Lightroom: mathematics of Exposure Control

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  • Updated 7 years ago
The exposure slider is said to control the exposure of the photo: e.g. a value of +1 should do the same like doubling the exposure time when taking the picture. While the result is visually somehow pleasant, the RGB values in the picture aren't manupulated like mentioned: e.g. an exposure control value +1 should double the XYZ values (XYZ values are those values that can be calculated from the RGB vals using the correct ICC-Profile). The result of "exposure control" is quite far away from doing that. I assume that this behavior is intendet.
But, is there a way to have something like a "real" (mathematically) exposure control? E.g. a linear scaling of the XYZ tone curves?
Thanks for any suggestion.


PS I'm using LR as a measurement tool
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Edgar Loser

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Posted 7 years ago

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jdv, Champion

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This would only apply to imported JPEGs with embedded profiles, right? Because raw images are always in the same space.
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Dorin Nicolaescu-Musteață, Champion

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Use PV 2010, perhaps...
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Rob Cole

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Yeah, PV2012 exposure is not a simple linear scaling (very much intentional) - not suited for use as a measurement tool. - works pretty darn good for qualitative editing of photographs though, in my opinion.
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I wouldn't use LR as a measurement tool. The non-standard behaviour of the the exposure control (non-optional highlight protection) is just one of many things were the designers felt they need to make editing less painful (achieving the opposite for those who know what they are doing, AFAIC).

There are hidden tone curves in camera profiles, etc.

You'll have to jump to a number of hoops to get LR to be neutral and then you'll still be facing non-optional auto-this and auto-that behaviour.

I didn't upgrade to LR4 and its patronising attitude towards users that know what they are doing was one of the reasons.

P.S.: If you switch to PV 2010, you practically get LR3 back, but I wonder if there is a way to get rid of the warning triangles that signal that you are not using PV 2012. I wouldn't want them to be in my face all the time.
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Edgar Loser

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Thanks for the hints about PV2003 and PV2010. Unfortunately the behave different, but still no linear XYZ scaling...
But I found a workaround using Photoshop:
1. (if not done before) change RGB Format to 16 Bit
2. convert to "own RGB Profile": for this profile I used a gamma of 1.0 (i.e. RGB is linear to XYZ)
3. do the scaling by using Tone Curves
4. convert back to original RGB profile

@TK: I know about "hidden tone curves" of the LR camera profiles. But you can get rid of then using the Adobe DNG Profile editor (free download). Howto can be found here (in german but the pictures may be sufficient):