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chrhar
chrhar
Participant
January 11, 2017
Answered

Camera raw: Exposure impact on RGB numbers - formula?

  • January 11, 2017
  • 1 reply
  • 760 views

Is there a formula for calculating the impact that a exposure change has on rgb values? When working with "dark colors" a 0.05 raise typically means 1-1.5 points higher rgb value, with "brighter colors" the same raise of exposure means 2.5-3 points impact.

Edit: I missed to mention that I'm working in AdobeRGB...

    This topic has been closed for replies.
    Correct answer D Fosse

    I don't know of any formula.

    However, I'm pretty sure the underlying reason is that adjustments are applied in ACR's internal working color space - ProPhoto with linear tone response curve. For output this is converted into a standard color space such as Adobe RGB, with gamma 2.2 TRC.

    Going from gamma 1.0 to gamma 2.2 should - as far as I can tell - produce this effect.

    You can try to output to (1.8) ProPhoto just to see if the relationship is the same. If my "theory" holds, the effect should be slightly less pronounced.

    1 reply

    D Fosse
    Community Expert
    D FosseCommunity ExpertCorrect answer
    Community Expert
    January 11, 2017

    I don't know of any formula.

    However, I'm pretty sure the underlying reason is that adjustments are applied in ACR's internal working color space - ProPhoto with linear tone response curve. For output this is converted into a standard color space such as Adobe RGB, with gamma 2.2 TRC.

    Going from gamma 1.0 to gamma 2.2 should - as far as I can tell - produce this effect.

    You can try to output to (1.8) ProPhoto just to see if the relationship is the same. If my "theory" holds, the effect should be slightly less pronounced.

      D Fosse
      Community Expert
      D FosseCommunity Expert
      Community Expert
      January 11, 2017

      ...and of course, the "rubber band"-effect, where midtones are more affected than endpoints, as well as ACR's image adaptive process, which renders all bets off in terms of any number predictability.

      But all else being equal, the gamma encoding should do it.

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