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I have a Canon Rebel T5. I can shoot in RAW or RAW+JPEG. When I shoot in the latter, it stores both a preview with the RAW file, as well as the JPEG.

I've been using Digikam to manipulate my photos, but converting from RAW to JPEG or png takes forever. I didn't seem to get much out of it (like fixing exposure), except for the cases where I set my camera to shoot in black & white. Then I have the RAW if I decided that I did want the actual colors.

I did notice that the RAW image tended to have a lot more noise, though running wavelet noise reduction seemed to produce image quality roughly equivalent to the embedded JPEG. But this seemed like I was just taking a lot of time when I could just shoot in JPEG and get the same result.

Though today, I just discovered how to use exiftool to extract the preview from my RAW file. To test it out, I shot RAW+JPEG, and extracted the preview from my RAW file. I got an interesting surprise - the extracted file was almost 1/3rd the size.

Comparing the no-longer-embedded preview and the high quality JPEG, I couldn't spot any differences visually at 100% crop:the same? images

So then I ran ImageMagick's compare tool with fuzz. Using a fuzz value of 5% gives me almost no differences.

One difference I did notice is that the preview has almost no EXIF data. But surely it's not 4MB worth of EXIF data, right?

So why does it appear that there is little difference in the images, despite having two wildly different filesizes?

  • JPEG compression throws away information hard to discern by the human eye. Usually this is evident in sharp contrasts being slightly less sharp. This process, however, is irreversible, so if you want to manipulate the image later, the information thrown away is needed to do this well. The harder the compression, the worse this gets. – Thorbjørn Ravn Andersen Mar 8 '16 at 8:36
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The JPEG image saved by the camera uses a JPEG quality level of 98, while the preview extracted from the RAW file only uses a quality level of 81. You can check these values with ImageMagick: "identify -verbose foo.jpg"

Depending on the image content, it can be quite hard to visually tell the difference between these compression levels, but the file size difference can be significant.

  • The difference is likely to become much more apparent if you decide to do further processing on the files, so keep that in mind, too. – junkyardsparkle Feb 23 '16 at 0:37
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    The difference will also be more apparent in areas of an image with hard edges between bright and dark areas that are in sharp focus. Lack of detail and contrast, such as the area in the example, will tend to minimize the differences in the quality settings. – Michael C Feb 23 '16 at 3:48
  • I'll have to try some other tests - I couldn't find any spot in this shot that seemed sharp so I probably wasn't very steady. I did look at at least one sharper images that I took and I seemed to get a similar comparison. – Wayne Werner Feb 23 '16 at 13:30
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In my opinion, the preview image looks blotchy when compared to the high-quality version. Granted, you'll have to look for it, but you'll see coloured JPEG compression artifacts, most noticably (IMO) in the top left corner of the frame.

  • I don't think that the chroma noise is a JPEG compression artifact. Taking the original image and saving it with the same quality setting as the preview image does not introduce this kind of noise. In fact, this allows the file size to become even smaller. I'm pretty sure that the camera did apply some noise reduction to the JPEG image that it did not apply to the preview embedded in the RAW file. – Jules Feb 23 '16 at 0:44
  • But the blotches don't look like noise to me, they appear rather larger-scale. Maybe the camera even did apply some kind of noise filter to the preview that it didn't to the JPEG image; I seem to remember similar blotchy effects from too strong chroma smoothing in post-processing. – Thomas Lotze Feb 23 '16 at 0:53

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