^(0.5167 * ln(Tm) - 0.2)

with Ts the new corrected shutter open time and Tm the measured time for the scene.
Hope this helps clarify it's use.
raul

PS: I have not enough experience, but is it a matter of fact that the two Portras fail in the same way?

[jbenedict]

I have a question: In using this chart, are you adjusting the aperture by the given number of stops?

For the purposes of exposure, "a stop is a stop" whether adjusted via aperture or shutter.

[polyglot]

For the purposes of exposure, "a stop is a stop" whether adjusted via aperture or shutter.

Not when reciprocity is failing! That's the whole thing, it's reciprocity failure, i.e. the film fails to follow the whole a-stop-is-a-stop reciprocity rule wherein film-plane illuminance (~= scene brightness * effective aperture area) and time can be traded off as equivalent.

Reciprocity corrections are time corrections. Correcting the aperture increases the illumination on the film and therefore reduces the amount of failure.

[BM PHOTOS]

Ts = Tm*2^(0.5167 * ln(Tm) - 0.2)
Ts the new corrected shutter open time and Tm the measured time for the scene.
Hope this helps clarify it's use.

Thanks a lot Raul! This helps tremendously

Here are a graph and a Table I made using your formula and rounding the numbers.
Hope it's useful for people in the field.

Best,

Nico

Reciprocity_Kodak_Portra.pdf

Reciprocity_Kodak_Portra_Table.pdf

[StoneNYC]

There's a member here who did a lot of testing with seemingly accurate results and those numbers were added to the app "Reciprocity Timer" for iPhone I highly recommend it, works great every time.

[Christopher Barrett]

There's a member here who did a lot of testing with seemingly accurate results and those numbers were added to the app "Reciprocity Timer" for iPhone I highly recommend it, works great every time.

Yeah, the app has worked awesomely for my lakeshore images shot on Portra 160, usually 3-5 minutes. It also handles bellows factors really nicely.

[Serge S]

Hi Stone,

I can't find that app...does it go by another name?

There's a member here who did a lot of testing with seemingly accurate results and those numbers were added to the app "Reciprocity Timer" for iPhone I highly recommend it, works great every time.

[StoneNYC]

Hi Stone,

I can't find that app...does it go by another name?

Link...

Reciprocity Timer by Pump Interactive
https://appsto.re/us/-tLzB.i

App looks like this...

[Serge S]

Link...

Reciprocity Timer by Pump Interactive
https://appsto.re/us/-tLzB.i

App looks like this...

Great thanks a bunch. Got it. Not sure why the search function was not pulling it up.