Doug Kerr
Well-known member
A while ago, Asher mentioned, rather in passing, while discussing some topic, that in some Leica cameras, a correction for the "color cast" of the lens in place was applied to the raw data before it was delivered in the raw file. In later discussions, he told me that he thought the same might be so of Hasselblad digital cameras and PhaseOne digital camera backs.
I wasn't aware of this kind of arrangement (there are so many things of which I am not aware!), and I was interested to learn more about it. I had a number of questions, including:
• If there any chance that what actually happens here is not that the correction is made to the raw data but rather that a description of the needed correction (based on the lens in place) was included in the raw file so the correction could be applied by the raw development software. (Not of course that I thought Asher was possibly confused as to the details here, but I just like to be sure.)
• Is this same thing done with regard to other lens-dependent corrections, such as vignetting and geometric distortion?
On my way to learning more about this matter, I came to realize that many (if not all) Leica cameras (not including those made by Panasonic) deliver their raw data as a DNG file.
Today I got very complete information on this from Sandy McGuffog, host of the ChromaSoft blog, and the developer of AccuRaw, a raw developing program for the Mac. Here's the scoop.
• In the Leica M system, the lenses, having mostly been designed for film cameras, typically have very little geometric distortion, but may exhibit a substantial color cast and/or (especially for wide-angle lenses) significant vignetting.
In the digital context, it is very practical, technically, to correct for both lens color casts and vignetting in the raw data (that is, to modify the raw data to effect the corrections before the raw data is delivered in the DNG file), and Leica in fact typically does that (assuming that the lens involved identifies itself to the body, which is done with an optical coding system).
Then, no later part of the processing chain need be concerned with this matter. It is as if the lens exhibited no color cast nor vignetting.
• In other Leica cameras, such as the Q series, the lenses are designed for the digital context. They are typically very low in color casts and vignetting, but may have significant geometric distortion.
It is not really practical to correct for geometric distortion in the delivered raw data. Rather, in those Leica cameras, "instructions" for applying that kind of correction are included in the raw (DNG) file, They are used to control the application of the correction while the image is being developed by external raw development software. These instructions are in the form of "opcodes" (short for "operation codes"), which we can think of as macros that perform the corrections. (The DNG format, in its present from, gladly accommodates such opcodes.)
Now, while I was seeking to lean more about this whole topic, I asked Check Westfall of Canon USA whether any Canon cameras make any kinds of "corrections" to the raw data before it is delivered in a raw file. His answer was essentially, "not that he knows of", with the caveat that Canon does not tell even him everything that goes on in a Canon raw file.
So that's what we now know here about that.
Best regards,
Doug
I wasn't aware of this kind of arrangement (there are so many things of which I am not aware!), and I was interested to learn more about it. I had a number of questions, including:
• If there any chance that what actually happens here is not that the correction is made to the raw data but rather that a description of the needed correction (based on the lens in place) was included in the raw file so the correction could be applied by the raw development software. (Not of course that I thought Asher was possibly confused as to the details here, but I just like to be sure.)
• Is this same thing done with regard to other lens-dependent corrections, such as vignetting and geometric distortion?
On my way to learning more about this matter, I came to realize that many (if not all) Leica cameras (not including those made by Panasonic) deliver their raw data as a DNG file.
Today I got very complete information on this from Sandy McGuffog, host of the ChromaSoft blog, and the developer of AccuRaw, a raw developing program for the Mac. Here's the scoop.
• In the Leica M system, the lenses, having mostly been designed for film cameras, typically have very little geometric distortion, but may exhibit a substantial color cast and/or (especially for wide-angle lenses) significant vignetting.
In the digital context, it is very practical, technically, to correct for both lens color casts and vignetting in the raw data (that is, to modify the raw data to effect the corrections before the raw data is delivered in the DNG file), and Leica in fact typically does that (assuming that the lens involved identifies itself to the body, which is done with an optical coding system).
Then, no later part of the processing chain need be concerned with this matter. It is as if the lens exhibited no color cast nor vignetting.
• In other Leica cameras, such as the Q series, the lenses are designed for the digital context. They are typically very low in color casts and vignetting, but may have significant geometric distortion.
It is not really practical to correct for geometric distortion in the delivered raw data. Rather, in those Leica cameras, "instructions" for applying that kind of correction are included in the raw (DNG) file, They are used to control the application of the correction while the image is being developed by external raw development software. These instructions are in the form of "opcodes" (short for "operation codes"), which we can think of as macros that perform the corrections. (The DNG format, in its present from, gladly accommodates such opcodes.)
Now, while I was seeking to lean more about this whole topic, I asked Check Westfall of Canon USA whether any Canon cameras make any kinds of "corrections" to the raw data before it is delivered in a raw file. His answer was essentially, "not that he knows of", with the caveat that Canon does not tell even him everything that goes on in a Canon raw file.
So that's what we now know here about that.
Best regards,
Doug
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