Exposing To The Right

[ejmartin]

Again, for this example all other tonal controls were zeroed in ACR, which is supposed to provide a "linear rendition". Point curve linear as well. ProPhoto RGB selected for output, which to my knowledge is what the RGB readings refer to. Further, the baseline matrix profile was used to avoid any related hue twists. Also, Rawnalyze does not seem to provide any indication for Raw clipping in this mid part of the sky.

Zero'd tone controls and 'linear' tone curve don't produce a linear output in ACR; gamma is still applied according to the chosen color space.  So lower tones will be amplified more than higher tones, simply because that's the way a gamma curve works.  And there will be hue/saturation shifts as a result.  Only the sort of test Sandy is doing, where one tries to achieve the same output density with shots taken at different ISO, should be independent of the applied gamma, which takes place after the exposure slider is applied in the processing pipeline.

[bjanes]

Zero'd tone controls and 'linear' tone curve don't produce a linear output in ACR; gamma is still applied according to the chosen color space.  So lower tones will be amplified more than higher tones, simply because that's the way a gamma curve works.  And there will be hue/saturation shifts as a result.  Only the sort of test Sandy is doing, where one tries to achieve the same output density with shots taken at different ISO, should be independent of the applied gamma, which takes place after the exposure slider is applied in the processing pipeline.

Emil is correct about the gamma question. If you want a linear tone curve (scene referred), you can convert from ProPhotoRGB to ISO22028-3_RIMM-RGB-exCR color space as described in this ICC paper.

Regards,

Bill

[Peter_DL]

Zero'd tone controls and 'linear' tone curve don't produce a linear output in ACR; gamma is still applied according to the chosen color space.  So lower tones will be amplified more than higher tones, simply because that's the way a gamma curve works.  And there will be hue/saturation shifts as a result.

Sorry, that’s a common misunderstanding about gamma.

With a regular gamma, gamma-encoding and linear scaling (multiplication) are commutative operations. Means that the sequence can be exchanged – without changing the result – provided that the absolute value of the factor f can be adapted:

(f x rgb)^(1/gamma) = f^(1/gamma) x rgb^(1/gamma)

= f* x rgb^(1/gamma)

Any linear scaling applied on an RGB triplet which is done in one gamma sphere, can be undone by linear scaling in another gamma sphere. Color Integrity in terms of R:G:B is maintained.


But I’ll be happy to transfer the numerical comparison with the given example and measuring point to any 1.0 gamma space, if needed.

Peter

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[sandymc]

Zero'd tone controls and 'linear' tone curve don't produce a linear output in ACR; gamma is still applied according to the chosen color space.  So lower tones will be amplified more than higher tones, simply because that's the way a gamma curve works.  And there will be hue/saturation shifts as a result.  Only the sort of test Sandy is doing, where one tries to achieve the same output density with shots taken at different ISO, should be independent of the applied gamma, which takes place after the exposure slider is applied in the processing pipeline.

Doubt that this is gamma related - the exposure controls really should be gamma independent, and so far as I am aware LR/ACR operate in a linear light (gamma 1) internal space anyway. But Eric may be able to throw more light on exactly what the mechanism is. Eric's "per channel blacks" response makes sense as regards blacks (although it makes me wonder why the LR default is 5, if that's the way LR/ACR operate - seems like "color offset by default", unless there's compensation somewhere else in the chain), but there seems to be something else also going on in the blue channel.

Sandy

[Guillermo Luijk]

I gree with Peter. In fact as Eric Chan explained, ACR doesn't use a standard output gamma in the shadows, but the effect of an exposure change seems to be independent from it.

[ejmartin]

Preservation of hue/saturation only holds when gamma is a pure power law, not two different power laws, one for shadows and another for everything else.  Though it seems that Peter's example doesn't range down into the linear part of Adobe's gamma function, so it's still a puzzle.

[madmanchan]

Sandy, that does sound unusual. You mentioned earlier that when Blacks, Brightness, and Contrast were all 0 (i.e., no-ops) that the hue shift wasn't there. Now it seems that with Blacks 0, but the other two at defaults, there is a hue shift (due to a blue difference).  I don't have an explanation for that.  But I'd be happy to study your test images (you have my email).

Eric

[sandymc]

Sandy, that does sound unusual. You mentioned earlier that when Blacks, Brightness, and Contrast were all 0 (i.e., no-ops) that the hue shift wasn't there. Now it seems that with Blacks 0, but the other two at defaults, there is a hue shift (due to a blue difference).  I don't have an explanation for that.  But I'd be happy to study your test images (you have my email).

Eric

Eric, yes, it's odd - each control in isolation seems ok if all others are at zero. But more than one away from zero, and this slight hue shift appears. Anyway, files sent.

Regards,

Sandy

[Peter_DL]

Perhaps ACR's highlight strategy extended beyond the strictly RAW-clipped area. In this thread you can see an example.

Yes, interesting,
I'll go along with this explanation.

Many thanks.
& Best regards, Peter

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[sandymc]

Ok, so Eric and I have spent some time on trying to chase down the elusive blue shift. (Thanks Eric!)

After a few blind alleys, I ended up reimporting the images in LR, and the residual blue shift vanished. Reason unclear

But, the bottom line is that ACR/LR's basic exposure controls (Exposure, brightness, contrast) don't introduce color shifts IF:

• The camera profile you use doesn't have 3-D HueSatMap tables - according to Eric, no current Adobe profiles, either prepackaged or as produced by the Profile editor, have such tables. Your mileage may of course vary for profiles from other sources
• The "Blacks" slider must be at zero. Note this isn't the default, so you need to change that, or set up a preset

So, if you really have to use ETTR, on ACR or Lightroom: (a) "Blacks" slider to zero, then (b) dial out the ETTR with the "Exposure" slider

Sandy

[stamper]

In another post I asked why ACR and Lightroom aren't set for zero settings as base and the answer that Eric provided was that most owners who don't know much about the programs wouldn't like that. I am paraphrasing him. For advanced users it is important to know they aren't zerod.

[madmanchan]

Put another way, we've found that users prefer that their raw files are tone- and color-rendered by default (i.e., have a film-like S-curve with shoulder and toe applied), rather than a straight scene-referred rendering which usually looks flat.

[stamper]

Put another way, we've found that users prefer that their raw files are tone- and color-rendered by default (i.e., have a film-like S-curve with shoulder and toe applied), rather than a straight scene-referred rendering which usually looks flat.

Fine .... as long as they know about it.