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Subject: Re: Exposure to the right and tone placement



I believe that Jeff is referring to the clipping display in Camera Raw, obtained by Option/Alt-dragging the Exposure and Brightness sliders.

There's no way to say this particularly gently. Rags' article is a lengthy chunk of solemn nonsense. If you can swallow statements like "the gamma of light, based on the inverse square law, is 2." without shooting coffee down your nose, you need to do a good bit of basic research.

If you can accept (or even interpret as meaningful) statements like "Tonal values are simply integer numbers and the values are linear. How they are encoded in the computer is irrelevant." you need to refine your understanding of how computers are used to edit images by changing numbers.

"Brightness" is the attribute of a visual sensation according to which an area appears to emit more or less light. As such, it's a psychophysical quantity that can only be recorded by interrogating subjects who are experiencing said visual sensation. We can correlate these reported sensations with things that we can measure physically—photon count, illuminance (which is what incident meters measure) or luminance (which is what reflective meters measure), but the relationship varies at different brightness levels. If we take photon count as the input and the reported sensation of brightness as the output, the relationship can be described reasonably well by a gamma curve somewhere between 1.8 and 2.4, depending on the absolute brightness.

The Zone scale deals with lightness, which is the perception of how dark or light a tone is relative to some absolute brightness. Again, lightness is logarithmic in relation to photon count. Rags calls this "Tonal value."

Digital cameras just count photons, and record a value exactly proportional to the number of photons that impinge on the sensor. That's what we mean when we say that digital capture is linear, or has a gamma of 1.0. Digital cameras and other photon counters know zilch about lightness, which is an attribute of human sensation.

Underexposure does not retain shadow detail, nor does it make shadow detail easier to recover. It increases the number of values recorded in the shadows only because most of these values are noise. The only thing in the universe that is black is the event horizon of a black hole. If you get close enough to photograph one of these, you'll have other issues to worry about, but even although no photons are present, the camera will record some values. Those values represent the internal noise of the system, and they overwhelm weak photon counts. If you want to capture shadow detail (which by definition is not black), you'll do a better job the further up the tone scale you place it. The question is, what highlight do you want to retain while doing so?

It's totally unclear what is meant in this context by "overexposure" "normal exposure" or "underexposure." The definitions are circular.

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