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Author Topic: exposing to the right  (Read 34063 times)
madmanchan
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« Reply #20 on: April 01, 2008, 08:35:07 AM »
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Hi Eric,

As far as I'm aware the only recent cameras with live histograms are non-DSLRs. Even my 1DsMk3, which has live-view, does NOT (much to my chagrin) have a live histogram. I wish..........

Mark
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Are you absolutely sure about this, Mark?

I recently used a 1D Mark III in Live View mode and it had a live three-channel RGB histogram. I am fairly certain that if the 1D III has it, then the 1Ds III should as well.

The new Rebel XSi also has it.

Please check again.
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madmanchan
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« Reply #21 on: April 01, 2008, 08:39:31 AM »
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IMO, any test about ETTR to analyse if a RAW file was clipped, needs neutral (1.0) white balance multipliers. Since ACR and any other commercial developer don't allow them, I would not take too much seriously precise results obtained using them.

DCRAW achieves neutral WB by -r 1 1 1 1

Regards.
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For shooting step wedges such as what Bill is doing, it doesn't really matter since the green channel will be the first to clip, and the green multiplier is nearly always 1 anyways.
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francois
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« Reply #22 on: April 01, 2008, 08:41:08 AM »
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Hi Eric,

As far as I'm aware the only recent cameras with live histograms are non-DSLRs. Even my 1DsMk3, which has live-view, does NOT (much to my chagrin) have a live histogram. I wish..........

Mark
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Mark,
Well, my 1Ds3 does display live histogram in live view mode. Read page 114 of your manual.

You need to set C.Fn IV - 16 to 1 (simulate exposure) to activate it. If it isn't displayed, then press the info button (once or twice).
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Francois
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« Reply #23 on: April 01, 2008, 08:53:20 AM »
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Bill, as we discussed briefly on the Adobe User-to-User forums, the issue here is the baseline exposure differences between various cameras and across vendors. There is a fundamental tradeoff between (a) honoring the original distribution of raw values (between black point and white point) and ( b ) having a given exposure (e.g., f/8, 1/10th sec, ISO 400) be rendered the same way across different cameras. In order to have common controls such as Exposure compensation behave the same way across the different cameras, Camera Raw applies a baseline exposure compensation that varies from model to model in order to get them all to behave similarly when the exposure compensation control is set to its default value of zero.

In the case of the D3, the baseline exposure is set to +0.5. You can "undo" this by setting CR's exposure compensation to -0.5 (and save that as your camera default if that's what you prefer).
« Last Edit: April 01, 2008, 09:01:01 AM by madmanchan » Logged

Mark D Segal
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« Reply #24 on: April 01, 2008, 09:12:03 AM »
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Mark,
Well, my 1Ds3 does display live histogram in live view mode. Read page 114 of your manual.

You need to set C.Fn IV - 16 to 1 (simulate exposure) to activate it. If it isn't displayed, then press the info button (once or twice).
[a href=\"index.php?act=findpost&pid=186095\"][{POST_SNAPBACK}][/a]

Eric, Francois - thanks - yes indeed it is there - we live and learn. Even the Canon rep at a trade show couldn't find it - neither of us had read page 114!  
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Mark D Segal (formerly MarkDS)
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francois
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« Reply #25 on: April 01, 2008, 09:34:55 AM »
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Eric, Francois - thanks - yes indeed it is there - we live and learn. Even the Canon rep at a trade show couldn't find it - neither of us had read page 114!
[a href=\"index.php?act=findpost&pid=186107\"][{POST_SNAPBACK}][/a]
Great! Intuitive is not always part of the dictionary of modern electronic equipment manufacturers.
« Last Edit: April 01, 2008, 09:35:15 AM by francois » Logged

Francois
bjanes
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« Reply #26 on: April 02, 2008, 09:13:31 AM »
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Bill, as we discussed briefly on the Adobe User-to-User forums, the issue here is the baseline exposure differences between various cameras and across vendors. There is a fundamental tradeoff between (a) honoring the original distribution of raw values (between black point and white point) and ( b ) having a given exposure (e.g., f/8, 1/10th sec, ISO 400) be rendered the same way across different cameras. In order to have common controls such as Exposure compensation behave the same way across the different cameras, Camera Raw applies a baseline exposure compensation that varies from model to model in order to get them all to behave similarly when the exposure compensation control is set to its default value of zero.

In the case of the D3, the baseline exposure is set to +0.5. You can "undo" this by setting CR's exposure compensation to -0.5 (and save that as your camera default if that's what you prefer).
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Eric,

As long as we understand what is going on behind the scenes so to speak, we can deal with the issues as you suggest. My question is from where does the BaseLine exposure value come? From the DNG specification I see that it is one of the metadata values. Does this value come from the camera maker or is it filled in by the DNG converter from its own database? Also, how does one examine the DNG metadata fields to determine their value?

The point of my original post here was that without this knowledge, one can not really use ACR or Lightroom to determine if there is clipping in the raw file. IMHO it is best to examine the raw data directly with a program such as Iris. Another good way to look at the raw data is another freeware program [a href=\"http://www.cryptobola.com/photobola/rawnalyze.htm]Rawanalyze[/url].

DigitalDog does not go into these issues in his referenced article.

Bill
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Mark D Segal
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« Reply #27 on: April 02, 2008, 09:38:58 AM »
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Eric,

As long as we understand what is going on behind the scenes so to speak, we can deal with the issues as you suggest. My question is from where does the BaseLine exposure value come? From the DNG specification I see that it is one of the metadata values. Does this value come from the camera maker or is it filled in by the DNG converter from its own database? Also, how does one examine the DNG metadata fields to determine their value?

The point of my original post here was that without this knowledge, one can not really use ACR or Lightroom to determine if there is clipping in the raw file. IMHO it is best to examine the raw data directly with a program such as Iris. Another good way to look at the raw data is another freeware program Rawanalyze.

DigitalDog does not go into these issues in his referenced article.

Bill
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Bill, at a purely operational level without the benefit of having those applications you recommend, would you agree that when you open a file in ACR or Lightroom with all the luminosity information in the Basic tab zeroed and the two tone curves set to linear, this should give a fairly reliable interpretation of whether or not the raw file has suffered from highlight clipping during exposure?

Mark
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Mark D Segal (formerly MarkDS)
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Jonathan Wienke
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« Reply #28 on: April 02, 2008, 09:39:32 AM »
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The point of my original post here was that without this knowledge, one can not really use ACR or Lightroom to determine if there is clipping in the raw file.

It's really not that hard to check for RAW clipping in ACR. Slide the exposure control to -4, and if the histogram data still is touching the right edge, or any of the channels exhibit spikes at their maximum values as shown below even if the spike isn't touching the right edge, then the RAW is clipped.



Rawanalyze is also an excellent tool for evaluating exposure. It is particularly useful for determining the exposure offset between the camera histogram's clip indication and actual RAW clipping. It does so more precisely than ACR, and if you have underexposed, Rawanalyse will show you exactly how much.
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bjanes
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« Reply #29 on: April 02, 2008, 12:42:19 PM »
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Bill, at a purely operational level without the benefit of having those applications you recommend, would you agree that when you open a file in ACR or Lightroom with all the luminosity information in the Basic tab zeroed and the two tone curves set to linear, this should give a fairly reliable interpretation of whether or not the raw file has suffered from highlight clipping during exposure?

Mark
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Mark,

Not entirely, unless you use the BaseLine exposure correction. As shown below with a file that is not clipped when one examines the raw file directly, ACR with the neutral settings you suggest (and without the baseline correction) still indicates that steps 1 and 2 in the wedge are blown. That is 2/3 of an f/stop. This is not surprising since contrast affects the mid-tones and three quarter tones but should have no effect when the normalized pixel value is 1.0 (255/255, 16383/16383, etc).

As Jeff Schewe points out in p. 73 of his ACR book, increasing brightness to values over 100 can push the highlights to 255 in an 8 bit file. This can look like highlight clipping, but he says if you look at the converted 16 bit file, there usually is not clipping. The brightness control adjusts the mid-tones without clipping the highlights or shadows and determines the point about which the contrast slider operates.

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bjanes
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« Reply #30 on: April 02, 2008, 01:14:13 PM »
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It's really not that hard to check for RAW clipping in ACR. Slide the exposure control to -4, and if the histogram data still is touching the right edge, or any of the channels exhibit spikes at their maximum values as shown below even if the spike isn't touching the right edge, then the RAW is clipped.



Rawanalyze is also an excellent tool for evaluating exposure. It is particularly useful for determining the exposure offset between the camera histogram's clip indication and actual RAW clipping. It does so more precisely than ACR, and if you have underexposed, Rawanalyse will show you exactly how much.
[a href=\"index.php?act=findpost&pid=186405\"][{POST_SNAPBACK}][/a]

I would prefer to use a better documented method of checking for clipping. Here is a file that is clipped in steps 1, 2, and 3 as shown on the plot below and by visual inspection (green1 channel only is analyzed):



If I use your method in ACR with exposure of -4, the histogram does not touch the right and whether or not those spikes at maximum are significant is open to question.



Rawanalyze shows clipping in the raw file step 3 at about 15970:



For those who are interested, here is the raw histogram:

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Mark D Segal
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« Reply #31 on: April 02, 2008, 01:18:05 PM »
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Thanks Bill,

By "use the BaseLine exposure correction" - what do you mean?

Not clear to me what pg 73 of Jeff's book has to do with this. What he says about the Brightness slider is of course correct, but how is it relevant to the problem we are discussing, insofar as we have the Brightness slider at zero? Something I'm not understanding here?

Mark
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Mark D Segal (formerly MarkDS)
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« Reply #32 on: April 02, 2008, 01:24:31 PM »
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I would prefer to use a better documented method of checking for clipping. Here is a file that is clipped in steps 1, 2, and 3 as shown on the plot below and by visual inspection (green1 channel only is analyzed):

...

If I use your method in ACR with exposure of -4, the histogram does not touch the right and whether or not those spikes at maximum are significant is open to question.

I agree that Rawanalyze is much more precise; when the RAW is only slightly clipped, ACR's highlight recovery may disguise that. But in most such cases, the clipping doesn't detract from the image significantly after the highlight recovery process. But whenever you see the indications I described in ACR, the RAW is clipped.
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bjanes
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« Reply #33 on: April 02, 2008, 01:44:46 PM »
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Thanks Bill,

By "use the BaseLine exposure correction" - what do you mean?

Not clear to me what pg 73 of Jeff's book has to do with this. What he says about the Brightness slider is of course correct, but how is it relevant to the problem we are discussing, insofar as we have the Brightness slider at zero? Something I'm not understanding here?

Mark
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Mark,

For the baseline exposure correction, see Eric Chan's [a href=\"http://luminous-landscape.com/forum/index.php?showtopic=24354&view=findpost&p=186098]Post #27[/url]. Unfortunately, I am not that familiar with that correction and did not even know of its existence prior to this discussion. I posted a couple of questions for Eric and perhaps he will reply.

I think that some important concepts are involved here and it is strange that Schewe and his colleagues do not lend us their expertise. The last I heard, Schewe was ignoring me   and DigiDog never admits error  .

My point was that you see about the same highlight clipping information in ACR regardless of reasonable settings for contrast and brightness. It is only when the BaseLine exposure correction of -0.5 is applied in ACR that the ACR clipping results are accurate.

Bill
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Panopeeper
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« Reply #34 on: April 02, 2008, 02:21:24 PM »
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By "use the BaseLine exposure correction" - what do you mean?

BaselineExposure is a DNG specific TIFF tag indicating, that an "exposure adjustment" should be carried out by the raw converter. This tag is inserted in the DNG file by the Adobe DNG converter with the value of +0.5 EV for the Nikon D3 and D300, -0.5 EV for the D200, and D2X, -0.75 EV for the D70 and D100. The same "correction" values are applied by ACR automatically.

The automatic adjustment has legitimate usages. For example the Highlight Tone Protection feature of newer Canon cameras "requires" +1 EV adjustment in raw processing (and pulling back the highlights), because HTP is supposed to underexpose the shot in order to prevent clipping.

The problems with these adjustments are:

1. they are not always "legitimate". The argument

In order to have common controls such as Exposure compensation behave the same way across the different cameras, Camera Raw applies a baseline exposure compensation that varies from model to model in order to get them all to behave similarly when the exposure compensation control is set to its default value of zero

is plain bullshit. What we see here is a "correction" of the camera's ISO values. Some camera's gains are not corresponding to ISO 100, 200, etc.  but to ISO 80 or ISO 135 or whatever. Camera manufacturers don't like to admit  this, and call the settings "ISO 100" etc.

It is not the task of the raw processor to "correct" the camera's characteristics.

2. the adjustment can be destructive: if the raw data "encroaches" into the top 0.5 EV (with a Nikon D3 or D300), the adjustment causes clipping in the conversion. On the other hand, a negative adjustment can ruin the shadows.

In both cases the adjustment is without any reason. The numerical range of the pixel values for a camera (and in some cases for the actual ISO) is given, and the camera can occupy that full numerical range, depending on the exposure. There is no basis to remoive part of those values.

3. The worse on this habit of mutilation is, that ACR does not give any indication of the automatic adjustment: the "Exposure" slider is at 0, even though an adjustment occured. Therefor all those users, who don't know about the mutilation are led to believe, that their shot is over- or underexposed, even if the shot is perfect.
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Gabor
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« Reply #35 on: April 02, 2008, 02:55:03 PM »
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Pano,

Thanks for pitching in here. This is a very interesting perspective on the issue. If correct, I'm beginning to see the solution to this problem. There are a couple of layers to it, leaving out the DNG complication for the moment: (1) What the camera's JPEG is showing relative to what the raw file is really like; and (2) What the raw file is really like compared with how ACR portrays it, with all user-defined settings as neutral as they can be (i.e. zeros and straight lines).

It seems that the RawAnalyze program Jonathan recommends - according to Jonathan - is the one that "tells the truth" about whether the raw file actually has clipping or not.

Therefore the solution to this issue would seem to be to find a JPEG setting for the camera (by making test exposures ostensibly using ETTR) which comes as close as possible to a no-clipping histogram both on the camera LCD and in RawAnalyse. Then open that image in ACR and set ACR's Basic Tab luminosity controls to reference values which come as close as possible to replicating the "correct" histogram in ACR, and use those as one's defaults for that particular camera.

Perhaps further adjustments would be needed to deal with any baked-in compensation if one converts to DNG.
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Mark D Segal (formerly MarkDS)
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« Reply #36 on: April 02, 2008, 03:47:26 PM »
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Mark,

For the baseline exposure correction, see Eric Chan's Post #27. Unfortunately, I am not that familiar with that correction and did not even know of its existence prior to this discussion. I posted a couple of questions for Eric and perhaps he will reply.

I think that some important concepts are involved here and it is strange that Schewe and his colleagues do not lend us their expertise. The last I heard, Schewe was ignoring me   and DigiDog never admits error  .

My point was that you see about the same highlight clipping information in ACR regardless of reasonable settings for contrast and brightness. It is only when the BaseLine exposure correction of -0.5 is applied in ACR that the ACR clipping results are accurate.

Bill
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Bill,

I think it would be very helpful to get Jeff's and Andrew's technical input here; and to this objective it could be more useful to encourage them to participate rather than to presume upon their motives vis a vis yourself, the basis of which certainly isn't apparent to me.

So getting back to the substance of the issue, I agree, the Brightness and Contrast settings largely affect the midtones - though both can impact on highlights where there are tones near clipping before these adjustments are made. It is normal to expect that an Exposure change will have a greater impact on highlight clipping. All that said, in my just previous post, based on a contribution from Pano, I suggested an empirical procedure for being able to manage one's exposures between the histogram view, the raw file and ACR. Feedback on these suggestions would be most interesting.

Cheers,

Mark
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Mark D Segal (formerly MarkDS)
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« Reply #37 on: April 02, 2008, 03:51:13 PM »
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Bill, I should have also have mentioned that Jeff and Andrew are probably both immersed or about to be immersed in PhotoshopWorld at Orlando, so that may explain why we're not hearing from them. I think if either of them sees this thread and they have the time we may hear from one or both as and when they can manage it. Let's just wait and see.

Mark
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Mark D Segal (formerly MarkDS)
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madmanchan
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« Reply #38 on: April 02, 2008, 05:23:03 PM »
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It is not the task of the raw processor to "correct" the camera's characteristics.

Disagree.

I think it's one of the raw converter's most important features, to compensate for differences among cameras including deficiencies. There are characteristics of cameras & lenses, such as hot pixels, chromatic aberration, noise, excessive infrared response, etc. which are all useful to treat in a raw processor. Most people consider these attributes "problems" or "deficiencies" and would like to be able to tame them in the raw converter.

There are also widely differing characteristics in sensor response which requires normalization (through the use of a color matrix or color lookup table).

If you want to examine raw data then I suggest ACR isn't the right place to do that even with the tone controls 'linearized' because ACR will still apply the color transform.

If you really want to start with the original data, run dcraw in document mode (-D) and enjoy.

Some (many, I would argue) people find it useful to be able to shoot a given job with two different camera models set at the same exposure and get the same results (in terms of tonality).

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2. the adjustment can be destructive: if the raw data "encroaches" into the top 0.5 EV (with a Nikon D3 or D300), the adjustment causes clipping in the conversion. On the other hand, a negative adjustment can ruin the shadows.

I fail to see how a negative adjustment 'ruins' the shadows. It should not ruin the shadows any more than a conversion in which the baseline exposure compensation of +0.5 omitted.

Using negative EC in this case of -0.5 should not cause the shadows to clip. Perhaps you can clarify what you mean.

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In both cases the adjustment is without any reason. The numerical range of the pixel values for a camera (and in some cases for the actual ISO) is given, and the camera can occupy that full numerical range, depending on the exposure. There is no basis to remoive part of those values.

As noted earlier, the data isn't really lost. Yes, those extreme highlights may be clipped in the default rendering, but that is easily adjusted (with no data loss).

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3. The worse on this habit of mutilation is, that ACR does not give any indication of the automatic adjustment: the "Exposure" slider is at 0, even though an adjustment occured. Therefor all those users, who don't know about the mutilation are led to believe, that their shot is over- or underexposed, even if the shot is perfect.

Your point is well taken in the context of considering the "perfect shot" to be the one in which the raw data is not clipped and well exposed.

However, I don't consider this to be an issue, and I certainly think the use of the word 'mutilation' overstates the case (esp. since no real data is lost).

Eric
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madmanchan
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« Reply #39 on: April 02, 2008, 05:47:26 PM »
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As long as we understand what is going on behind the scenes so to speak, we can deal with the issues as you suggest. My question is from where does the BaseLine exposure value? From the DNG specification I see that it is one of the metadata values. Does this value come from the camera maker or is it filled in by the DNG converter from its own database? Also, how does one examine the DNG metadata fields to determine their value?

Bill, the camera vendors who have their own raw formats (such as Canon and Nikon) do not provide this information. The metadata is filled in by the DNG Converter from its own database. I think the data comes from measurements that Thomas makes when he's adding new camera support (I don't know all the details here).

You should be able use any tool that can inspect TIFF tags to examine DNG metadata, since DNG is a TIFF extension.

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The point of my original post here was that without this knowledge, one can not really use ACR or Lightroom to determine if there is clipping in the raw file. IMHO it is best to examine the raw data directly with a program such as Iris. Another good way to look at the raw data is another freeware program Rawanalyze.

I agree with you on this point. Given that ACR always performs some initial processing (besides baseline exposure compensation; things like white balance, etc.) I would suggest that if you're really interested in examining the raw data that you use Rawnalyze or dcraw for that purpose.
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