What is the digital camera equivalent of reciprocity failure?

Reciprocity failure occurs when there's a significant discrepancy between the exposure time and the resulting image brightness, particularly in film photography. This phenomenon happens when film does not respond uniformly to light under long exposure times or very short exposure times, often leading to images that are either overly dark or washed out.

In the context of digital photography, dark noise is the most fitting equivalent of reciprocity failure. Dark noise refers to the unwanted noise that is present in images taken with a digital sensor due to the sensor's inherent characteristics, especially at low-light levels or during long exposure times. As the exposure time increases, the sensor can generate noise that can affect the overall quality of the image, manifesting itself similar to how reciprocity failure affects film. This noise is more pronounced under conditions where the signal (the light captured) is low, akin to certain exposure times leading to inadequate exposure under film.

Thus, dark noise accurately represents the issues that arise in digital imaging that parallel the problems of reciprocity failure in film.