Exposure shift for incident light metering

There is a lot of confusion in the sentence above.

First, the camera exposure is the combination of aperture, shutter speed, and (to a lesser extent, because of ISO invariance) the ISO you shot the image with. You can use an exposure meter, but it does not make much sense for digital cameras, especially MILCs, as they have pixel-level “exposure meters” built in (visualized with histograms, zebra patterns, etc).

side note

Global exposure meters, either built in or as an accessory, made more sense for film, which is very tolerant of overexposure. For most film types, you still get to see a lot of details at significant overexposure, which even high DR digital cameras would clip hopelessly.

Second, the exposure you set in Darktable is just, more or less, a number that multiples the values recorded by the camera sensor. (Some argue that calling it exposure is a misnomer, but that is water under the bridge now, the term is stuck.) It is not unlike setting the ISO in your camera, except that it does not introduce additional noise or clipping.

That term is ill-defined for two reasons. First, unless your image has very little dynamic range, camera exposure is almost always a compromise between getting some detail in shadows without blowing the highlights. There is no “perfect” exposure — it depends on what you want to do with your image.

Similarly, exposure depends on processing intent: where you want your middle grays, which part of the image you want to emphasize, etc. To confuse matters, a lot of other modules can counteract exposure, ie a more or less identical image can be achieved with a wide range of exposure settings (eg high exposure, tame highlights in tone equalizer, or low exposure, lift shadows in the same module).

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