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Author Topic: Understanding Camera RAW article  (Read 4695 times)
mlfrost
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« on: September 30, 2013, 01:30:22 PM »
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I think there may be a misprint in this article.  It reads:
 (The remaining levels out of 256 are for the f-stops beyond the 5 in this example)

 I think it means that the remaining levels out of 256 are for the f-stops beyond the lowest 2 in this example.

Am I correct or did I misunderstand the wording?

http://www.luminous-landscape.com/tutorials/understanding-series/u-raw-files.shtml

Mary Lou
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Eric Myrvaagnes
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« Reply #1 on: September 30, 2013, 03:50:14 PM »
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I think you are correct. The passage you cite seems to refer to figure 2, showing the light levels for an 8-bit jpeg image, and in that table the first two steps have a total of 20+27=47 levels. Note that the total for all five steps in that table comes to 203 levels, so 53 levels presumably are squeezed to the edges at top and bottom and don't really hold detail.

Does this help?
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mlfrost
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« Reply #2 on: September 30, 2013, 05:14:21 PM »
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Yes, that helps.  I hadn't noticed that the total was less than 256.  Thanks for pointing that out.

Perhaps you can answer a related question on Camera RAW.  When I open an image captured in RAW format, I see an image and histogram in Adobe Camera Raw.  Am I correct in assuming that the opening image and histogram are created from the camera's jpeg settings?

I know that it is critical to keep the camera's jpeg settings as neutral as possible because the LCD image on the back of the camera and its histogram come from those jpeg settings and don't directly relate to the image capture in RAW.

Thanks for your help.

Mary Lou
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Schewe
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« Reply #3 on: September 30, 2013, 05:20:22 PM »
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Perhaps you can answer a related question on Camera RAW.  When I open an image captured in RAW format, I see an image and histogram in Adobe Camera Raw.  Am I correct in assuming that the opening image and histogram are created from the camera's jpeg settings?

No...Camera Raw rips the entire raw image and does a demosaic and application of ACR defaults to generate the preview and the histogram (which is displaying the resulting graph based on the output color space set in ACR Workflow Options). ACR doesn't use ANY settings from the camera's JPEG settings and the only thing that ACR uses from the raw file is white balance info and the ISO metadata...
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bjanes
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« Reply #4 on: September 30, 2013, 06:09:21 PM »
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I think there may be a misprint in this article.  It reads:
 (The remaining levels out of 256 are for the f-stops beyond the 5 in this example)

 I think it means that the remaining levels out of 256 are for the f-stops beyond the lowest 2 in this example.

Am I correct or did I misunderstand the wording?

http://www.luminous-landscape.com/tutorials/understanding-series/u-raw-files.shtml

Mary Lou


The number of levels in each f/stop has been misunderstood by those advocating expose to the right and others. In a 14 bit capture, the brightest f/stop contains 8192 potential levels, but those are dithered by noise and the actual raw file contains far fewer effective levels. Furthermore, the human visual system (Weber-Fechner law) is sensitive only to a 1% change in luminance (see Norman Koren). He also has an expanded explanation of levels and the effect of gamma on the distribution of levels in each stop. If one needs to tease out highlight detail with editing, it is useful to have more than the required number of levels but there is no way to use 8192 levels even if they were attainable.

DXO measures the tonal range, which is the number of bits necessary to encode the range of a sensor. The Nikon D800e has 14 bit files but the tonal range is only 9.83 bits at ISO 100, and this is a highly rated camera.

Bill
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mlfrost
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« Reply #5 on: October 05, 2013, 12:02:14 AM »
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What is the explanation for why the brightness levels don't add up to 256 for jpeg, or 4,096 for 2 bit capture?

What happens to the other brightness levels that are quoted but not accounted for?

Mary Lou
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elied
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« Reply #6 on: October 05, 2013, 05:43:19 PM »
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For the most part they are lost because of quantization (rounding off) errors that accumulate during the initial rendering of the analog sensor data as digital Raw data and then again during the processing of that data to create the jpg.
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mlfrost
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« Reply #7 on: October 05, 2013, 09:51:12 PM »
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Thanks for the information.

Mary Lou
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