What does the Exposure slider do in Adobe Camera Raw/Lightroom, and does +1.0 mean one stop?
Asked 12/7/2017
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In Adobe Camera Raw/Lightroom, the Exposure slider doesn’t seem to behave like a simple brightness control, because highlights often don’t clip as quickly as expected. Is the adjustment linear, and is there any documented tone-compression curve behind it? Also, do the slider values have an absolute meaning—for example, does +1.0 or -1.0 correspond to one stop of exposure change?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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I don't have a proper answer yet, but I have a partial answer.
It seems the parameters in the DNG map directly to values in ACR, sort of making DNG an internal serialization format for ACR. DNG is documented, but there's no explanation how to compute these specific transformations (which also means any other DNG program will not render the same as ACR without reverse engineering ACR; quality archival format!).
Adobe also provides an SDK (unfortunately C++) for working with DNG images. It appears this SDK is pretty complete and will do DNG "exposure" adjustment. There is no reason why ACR would use the same library, or even the same algorithm, but it's the best we got so far.
I have not yet read the code in depth to answer my question precisely (the code is a mess), but I will do so and update this answer.
Originally by user49699. Source · Licensed CC BY-SA 4.0
user49699
8y ago
0
Generated from our catalog & community — verify before relying on it.
In ACR/Lightroom, the Exposure control is generally understood to represent a linear exposure adjustment, where +1.0 is effectively one stop brighter and -1.0 is one stop darker. A practical check is to compare two otherwise identical shots taken one stop apart: applying +1.0 Exposure to the darker file should make it closely match the brighter one.
Why it doesn’t always look like simple linear scaling is that Exposure is only part of the rendering pipeline. After the raw data is scaled, Lightroom/ACR also applies tone mapping and other non-linear adjustments, so highlights may be compressed and midtones may appear to change more than extremes. That’s why you can often move the slider quite a bit without immediately blowing highlights.
So, in short: the slider values do have stop-like meaning, but the final visual result is influenced by additional non-linear processing in the raw converter. Adobe’s exact internal rendering behavior is not clearly documented in the provided answers.
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