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Reciprocity Law in Photography – Exposure Therapy

exposuretherapy.ca · 719 words · saved by 1 readers

At the begin­ning of this chap­ter, you were intro­duced to the for­mu­la Expo­sure = Inten­si­ty × Time. This equa­tion express­es a rec­i­p­ro­cal rela­tion­ship between the inten­si­ty of light pass­ing through the lens and the dura­tion that light is per­mit­ted to fall upon the image sen­sor. It demon­strates that you can achieve equiv­a­lent expo­sures by vary­ing both the aper­ture and shut­ter speed in inverse pro­por­tion to one anoth­er. Giv­en their rec­i­p­ro­cal rela­tion­ship, we can make the fol­low­ing state­ments: To deter­mine whether the cor­re­spond­ing changes are inverse­ly pro­por­tion­al, refer back to the con­cept of the pho­to­graph­ic stop as a unit of rel­a­tive change. When con­sid­er­ing the total expo­sure of the image sen­sor (and even effec­tive expo­sure, if dis­cussing ISO), a stop is a stop—it rep­re­sents either dou­bling or halv­ing the exposure—irrespective of whether you’re chang­ing the aper­ture or shut­ter speed. When con­sid­er­ing the total

This ani­ma­tion demon­strates the rec­i­p­ro­cal rela­tion­ship between the aper­ture, shut­ter speed, and ISO and its effect on effec­tive expo­sure (image bright­ness). The first two frames start at ƒ/5.6, ISO 400, and 1/500 s. Rais­ing the ISO to 800 (and then 1600) requires adjust­ing the shut­ter to 1/1000 s (and then 1/2000 s) to main­tain con­stant effec­tive expo­sure. In the frames where the ISO remains sta­t­ic, a con­stant expo­sure is main­tained despite adjust­ments because as the aper­ture f‑number is increased (reduc­ing light), the shut­ter speed is slowed (increas­ing light)

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