Quote Originally Posted by TheFlyingCamera View Post
An f-stop in this context is a quantity of light. It can be achieved by opening the lens further or it can be achieved by adding time.
Woah. That's true, but only when reciprocity holds and we're specifically talking about the regime where it does not. A stop is a factor of two and for long exposures, it matters where you apply that factor of two, because time and aperture are no longer equivalent.

If you give something a stop more aperture, you are increasing the luminous flux on the film, which will reduce the effects of reciprocity failure. If you give something a stop more time, the luminous flux is unchanged and you're still in the realm of reciprocity failure. Fixing RF through additional time requires a larger correction than fixing it through larger apertures because the latter actually helps to solve the problem.

Secondly, RF causes a contrast increase. Because RF is worse at lower fluxes, the shadows are affected more severely than the highlights. If you want to maintain not only an appropriate exposure but also an appropriate contrast, you will need to pull the film (overexpose+underdevelop). This means that there are now effectively three parts to correcting your image:
- additional exposure sufficient to bring your shadows up off the toe
- a reduction in development time in order to tame the contrast increase
- yet more exposure to deal with the loss of speed arising from the reduction in development

When you read a reciprocity table and it says "add X extra", it matters where you add it. 90% of the time they will recommend a (much?) longer time but sometimes, the corrections list a larger aperture. Using the aperture correction is NOT sufficient if applied to time, using a larger aperture is basically a cheat and may well not be possible (max aperture limits) or artistically desirable (DOF). If you could possibly get away with a larger aperture, you would have done that for yourself and started your reciprocity calculations from a different (less severely affected) point on the curve.

It's all too hard? Go shoot some Acros or Provia