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I was reading in a book about the dynamic range and several times I find the expression "responsive curve" or "image's responsive curve"?

What does the responsive curve represent? What is the responsive curve?

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A response curve is the relation between what the input and output of a sensor or even film.

As you may know, a sensor measures light by creating an electrical charge in response to photons hitting a photosite. One would expect in a perfect sensor that the amount of light falling on a photosite would be directly proportional to the charge it creates. If this were the case, the response-curve would not be a curve at all and instead it would be a line.

In practice, you have a curve. It is almost flat for most of the range but usually tapers off at the low end and, usually for film, at the high end too. This called the shoulder or roll-off.

Things get more complex as RAW data is transformed into pixels because that transformation is not at all linear either. JPEG files for example use a gamma curve which varies between cameras and converters.

Where it relates to dynamic-range is that the curve represents how various tones of the scene are mapped into an image. With a steep curve, dynamic-range would be narrower but the image would have higher contrast (and vice-versa).

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  • I read that values in a RAW are proportional to light intensity. Are you saying that's not true at the low end? Usually what values? 5% of the maximum light intensity? 1%? – Kartick Vaddadi Jul 29 '17 at 2:06

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