Are equivalent aperture and shutter-speed settings always truly equivalent?
Asked 8/22/2017
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If two exposure settings deliver the same total amount of light to the film or sensor—for example, stopping down one stop and lengthening the shutter by one stop—will the resulting image always be the same? I'm wondering whether a short, bright exposure differs from a longer, dimmer one in practice, especially for film versus digital. Are there situations where reciprocity failure, noise, blooming, or other effects mean that equivalent exposure settings are not actually equivalent?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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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.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
8y ago
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Generated from our catalog & community — verify before relying on it.
Not always.
In the normal exposure range, equivalent aperture/shutter combinations are treated as equivalent because they deliver the same total exposure. But there are important exceptions.
For film and photographic paper, very long or very short exposures can cause reciprocity failure. In those cases, the material no longer responds linearly to light, so the stated ISO effectively drops and you need extra exposure beyond the simple stop-for-stop calculation.
Digital sensors generally do not suffer reciprocity failure in the same way. For normal use, equal total exposure is effectively equal exposure. However, very long digital exposures can introduce other problems, especially increased noise and hot pixels. That’s why long-exposure and astronomy shooters often use cooling or noise-reduction techniques.
So: within normal ranges, yes, an exposure is basically an exposure. Outside those ranges, film and digital can behave differently, and “equivalent” settings may no longer produce identical results.
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