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Author Topic: Separating 'RAW' functions in a RAW converter  (Read 27599 times)
Panopeeper
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« Reply #60 on: May 05, 2008, 09:16:27 PM »
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Now wouldn't it be nice if we could have some documentation from Adobe explaining what is going on here?

Well, yes, but of course you can't expect a useful documentation of a product for only a few hundred bucks, can you?

Anyway, I guess the Bruce/Schewe book does contain specific information. I learned, that the best is to test such issues. I just did test this; you don't need to touch the recovery slider in order to activate the guessing.
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« Reply #61 on: May 05, 2008, 11:19:14 PM »
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What makes you think it isn't? Pretty neat feature when you see it working, isn't it?
[a href=\"index.php?act=findpost&pid=193701\"][{POST_SNAPBACK}][/a]

I don't know, because I don't know what it is that it is supposed to be doing.  I find I can achieve most things with the Exposure slider anyway and that way still retain somewhat normal contrast in the image (as opposed to the recovery slider which just seems to dull the highlights).
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NikosR
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« Reply #62 on: May 06, 2008, 03:30:34 AM »
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NikosR, your recent questions seem to be more about understanding how to use CR's controls, rather than needing to know the underlying details of how CR works.[a href=\"index.php?act=findpost&pid=193616\"][{POST_SNAPBACK}][/a]
[/quote]

Please pay attention to the fact that I stressed these are posed as rhetorical questions and food for thought. Having a clearer high level (no need to know about the algorithmic details - it is these I would consider highly proprietary stuff) helps a user get a grasp of the proper way of using controls or even perform educated rather than random tests.

And please remember that in my OP I was referring to virtually all raw converters out there. Not just Lightroom / ACR.

The fact that not many people, even in this forum frequented by users of higher than average level, understand, for example, how WB is accomplished in a non-demosaiced space is again food for thought.
« Last Edit: May 06, 2008, 03:35:18 AM by NikosR » Logged

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« Reply #63 on: May 06, 2008, 03:34:03 AM »
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For example, with regards to your question about Exposure vs. Brightness -- Panopeeper covered it: the former is linear and hence preserves linear tonal relationships, but can cause clipping. The latter is a non-linear tone curve adjustment (and hence doesn't preserve linear tonal relationships) which doesn't clip. Two different ways to adjust tonality, with pros and cons of each.
[a href=\"index.php?act=findpost&pid=193616\"][{POST_SNAPBACK}][/a]

What makes you assume that Exposure is always adjusted in a demosaiced space, if that's what you are alluding to?
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Mark D Segal
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« Reply #64 on: May 06, 2008, 06:48:22 AM »
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I don't know, because I don't know what it is that it is supposed to be doing.  I find I can achieve most things with the Exposure slider anyway and that way still retain somewhat normal contrast in the image (as opposed to the recovery slider which just seems to dull the highlights).
[a href=\"index.php?act=findpost&pid=193716\"][{POST_SNAPBACK}][/a]

Bernie, yes, the Exposure slider is critical, but achieving "most things" with it is another matter. It or Recovery, depending, is the first step after White Balance, but I find often that Fill, Blacks and less often Brightness also make a huge difference to the quality of the image's tonality - before getting to the Tone Curves. The purposes and roles of Exposure and Recovery are explained with rather more precision on page 38 and on pages 141/142 of Fraser/Schewe. I really suggest to one and all interested in this topic, if you haven't done so already, to read this book. Believe me I get nothing whatever for recommending it - it's just "essential reading" if the subject is essential to you, as it is to me.
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Mark D Segal (formerly MarkDS)
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bernie west
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« Reply #65 on: May 06, 2008, 06:53:45 AM »
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By "most things" I meant in terms of highlight recovery.  I probably should have said "most images".  Fill, Blacks and brightness certainly have a roll.  Infact, since recently trying LR's auto adjustment on a few images lately (and liking the result), I have been a bit of a convert to the brightness slider.  I like to use a little less Fill, but add in a bit of Brightness.
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bjanes
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« Reply #66 on: May 06, 2008, 06:53:50 AM »
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Now wouldn't it be nice if we could have some documentation from Adobe explaining what is going on here?  Surely this isn't a commercial confidence issue?
[a href=\"index.php?act=findpost&pid=193697\"][{POST_SNAPBACK}][/a]

More documentation regarding the Recovery and Exposure sliders would be welcome. The documentation does state that Exposure is in increments of f/stops and has the same effect as changing the f/stop by an equal increment. This implies that the adjustment is linear. The PS documentation says use recovery to bring the highlights down. Experience demonstrates that it affects highlights more than mid tone and dark tones and is thus nonlinear.

When dealing with overexposure, when does one use recovery and when does he use exposure? A simple test can help here. I Exposed a MacBeth color checker with my Nikon D3 according to the light meter and in one stop increments and used ACR for rendering. With the D3, ACR uses a base line exposure of +0.5 EV and one must use a negative exposure of -0.5 EV in ACR to cancel this.

According to Bruce Lindbloom, the proper values in ProPhotoRGB for the neutral squares of the color checker are 233, 189, 144, 102, 66, and 37. For the nominal exposure, I used Exposure at -0.3 (close to the -0.5 mentioned above) to get the white patch to a pixel value of 233:
[attachment=6433:attachment]

For a one stop overexposure, it would make sense to use -1 EV of exposure compensation added to the -0.3 value used for the nominal exposure development:
[attachment=6434:attachment]

Recovery can't do the job. Using 100% recovery, the white patch comes down to 246, but the other patches are too light and the color is washed out. As documented, recovery affects the highlights more than the remaining tones. The -0.3 exposure adjustment is to account for the baseline exposure that ACR uses for the D3.
[attachment=6435:attachment]

From these tests, I conclude that when correcting overexposure, the Exposure slider is the primary tool. The recovery slider would be useful in high dynamic range shots where the highlights are burnt out but the midtones are properly exposed. Further discussion is invited.

Bill
« Last Edit: May 06, 2008, 06:55:29 AM by bjanes » Logged
Mark D Segal
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« Reply #67 on: May 06, 2008, 07:01:42 AM »
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By "most things" I meant in terms of highlight recovery.  I probably should have said "most images".  Fill, Blacks and brightness certainly have a roll.  Infact, since recently trying LR's auto adjustment on a few images lately (and liking the result), I have been a bit of a convert to the brightness slider.  I like to use a little less Fill, but add in a bit of Brightness.
[a href=\"index.php?act=findpost&pid=193750\"][{POST_SNAPBACK}][/a]

Bernie, Exposure isn't ideal for highlight recovery because you often find yourself dulling the rest of the image to ramp-down a few partially blown highlights in very small parts of the image. But Exposure is best where correcting an overall under-exposure, for example. That's why Recovery is there - to deal in a targeted manner with highlights. As for Fill, Blacks and Brightness, I don't think it optimal to be a convert to any particular control on this panel - one uses what works best for the image at hand. For example, if you want more granularity in the separation of shadow tones but the mid tones upward are OK, you'll get much more mileage playing between Fill and Blacks. And better yet, if the luminosity problem is confined to one colour group, adjusting the Luminosity of the offending colour group in the HSL tab can work wonders - for example overly dark skies are well-handled by lightening the Blues in the L tab of the HSL panel.
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Mark D Segal (formerly MarkDS)
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« Reply #68 on: May 06, 2008, 07:16:04 AM »
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More documentation regarding the Recovery and Exposure sliders would be welcome. The documentation does state that Exposure is in increments of f/stops and has the same effect as changing the f/stop by an equal increment. This implies that the adjustment is linear. The PS documentation says use recovery to bring the highlights down. Experience demonstrates that it affects highlights more than mid tone and dark tones and is thus nonlinear.

When dealing with overexposure, when does one use recovery and when does he use exposure? A simple test can help here. I Exposed a MacBeth color checker with my Nikon D3 according to the light meter and in one stop increments and used ACR for rendering. With the D3, ACR uses a base line exposure of +0.5 EV and one must use a negative exposure of -0.5 EV in ACR to cancel this.


From these tests, I conclude that when correcting overexposure, the Exposure slider is the primary tool. The recovery slider would be useful in high dynamic range shots where the highlights are burnt out but the midtones are properly exposed. Further discussion is invited.

Bill
[a href=\"index.php?act=findpost&pid=193751\"][{POST_SNAPBACK}][/a]

Bill, take a rather heavily under-exposed image into ACR and normalize the histogram with the Exposure slider. You will see that as it shifts the histogram to the right, the very darkest tones hardly budge, the mid-tones spread somewhat and the right tail of the distribution moves the most. Not sure whether this fits a linear behaviour pattern, but whatever, that's what happens.

Test charts is one way of exploring these things and they can provide some useful guidance, but honestly, there's only a few sliders in that tab, it has a logical workflow layout, and the optimal combination of adjustments really tends to be very image-specific, so my more pedestrian approach is simply to pull-up the real-world images on a well calibrated/profiled display and just go down and up the page to get things either just-right (to the extent feasible in ACR) or almost just-right, and in the latter case revert to Curves and HSL for the finishing touches. What's really nice about this "little" program is that the rocket-science is under the hood and the U.I. very friendly. Some basic instruction along with experience and experimentation on real-world photos takes one a very long way to getting the most it can offer.
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Mark D Segal (formerly MarkDS)
Author: "Scanning Workflows with SilverFast 8....." http://www.luminous-landscape.com/reviews/film/scanning_workflows_with_silverfast_8.shtml
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« Reply #69 on: May 06, 2008, 07:19:38 AM »
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What makes you assume that Exposure is always adjusted in a demosaiced space, if that's what you are alluding to?
[{POST_SNAPBACK}][/a]

Nikos, some research (open info from his website) about the individual to whom you are asking this question: [a href=\"http://people.csail.mit.edu/ericchan/]Eric Chan[/url]
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Mark D Segal (formerly MarkDS)
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bernie west
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« Reply #70 on: May 06, 2008, 07:41:52 AM »
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Bernie, Exposure isn't ideal for highlight recovery because you often find yourself dulling the rest of the image to ramp-down a few partially blown highlights in very small parts of the image. But Exposure is best where correcting an overall under-exposure, for example. That's why Recovery is there - to deal in a targeted manner with highlights. As for Fill, Blacks and Brightness, I don't think it optimal to be a convert to any particular control on this panel - one uses what works best for the image at hand. For example, if you want more granularity in the separation of shadow tones but the mid tones upward are OK, you'll get much more mileage playing between Fill and Blacks. And better yet, if the luminosity problem is confined to one colour group, adjusting the Luminosity of the offending colour group in the HSL tab can work wonders - for example overly dark skies are well-handled by lightening the Blues in the L tab of the HSL panel.
[a href=\"index.php?act=findpost&pid=193754\"][{POST_SNAPBACK}][/a]

I don't know whether I am being sloppy, or you are being extra critical, but I am not being absolutist here.  I am just saying that for some of my (landscape) images of late, I have found the brightness slider usefull.  Previous to this, I had never touched it.

As for your (perhaps absolutist??) statement that the Exposure slider isn't ideal for highlight recovery, I'd say this: Using the Exposure slider to darken everything and then use the Fill or Brightness to bring the darker tones back up (to where they were), OR, use the Recovery slider to drop the highlights and leave the other tones where they were; you say Tomato, I say Toma®to.  However, like I said, I feel using the Recovery slider method leads to flat dull highlights.  I find using the Exposure slider retains good contrast in the brighter tones.

One other thing.  I think (although I am happy to be corrected) Nikos's question was rhetorical.
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bjanes
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« Reply #71 on: May 06, 2008, 07:57:30 AM »
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Bill, take a rather heavily under-exposed image into ACR and normalize the histogram with the Exposure slider. You will see that as it shifts the histogram to the right, the very darkest tones hardly budge, the mid-tones spread somewhat and the right tail of the distribution moves the most. Not sure whether this fits a linear behaviour pattern, but whatever, that's what happens.
[a href=\"index.php?act=findpost&pid=193759\"][{POST_SNAPBACK}][/a]

Mark,

Since we are talking about highlight recovery, I'm not sure why you bring up an example with underexposure. In the example you cite, the exposure control is not equivalent to increasing exposure in the camera as advertised. There are limitations to what it can do. In the example I gave, exposure does work in a more or less linear fashion.

Quote
Test charts is one way of exploring these things and they can provide some useful guidance, but honestly, there's only a few sliders in that tab, it has a logical workflow layout, and the optimal combination of adjustments really tends to be very image-specific, so my more pedestrian approach is simply to pull-up the real-world images on a well calibrated/profiled display and just go down and up the page to get things either just-right (to the extent feasible in ACR) or almost just-right, and in the latter case revert to Curves and HSL for the finishing touches. What's really nice about this "little" program is that the rocket-science is under the hood and the U.I. very friendly. Some basic instruction along with experience and experimentation on real-world photos takes one a very long way to getting the most it can offer.
[a href=\"index.php?act=findpost&pid=193759\"][{POST_SNAPBACK}][/a]

The ACR controls in the basic panel are arranged in a logical fashion, and it is often advised to work from top to bottom. The advice of Thomas Knoll in the LL ACR tutorial (CR05, around 5:30) is instructive. When correcting images, he uses exposure to set the white value and then brightness to adjust the mid tones. With the advent of Lightroom, he noted that some photographers use exposure to adjust the mid tones and then recovery to adjust the highlights.

He does not specifically state that he uses this approach for overexposed images where highlight recovery is needed, but recovery is not the tool that he would first use. According to his suggestions, exposure would be the first tool. Brightness or recovery could be used next. Fine tuning with Curves and HSL could come later.

There are only a few sliders on the basic panel, but the number of permutations is rather large, especially when the sub panels are brought into play. A random approach to use of these sliders could consume a great deal of time.

Test charts do not duplicate real world experience, but they are instructive in that they enable one to measure exactly what each control does. Once one understands the controls, then he can use this knowledge to adjust the image.
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Mark D Segal
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« Reply #72 on: May 06, 2008, 08:31:51 AM »
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I don't know whether I am being sloppy, or you are being extra critical, but I am not being absolutist here.  I am just saying that for some of my (landscape) images of late, I have found the brightness slider usefull.  Previous to this, I had never touched it.

As for your (perhaps absolutist??) statement that the Exposure slider isn't ideal for highlight recovery, I'd say this: Using the Exposure slider to darken everything and then use the Fill or Brightness to bring the darker tones back up (to where they were), OR, use the Recovery slider to drop the highlights and leave the other tones where they were; you say Tomato, I say TomaŽto.  However, like I said, I feel using the Recovery slider method leads to flat dull highlights.  I find using the Exposure slider retains good contrast in the brighter tones.

One other thing.  I think (although I am happy to be corrected) Nikos's question was rhetorical.
[a href=\"index.php?act=findpost&pid=193771\"][{POST_SNAPBACK}][/a]

No, I'm not being extra-critical and not absolutist. There is an unfortunate tendancy in this thread and in the previous one which Bill Janes referenced, for people to launch into attacks on the personae of the discussants rather than stick to the subject matter.

I agree with you that excessive adjustment of highlights with Recovery can overly flatten the highlights. Indeed, there is nothing absolutist about any of this - that's what I've been trying to say - maybe the message isn't communicating as it should. Like everything in Photoshop, there are umpteen ways to skin the cat, and it all depends on what works best for the image at hand; but there are some general principles one can start with, including the advice to use the program from top to bottom and left to right, but this isn't dogma either.
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Mark D Segal (formerly MarkDS)
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« Reply #73 on: May 06, 2008, 08:44:56 AM »
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Mark,

Since we are talking about highlight recovery, I'm not sure why you bring up an example with underexposure. In the example you cite, the exposure control is not equivalent to increasing exposure in the camera as advertised. There are limitations to what it can do. In the example I gave, exposure does work in a more or less linear fashion.
The ACR controls in the basic panel are arranged in a logical fashion, and it is often advised to work from top to bottom. The advice of Thomas Knoll in the LL ACR tutorial (CR05, around 5:30) is instructive. When correcting images, he uses exposure to set the white value and then brightness to adjust the mid tones. With the advent of Lightroom, he noted that some photographers use exposure to adjust the mid tones and then recovery to adjust the highlights.

He does not specifically state that he uses this approach for overexposed images where highlight recovery is needed, but recovery is not the tool that he would first use. According to his suggestions, exposure would be the first tool. Brightness or recovery could be used next. Fine tuning with Curves and HSL could come later.

There are only a few sliders on the basic panel, but the number of permutations is rather large, especially when the sub panels are brought into play. A random approach to use of these sliders could consume a great deal of time.

Test charts do not duplicate real world experience, but they are instructive in that they enable one to measure exactly what each control does. Once one understands the controls, then he can use this knowledge to adjust the image.
[a href=\"index.php?act=findpost&pid=193775\"][{POST_SNAPBACK}][/a]

You drew a conclusion that the Exposure slider works in a linear fashion. You based that on an example of an experiment you performed with a test chart. Fine as far as it goes, but the example I gave was intended to suggest that maybe it is less linear than suggested when called upon for other purposes. Perhaps someone who really KNOWS the answer to this could clarify it.

Regardless of who advises what, let us just explore the logic here, and as I keep saying, what's optimal will vary from one image to anther, albeit there are general principles guiding the workflow. So let's say we start with an image which has blown highlights and is otherwise under-exposed. Where would you start? Would you ramp-up the Exposure slider blowing the highlights further, or would you Recover the highlights and then ramp up the Exposure, working inter-actively between them till you reach a limit or a visually satifactory result? Not being dogmatic, I would say it doesn't matter much. In this kind of situation, it's just a matter of workflow convenience because ACR will process all the moves in its own determined sequence regardless of when you or I adjust what. For an image of this kind, I like playing with Recovery first, just to see how much this targeted adjustment can do for me, and to see how much head-room it gives me for then correcting the under-exposure in the rest of the image. But I could have started with Exposure and then applied more Recovery.

Finally, I hope you don't believe I do any of this randomly. I've processed literally thousands of images through this plug-in and I have neither the time, patience or absence of standards to do this kind of thing "randomly". Cummon Bill, get real!    OK, sermon over, back to processing!
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Mark D Segal (formerly MarkDS)
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« Reply #74 on: May 06, 2008, 09:04:41 AM »
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Nikos, some research (open info from his website) about the individual to whom you are asking this question: Eric Chan
[a href=\"index.php?act=findpost&pid=193761\"][{POST_SNAPBACK}][/a]

I know who Eric is, I'm not sure what this has to do with my question. Both raw and de-mosaiced linear spaces are, well, linear so my question still stands.
« Last Edit: May 06, 2008, 09:07:12 AM by NikosR » Logged

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« Reply #75 on: May 06, 2008, 10:02:40 AM »
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My response was merely intended to suggest that you can take his word for it, but as you wish to peak further under the hood - fine by me of course,  "over and out" to Eric.
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Mark D Segal (formerly MarkDS)
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« Reply #76 on: May 06, 2008, 11:12:09 AM »
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Both raw and de-mosaiced linear spaces are, well, linear so my question still stands.

Because noise reduction in ACR is integrated in the demosaicing process.
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« Reply #77 on: May 06, 2008, 01:04:32 PM »
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Because noise reduction in ACR is integrated in the demosaicing process.
[a href=\"index.php?act=findpost&pid=193854\"][{POST_SNAPBACK}][/a]

The question was about EV compensation, I'm not sure where NR fits in this question
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« Reply #78 on: May 06, 2008, 01:50:36 PM »
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The question was about EV compensation, I'm not sure where NR fits in this question
That was my answer to your question. I think this is the point, where the "legitimacy" of the curiosity ends, as this issue has absolutely no relevance to the outcome of raw processing, except, well, regarding the noise reduction.

With "legitimate curiosity" I mean questions regarding the outcome of the raw processing, i.e. what a certain action is doing, not how. The what should be part of the native documentation, the how is proprietory information. For example it is absolutely "legitimate" to ask, what effect the "recovery" slider is causing.

Pls note, that I have no insider information regarding ACR's processing logic; I am referring to what I regard logical under the given circumstances, but that may be incorrect regarding ACR's actual processing.
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« Reply #79 on: May 07, 2008, 10:03:19 AM »
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You drew a conclusion that the Exposure slider works in a linear fashion. You based that on an example of an experiment you performed with a test chart. Fine as far as it goes, but the example I gave was intended to suggest that maybe it is less linear than suggested when called upon for other purposes. Perhaps someone who really KNOWS the answer to this could clarify it.
[a href=\"index.php?act=findpost&pid=193793\"][{POST_SNAPBACK}][/a]

From its description in the Adobe help, the Exposure slider should perform a linear scaling of the luminance data, just as decreasing the exposure by 1.0 EV scales all the picture elements by a factor of 0.5.

This can be shown by experiment with Imatest and a Stouffer wedge. With the ACR defaults, the black point is 5, which rolls off the shadows. The default Contrast of +25, Brightness of +50, and the default point curve are applied with the exposure adjustment and complicate the resulting curve.

Here are the tone curves with ACR defaults and Exposure set to 0 and -1:
[attachment=6463:attachment]

Here are the tone curves with ACR defaults except for Black = 0:
[attachment=6464:attachment]

And finally, here are the tone curves with ACR linear settings (black = 0, contrast = 0, brightness = 0, point curve = linear):
[attachment=6465:attachment]

This last set of curves demonstrates that the Exposure adjustment is indeed linear over the entire range, except for the deepest shadows where the data are not good.

Panopeeper knows quite a bit in this area and also believes the Exposure adjustment is linear. Anyone who states otherwise should back up his statement with data.

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The so-called EV compensation of ACR (it is a misnomer) is a linear operation, at least that is my observation. The "brightness" adjustment is non-linear.
[a href=\"index.php?act=findpost&pid=193581\"][{POST_SNAPBACK}][/a]

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Regardless of who advises what, let us just explore the logic here, and as I keep saying, what's optimal will vary from one image to anther, albeit there are general principles guiding the workflow. So let's say we start with an image which has blown highlights and is otherwise under-exposed. Where would you start? Would you ramp-up the Exposure slider blowing the highlights further, or would you Recover the highlights and then ramp up the Exposure, working inter-actively between them till you reach a limit or a visually satifactory result? Not being dogmatic, I would say it doesn't matter much. In this kind of situation, it's just a matter of workflow convenience because ACR will process all the moves in its own determined sequence regardless of when you or I adjust what. For an image of this kind, I like playing with Recovery first, just to see how much this targeted adjustment can do for me, and to see how much head-room it gives me for then correcting the under-exposure in the rest of the image. But I could have started with Exposure and then applied more Recovery.

[a href=\"index.php?act=findpost&pid=193793\"][{POST_SNAPBACK}][/a]

I think Thomas Knoll's suggestions are for routine images in general, but there is no one more qualified to give such advise. I agree that one must evaluate the individual image. The example you gave with a generally underexposed image with blown highlights represents a high contrast scene whose dynamic range is greater than that of the camera. One must decide which luminances to favor in the final image. As you note, the order in which you do the recovery and exposure adjustments does not matter, since all these adjustments are concatenated and performed in the preferred order by ACR. As Schewe demonstrates in the tutorial, a curve applied to the highlights can also be useful and can give more control than the sliders.
« Last Edit: May 07, 2008, 10:07:42 AM by bjanes » Logged
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