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Aperture / shutter speed pairs that create the same exposure are theoretically equivalent because the same amount of light reaches the film/sensor. If you stop down the aperture while lengthening the time of exposure by one stop, then the total amount of light is the same in both cases. But will the two exposures really be the same?

I know in the old days photographers would often use dark rooms and small apertures and then just trickle in the light for a long exposure, and this would supposedly improve the tonality of the image.

Even though the amount of light is the same, there would seem to be a difference between a short blast of light versus a long trickle of light. For example, effects like bloom might be mitigated by a longer exposure.

So, is there a difference, or is an exposure just an exposure and that is all there is to it, no voodoo involved?

  • I have never heard of using small apertures combined with long exposure times to improve image tonality! – Alan Marcus Aug 23 '17 at 5:12
  • Reciprocity failure is a fact; it matters more in film than in digital (or rather the impact to film photography is different than to digital, as Alan neatly described). I once did a R project on the film reciprocity, you can have a look at the way the curve "breaks down" in a relatively predictable manner github.com/jlacko/Reciprocity :) – Jindra Lacko Aug 23 '17 at 13:56
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Digital image sensors do not suffer from reciprocity failure in the usual sense.

For chemical based photo films and papers, when the exposure time is lengthened or shortened so that it is outside the usual range, the ISO plummets. For this discussion I am referring to the “normal” range as between 1/1000 of a second and 1 second. This phenomenon forces the addition of either more time or more image brightness to maintain the effect of the exposure. This is a material specific situation.

Whereas, digital sensors are immune from reciprocity failure, they suffer increased noise and likely “hot” pixels when the exposure is prolonged. Astronomers, who are dependent on super long exposure, resort to super cooling their imaging sensors to mitigate this malady.

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