Exposure control in a raw developer: misnomer?

Started Jan 30, 2013 | Discussions thread
Jack Hogan
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Linear vs Non-linear Brightness
In reply to dosdan, Jan 30, 2013

dosdan wrote:

As I understand it, the "Exposure Triangle" is composed of Scene Luminance, Shutter Speed, Aperture (or should that be "F-number"?). These 3 alone determine the number of photons captured by the sensor.

Correct, but if we want to be precise we need to know what the light source is and speak of the average luminance at the sensor (L), exposure time (t) and f-number (N). For instance, with the spectral power distribution of a 5000K blackbody illuminant, the number of photons incident on the sensor is approximately:

13000 * Lt / N^2 photons per micrometer squared

ISO Sensitivity plays a role in the rendered brightness of the outputted image. (As well as helping to mitigate the contribution to the total read noise of the ADC noise floor in most cameras.)

In addition to OOC rendered brightness, depending on what definition of ISO your camera manufacturer chose, the ISO indicator of Sensitivity also interacts with your camera's metering and controls an intermediate step/choice, that of scaling (analogically and/or digitally) the photon count and storing it in the raw data file.

So, I take the phrase "exposing a shot" to refer to the capturing process, not the rendering process. With in-camera JPEG, these two processes are combined. When shooting raw, the development process is separated and postponed.

Correct, I would complement your statement by adding the scaling/storing component:

1) Expose (objectively determine the number of incoming photons)
2) Scale and Store (objectively scale the incoming photon count and store it in the raw data)
3) Render (present the raw data in a subjectively pleasing manner on the output medium of choice)

But most raw development programs have an "Exposure" adjustment control. But isn't this adjusting the brightness of the rendered output? If so, it has nothing to do with exposure and is a misnomer.

So what should this control be labelled? "Rendered Brightness"?

How about linear brightness (as opposed to non-linear, 'regular' brightness)? I think most people would be confused, and that's why Adobe combined the two in a single slider in LR4/ACR7 - misnaming it 'Exposure'. EC is really not that bad, as long as one uses the Saturation definition of ISO: within limits (and discounting noise and other effects), such a slider has in fact the same effect as dialing in an equivalent amount of in-camera Exposure Correction on the mean photon count as stored in the raw file.

Jack

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