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Author Topic: Shooting to the right  (Read 26295 times)
bjanes
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« Reply #40 on: June 21, 2006, 10:39:43 AM »
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I recall reports from people who had two identical bodies that metered up to a 1/2 stop different due to sloppiness in factory calibration. In the case of owning two bodies it may be worth sending them in to have them equalized, but for the typical user you just need to learn the behaviour of your's and adjust accordingly.
- DL
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As I read Bruce's post, he is talking about variations in the sensitivity of the sensor to light, not variations in metering. If the meter is well calibrated, but the sensor is not operating at the nominal ISO, then the exposure will be off. The exposure guided by an accurate hand held meter will also be off.

The simple test I mentioned with a gray card tests the system response, which is most important in an actual shooting situation.  One way to test the camera meter would involve comparing the camera reading with a that of an external meter which is known to be accurate. For accurate comparison, one would have to take into account the transmission of the lens and other factors.
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Jonathan Wienke
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« Reply #41 on: June 21, 2006, 12:14:20 PM »
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If you really want a color RAW histogram, you could have it look like this:

Ideal:


Over 2/3:


Under 2/3:


Just remember that the green channel scaling must be divided by two to match the red and blue channels. No bayer interpolation is necessary, simply sort the pixels by their filter color.
« Last Edit: June 21, 2006, 12:15:31 PM by Jonathan Wienke » Logged

Anon E. Mouse
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« Reply #42 on: June 22, 2006, 02:52:28 AM »
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Well guys, I am not convinced. Why don't you show me. Take a average scene - landscape on a sunny day with a few puffy clouds either 2 hours before or after noon. Then:

1. Post the image.

2. Post the RAW histogram with no changes to the data.

3. Post the histogram of the camera thumbnail.

4. Post the histogram of the converted RAW data.

Then lets see what we get.
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John Sheehy
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« Reply #43 on: June 22, 2006, 07:45:28 AM »
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Well guys, I am not convinced. Why don't you show me. Take a average scene - landscape on a sunny day with a few puffy clouds either 2 hours before or after noon.

You have chosen a poor example.  The biggest problems that a RAW histogram would address are with colored highlights.  For white clouds with my camera, the RAW data clips in the green channel about 1/3 stop higher than where the histogram clips, with contrast set to -2.

A red flower is a much more suitable subject.

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Then:

1. Post the image.

2. Post the RAW histogram with no changes to the data.
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What is "the image"?

Here's some red flowers, JPEG on one the right, and a linear RAW interpolation on the left:

[a href=\"http://www.pbase.com/jps_photo/image/61233204]crops[/url]

Here's a RAW RGB histogram (bottom), and an RGB histogram of the JPEG (top):

histograms

My camera doesn't have a color histogram, but if it did, the red channel would be clipped, which may cause someone to under-expose a second shot, when in fact, they should be increasing exposure by about 1.5 stops.
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bjanes
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« Reply #44 on: June 22, 2006, 08:29:25 AM »
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Well guys, I am not convinced. Why don't you show me. Take a average scene - landscape on a sunny day with a few puffy clouds either 2 hours before or after noon. Then:

1. Post the image.

2. Post the RAW histogram with no changes to the data.

3. Post the histogram of the camera thumbnail.

4. Post the histogram of the converted RAW data.

Then lets see what we get.
[a href=\"index.php?act=findpost&pid=68848\"][{POST_SNAPBACK}][/a]

I agree with John Sheehy that this scenario is not a good one to demonstrate the advantages of a raw histogram. However, it does demonstrate some disadvantages.

Here is such a scene exposed pretty much to the right. The highlights are just short of clipping in all channels.

[attachment=729:attachment]

Here is the raw conversion (DCRaw) with the histogram. In this case, I would have to agree with Bruce Fraser. Everything is to the left and the status of the highlights is very difficult to assess.

[attachment=728:attachment]
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Jonathan Wienke
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« Reply #45 on: June 22, 2006, 09:00:29 AM »
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Here is the raw conversion (DCRaw) with the histogram. In this case, I would have to agree with Bruce Fraser. Everything is to the left and the status of the highlights is very difficult to assess.

Which is why I proposed a bar chart format with each bar representing 1/3 stop of exposure range, instead of a linear or standard gamma-adjusted display. You are comparing apples to field mice.

A histogram is nothing more than a chart displaying a distribution of values over a given range. One can be made from any data set, whether audio, video, still image, uninterpolated RAW data values, ASCII text, or stock market quotes. There's absolutely no reason whatsoever RAW data values "must" be interpolated or otherwise processed into a visually pleasing image before a histogram can be made. The whole problem we're trying to solve here is the errors introduced into the exposure evaluation process when white balance and color interpolation is performed on the data prior to running the histogram generation routine. Making a histogram from the unprocessed RAW data (especially when the histogram is segmented to exactly match the camera's exposure adjustment interval) will make judging correct exposure from the histogram much easier, not harder.
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John Sheehy
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« Reply #46 on: June 22, 2006, 01:01:12 PM »
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In this case, I would have to agree with Bruce Fraser. Everything is to the left and the status of the highlights is very difficult to assess.
[attachment=728:attachment]
[a href=\"index.php?act=findpost&pid=68862\"][{POST_SNAPBACK}][/a]

I really don't have any problem looking at that histogram.  If it had an overlay that marked 1/3 stops and whole stops, it would be useable by anyone
The only issue I see with the linear histogram is that there are so many values represented by the right half, that minority highlights might be indistinguishable from the bottom line.  That can be addressed by scaling, for "volume-oriented" assessment.  Or, you can use a histogram that is logarithmic or gamma-adjusted, but still RAW.

I get the impression from some responses that I am begging for a linear RAW histogram.  That has never been the jist of my argument.  I said right from the start that linear, logarithmic (which Jonathan showed an example of), or gamma-adjusted would all be more useful than what we have now.

I do not think that RAW histograms have to be linear in the x axis.

If a histogram is to be logarithmic for the x axis, I would prefer a red, green, and blue line on a dark background, with added colors where they cross.  Jonathan's 1/3 stop bars seem unnecessarily coarse to me.  1/3 stop markers I would like, but data should be finer, IMO.
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bjanes
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« Reply #47 on: June 22, 2006, 06:00:41 PM »
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I really don't have any problem looking at that histogram. If it had an overlay that marked 1/3 stops and whole stops, it would be useable by anyone
The only issue I see with the linear histogram is that there are so many values represented by the right half, that minority highlights might be indistinguishable from the bottom line. That can be addressed by scaling, for "volume-oriented" assessment. Or, you can use a histogram that is logarithmic or gamma-adjusted, but still RAW.

I get the impression from some responses that I am begging for a linear RAW histogram. That has never been the jist of my argument. I said right from the start that linear, logarithmic (which Jonathan showed an example of), or gamma-adjusted would all be more useful than what we have now.

I do not think that RAW histograms have to be linear in the x axis.

If a histogram is to be logarithmic for the x axis, I would prefer a red, green, and blue line on a dark background, with added colors where they cross. Jonathan's 1/3 stop bars seem unnecessarily coarse to me. 1/3 stop markers I would like, but data should be finer, IMO.
[a href=\"index.php?act=findpost&pid=68892\"][{POST_SNAPBACK}][/a]

John,

I just put up what I had--the PSCS2 histogram of a linear raw capture. I'm entirely receptive to some adaption of a raw histogram and think that you and Johnathin have made some very good suggestions and I am in no way trying to shoot them down.

I have a gamma 2.2 curve that I constructed by reverse engineering of some stouffer step wedge conversions in DCRaw and ACR, which appears reasonably accurate. I applied it to the previous raw image and think that the expansion of the highlight tones makes the histogram quite a bit easier to read. I would prefer to have the x-axis scaled in f-stops as Jonathin suggests, and this would be easy to implement.

The image still appears dark and the histograms indicate that I could have given at least 0.5 EV more exposure without blowing the green channel, and a magenta filter would allow even more exposure to the right. The experiment and the discussions in this thead demonstrate why the standard gamma corrected white balanced camera histogram is not up to the task of ETTR.

While it might only be a matter of sematics, the concept of a gamma 2.2 "raw" image sounds a bit like an oxymoron to me.

Raw conversion with 2.2 gamma curve:
[attachment=730:attachment]

Straight raw conversion:
[attachment=731:attachment]
« Last Edit: June 22, 2006, 06:05:50 PM by bjanes » Logged
bjanes
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« Reply #48 on: June 22, 2006, 06:17:25 PM »
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Which is why I proposed a bar chart format with each bar representing 1/3 stop of exposure range, instead of a linear or standard gamma-adjusted display. You are comparing apples to field mice.

A histogram is nothing more than a chart displaying a distribution of values over a given range. One can be made from any data set, whether audio, video, still image, uninterpolated RAW data values, ASCII text, or stock market quotes. There's absolutely no reason whatsoever RAW data values "must" be interpolated or otherwise processed into a visually pleasing image before a histogram can be made. The whole problem we're trying to solve here is the errors introduced into the exposure evaluation process when white balance and color interpolation is performed on the data prior to running the histogram generation routine. Making a histogram from the unprocessed RAW data (especially when the histogram is segmented to exactly match the camera's exposure adjustment interval) will make judging correct exposure from the histogram much easier, not harder.
[a href=\"index.php?act=findpost&pid=68865\"][{POST_SNAPBACK}][/a]

Jonathan,

See my previous post to John Sheehy. I was just comparing what I had available and don't really what you meant by the metaphor.

You do not need to process the raw image into an image in order to make the histogram, but you do need to know about the image to interpret the histogram. Is the image low key or high key and is that why the histogram looks the way it does? Are those blown highlights expendable specular reflections and worth sacrificing for better shadow detail?

Thanks for the lession histograms 101.  

Bill
« Last Edit: June 22, 2006, 06:19:00 PM by bjanes » Logged
John Sheehy
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« Reply #49 on: June 23, 2006, 12:31:38 AM »
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I have a gamma 2.2 curve that I constructed by reverse engineering of some stouffer step wedge conversions in DCRaw and ACR, which appears reasonably accurate. I applied it to the previous raw image and think that the expansion of the highlight tones makes the histogram quite a bit easier to read. I would prefer to have the x-axis scaled in f-stops as Jonathin suggests, and this would be easy to implement.

Any RAW histogram should have subdivided f-stops indicated on it (at least for the top 3 or 4 stops).  That's a given.  The vertical marker lines would have different spacings for linear, gamma, and logarithmic (f-stop) displays.

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While it might only be a matter of sematics, the concept of a gamma 2.2 "raw" image sounds a bit like an oxymoron to me.
[a href=\"index.php?act=findpost&pid=68925\"][{POST_SNAPBACK}][/a]

When does changing the axis of a chart between linear, logarithmic, or gamma-adjusted ever change the nature of the data?
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bjanes
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« Reply #50 on: June 23, 2006, 08:56:23 AM »
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Any RAW histogram should have subdivided f-stops indicated on it (at least for the top 3 or 4 stops).  That's a given.  The vertical marker lines would have different spacings for linear, gamma, and logarithmic (f-stop) displays.
[a href=\"index.php?act=findpost&pid=68947\"][{POST_SNAPBACK}][/a]

The spacing for linear and gamma encoding can be deomonstrated by the Photoshop histograms of a Stouffer step wedge. The patches vary by 0.1 in density units, or 1/3 stop per patch. In the linear encoding, the first 3 patches (1 f/stop) occupy the right half of the histogram, the next stop the next fourth of the histogram, etc.

In the gamma encoded data, the distribution of the f/stops is more uniform, but there is still compression on the left. As you pointed out previously, this is a power function.

If one could plot the log base 2 of the exposure, the desired f/stop display would result, but I don't know of a way to do this in Photoshop.

[attachment=733:attachment]

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When does changing the axis of a chart between linear, logarithmic, or gamma-adjusted ever change the nature of the data?
[a href=\"index.php?act=findpost&pid=68947\"][{POST_SNAPBACK}][/a]

The underlying data are not changed. However, if you apply a log scale to the graph, this is the same as applying a log transformation of the data and plotting on a linear scale--one has effectively applied a transformation with the use of the log scale.

By your reasoning, a linear raw image and a gamma transformed image are the same, since the underlying raw data have not changed. However, they have a quite different appearance. Again, sematics and I would not press the point.
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John Sheehy
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« Reply #51 on: June 23, 2006, 01:43:04 PM »
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If one could plot the log base 2 of the exposure, the desired f/stop display would result, but I don't know of a way to do this in Photoshop.

You need something like Mathcad to do this.  You can make charts with an algebraic term as the that includes the array as the data source, with a full range of mathematical operators to work with.

IRIS has a logarithmic display function.

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By your reasoning, a linear raw image and a gamma transformed image are the same, since the underlying raw data have not changed. However, they have a quite different appearance. Again, sematics and I would not press the point.
[a href=\"index.php?act=findpost&pid=68971\"][{POST_SNAPBACK}][/a]

I don't think so.  A display and a histogram serve two completely different functions; a histogram is for analysis, and the scaling that best suites the analysis is the one you want to use.
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Jonathan Wienke
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« Reply #52 on: June 23, 2006, 05:45:00 PM »
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Jonathan,

See my previous post to John Sheehy. I was just comparing what I had available and don't really what you meant by the metaphor.

You do not need to process the raw image into an image in order to make the histogram, but you do need to know about the image to interpret the histogram. Is the image low key or high key and is that why the histogram looks the way it does? Are those blown highlights expendable specular reflections and worth sacrificing for better shadow detail?

Thanks for the lession histograms 101.  

Bill

OK, I misunderstood the point you were trying to make. The second paragraph of my post was directed at Anon E Mouse's objections to my RAW histogram proposal. He seemed to be laboring under the mistaken impression that an image had to be converted to a visually pleasing RGB format before a useful histogram could be made from it. That is obviously not the case, and I was attempting to explain why.
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Jonathan Wienke
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« Reply #53 on: June 23, 2006, 05:52:21 PM »
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Jonathan's 1/3 stop bars seem unnecessarily coarse to me.  1/3 stop markers I would like, but data should be finer, IMO.

I don't see much value in granulating the histogram any finer than the camera's minimum exposure adjustment. If the camera can adjust exposure in 1/10-stop increments, then a histogram that displays the data broken down in 1/10-stop segments makes sense. But if the camera only adjusts in 1/3-stop increments, anything finer than that is kind of a waste.
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John Sheehy
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« Reply #54 on: June 24, 2006, 03:09:55 PM »
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I don't see much value in granulating the histogram any finer than the camera's minimum exposure adjustment. If the camera can adjust exposure in 1/10-stop increments, then a histogram that displays the data broken down in 1/10-stop segments makes sense. But if the camera only adjusts in 1/3-stop increments, anything finer than that is kind of a waste.
[a href=\"index.php?act=findpost&pid=69016\"][{POST_SNAPBACK}][/a]

I don't see how more information, which takes up the same amount of room, is a waste.

Say you realize that your favorite RAW converter doesn't utilize the top 1/6 stop, and you want to draw the line somewhere else; way you want to see the finer histogram to try to figure out exactly what is close to clipping; say you want to know if that top 1/3 stop is populated more towards the top or bottom; in all these cases, 1/3 stop is too coarse.  I don't see the need to jump from our current histogram, with 200+ levels, to one with only 32.
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bjanes
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« Reply #55 on: June 26, 2006, 09:01:04 AM »
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I don't see how more information, which takes up the same amount of room, is a waste.

Say you realize that your favorite RAW converter doesn't utilize the top 1/6 stop, and you want to draw the line somewhere else; way you want to see the finer histogram to try to figure out exactly what is close to clipping; say you want to know if that top 1/3 stop is populated more towards the top or bottom; in all these cases, 1/3 stop is too coarse.  I don't see the need to jump from our current histogram, with 200+ levels, to one with only 32.
[a href=\"index.php?act=findpost&pid=69066\"][{POST_SNAPBACK}][/a]

If you want Jonathin's histogram to include color information as per his recent demonstration, then hundreds of levels would result in bars that are so narrow that they couldn't be seen on the camera display. If you want smaller bins, then the color histograms would have to be shown separately as they are on current cameras.

Personally, I think 0.3 EV resolution is sufficient. It is true that the manual settings can only be set in 0.3 EV increments, but if one is (horror of horrors) in an automatic mode (say aperture perferred) the shutter can be ajdusted in finer increments. Finer resolotion might be useful in a studio situation where one is shooting slowly with precision, but would have little use in field use.
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John Sheehy
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« Reply #56 on: June 29, 2006, 07:49:45 AM »
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If you want Jonathin's histogram to include color information as per his recent demonstration, then hundreds of levels would result in bars that are so narrow that they couldn't be seen on the camera display. If you want smaller bins, then the color histograms would have to be shown separately as they are on current cameras.

That's exactly what I want; 3 graphs for RGB (my preference) or one graph for greyscale RAW.  Bars would only be practical for greyscale; the colored bars as Jonathan posted them are too cluttered; single-line graphs super-impose more clearly.

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Personally, I think 0.3 EV resolution is sufficient.

Aren't many things already inferior because of what people thought would be sufficient?

Why repeat history?

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It is true that the manual settings can only be set in 0.3 EV increments, but if one is (horror of horrors) in an automatic mode (say aperture perferred) the shutter can be ajdusted in finer increments. Finer resolotion might be useful in a studio situation where one is shooting slowly with precision, but would have little use in field use.
[a href=\"index.php?act=findpost&pid=69159\"][{POST_SNAPBACK}][/a]

Not everything "in the field" is a moving target.

I want to see the fine humps in my histogram.  Regressing to big fat bars serves no useful purposes, and reduces information, and aliases the sampling.
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