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It has been hypothesized elsewhere that the reason why OS/X Preview program presents Apple iPhone DNGs in such a way that they look inaccurate (i.e. "bad") vis a vis the actual objective scene details is that these DNGs contain embedded JPEGs, whereas DNGs from other cameras e.g. Canon ELPH running CHDK freeware, do not look inaccurate and therefore look "better" in Preview despite these devices both having a similar sensor size.

Given this possibility (Apple DNGs having a JPEG preview), how can I remove (preferably from the command line) the included JPEGs so that I can see the true RAW pixels from the iPhone?

Preview's rendering of a DNG: https://imgur.com/TyGMxEs

Addendum: It's confirmed, CHDK DNGs do not have a preview JPEG. That's why they look good in OS/X Preview.

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So, good news and bad news. Good news is that you can replace the embedded JPEG with ExifTool (as shown in Can I embed existing JPEG's into DNG's as a preview?). Or, you can use dcraw -e to simply extract the preview to look at it separately.

The bad news is: this is extremely unlikely that Preview will fall back to doing its own demosaicing / RAW conversion. That would be a lot of work for a weird corner case, since all mainstream cameras produce RAW files with embedded previews. (That is: I am very sure of the premise of the question: Preview doesn't do conversion. It just shows the embedded preview.)

I would suggest using something other than Preview to do your comparisons. RawTherapee would be my choice for free software for OS X. That way, you're really comparing similar fruit, rather than being at the mercy of whatever processing is done to create the preview.

It's important to realize that there is no single "see the true RAW pixels" in a meaningful way. There needs to be some interpretation. The closest you are going to get is probably from FastRawViewer (proprietary software with a free trial for Mac and Windows), which presents the data with minimal interpretation. Or, you can use dcraw -h to get a quick and dirty conversion. But, I'm not really sure of the value of this; it's more practically useful to use a full program (whether RawTherapee or Lightroom or Darktable) to make the best conversion you can for each, and then compare those.

It's also very possible that the images from your Canon simply are better in the particular areas you are examining. Despite similar sensor size, the dedicated camera has the advantage of a much larger lens, and there's plenty of other factors that could come into play as well.

  • Not to mention that the DNGs from the Canon ELPH are almost certainly also being displayed by OS/X Preview from the JPEG preview embedded in them. The ELPH apparently does a better job at automatic conversion than the iPhone. – Michael C May 14 '17 at 22:46

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