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I would like to take long exposures with the Agfa vista 400, but I do not know how to calculate for the additional exposure time needed to compensate for reciprocity failure. I have also looked up the data sheet for the film, but instructions for compensation for exposures longer than 2 seconds only include recommended aperture adjustments. No guidance is provided regarding adjusting exposure times instead.

There is also this technical guide for many of Agfa's films, but the charts were incomprehensible to me.

A table of adjusted exposure times or any help would be much appreciated.

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I searched for the data sheet. It is linked from the official site here:
http://www1.lupus-imaging-media.com/images/downloads/vista_plus_400.pdf

Section 6 (Long Exposure Compensation) gives straightforward reciprocity failure details:

  • No exposure compensation required for exposures up to 2 seconds long
  • For a 4-second exposure, you need to open the aperture ⅓ of a stop (or equivalent)
  • For a 16-second exposure, you need to open the aperture ⅔ of a stop (or equivalent)
  • For a 64-second exposure, you need to open the aperture 1 full stop (or equivalent)
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  • \$\begingroup\$ What if the aperture is already wide open? What are the equivalent adjustments needed in terms of exposure time? The question specifically asks about compensating via exposure time, not via aperture. \$\endgroup\$
    – Michael C
    Dec 31, 2017 at 0:36
  • \$\begingroup\$ @MichaelClark: Then you'd translate the aperture change into an exposure time change. For example, the 16 second exposure says to open up by 2/3rds of a stop. If you're already wide open, you'd translate that into (around) 2/3rds of a stop longer exposure, which would be ~25 seconds. In the process, however, you're introducing still a bit more reciprocity failure, so it probably wouldn't hurt to give it 30 seconds instead (especially given that this is color negative film, which typically has quite a bit of latitude for overexposure). \$\endgroup\$
    – Jerry Coffin
    Dec 31, 2017 at 5:28
  • \$\begingroup\$ The translation from aperture to time is never linear. That's the entire source of reciprocity failure to begin with. That's why it is called "reciprocity" failure - because Av and Tv are not reciprocals at very long (or very short) exposure times. \$\endgroup\$
    – Michael C
    Dec 31, 2017 at 14:46
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Most film cameras have exposure times only in full stops. It is therefore easier to do the fine-tuning with aperture, which can usually be adjusted in thirds or half stops.
If you absolutely must work via exposure time you can ignore the failure up to a minute. For longer than a minute double your exposure time. When in doubt, bracket.
This may sound rough, but color negatives can take a lot...

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