hello,

i expect that this will be a rather dumb question but on the kodak site i can't see any reciprocity tables for the portra films 160 & 400.

i am planing to shoot in very low light and expect longish exposures and would like to find some guidelines.

any of you kind people got any experience?

thanks very much

adrian

-- adrian tyler (tyler@nova.es), April 25, 2002

Answers

Adjustments for Long and Short Exposures No filter correction or exposure compensation is required for PORTRA 160NC/VC and 400NC/VC Films for exposures from 1/10,000 second to 10 seconds. For PORTRA 800 Film, no adjustments are required for exposures from from 1/10,000 second to 1 second. For critical applications with longer exposure times, make tests under your conditions.

This is from the Kodak site.Whether this is truth or not,I cant say.

-- Edsel Adams (mrchippy628@aol.com), April 25, 2002.

Sorry that this doesn't answer your question directly Adrian.
It seems to me that the same mechanism that's responsible for reciprocity failure (electron recombination in the halide crystals, or latent image regression) is also the same one that gives the 'toe' to a film's characteristic curve.
If this is correct, it should be possible to predict a particular film's low-intensity exposure response from the toe part of its characteristic curve.

-- Pete Andrews (p.l.andrews@bham.ac.uk), April 26, 2002.

Can't give you any technical details, but I can tell you that my personal experience is that Portra VC colors remain true for exposures up to two hours (I'm a pinhole guy). Times of course get longer; my seat-of-the-pants formula (courtesy of Mr. Zernike Au of Zero Image pinhole cameras): exposures calculated to be 1-4 seconds, multiply by 2; 5-30 seconds, multiply by 5; >30 seconds, multiply by 12. It's a good starting point.

Generally you will have to experiment (write everything down) and/or bracket. Given the flexibility of color neg film, I bracket by 2 or 2.5 stops. But usually the middle exposure is the best.

Good luck and have fun,

CXC

-- Christopher Condit (cxc@pacbell.net), April 26, 2002.

thanks for the advice, i'll give it a test run.

adrian

-- adrian tyler (tyler@nova.es), April 29, 2002.