reciprocity
in photography, the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material
en-GB/ˌɹɛsɪˈpɹɒsɪtɪ/
en-US/ˌɹɛsəˈpɹɑsɪti/
in photography, the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material