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I typically do 5 or 6 minute long exposure with digital.

With film, technical specs usually go up to 2 minutes. Is the formula available for computing longer exposure?

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The information you require is available on the Ilford website: https://www.ilfordphoto.com/wp/wp-content/uploads/2017/06/Reciprocity-Failure-Compensation.pdf

It differs depending on the film in use, and as always, reciprocity and "correct" exposure are a little inexact, but there are factors listed there to help calculate exposure durations.

2

I recommend some experimentation to determine the exact reciprocity curve for your film and situation. There are many factors which influence a reciprocity curve so while bracketing a 10 minute exposure might seem laborious, you'll need to weigh the effort against the risk of losing a "perfect shot"

No one equation or chart exists because there are too many variables (and too little demand.) Zakia and Todd (1969) recognize the following effects on the reciprocity of film:

  • Exposure Time
  • Light Wavelength
  • Number of Exposures (intermittency effect)
  • Relative Intensity of Multiple Exposures (initial desensitization)
  • Herschel Effect
  • Solarization

Zakia and Todd fail to list these additional considerations for very long exposures

  • Decreases in base sensitization (E.G. it gets colder during exposure)
  • Increases in base sensitization (E.G. presence of ionizing radiation)

I'm not normally one to recommend sole source texts but Zakia and Todd's book "Photographic Sensitometry" is the largely undisputed bible of photographic science (NB in US academia photo science is the study of chemical photography whereas imaging science is the study of electronic photography) so if you're interested in precision with film, you ought to get a copy.

For a case study of reciprocity failure try LunarLight's primer. This resource doesn't need to be purchased from a used book store and includes a thorough briefing on how to make your own measurements as well as charts which can be used as "good guesses" for metered exposures up to 1 hour.

1

In short, you can expose film as long as you want - multiple hours, if you like - but colour may shift (on colour film) and you may not see a doubling of time equal one stop of additional exposure (reciprocity failure). As mentioned in the other posts, Ilford will have a table and some guidelines as to how its films suffer reciprocity failure, and you can do your own further experiments to see what happens.

For really long-exposure photographs such as star trails, with film, it's done in a single exposure rather than stacking multiple exposures as it's done with digital (which is necessary there due to sensor noise artifacts).

You'll need to tinker, but you can absolutely do it. Bracket your exposures, to be sure.

0

Here's a link to Illford's website where they provide documentation on how to properly calculate reciprocity failure - https://www.ilfordphoto.com/wp/wp-content/uploads/2017/06/Reciprocity-Failure-Compensation.pdf

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