Why is the JPEG preview embedded in my Canon RAW file much smaller than the separate high-quality JPEG?
Asked 2/22/2016
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2 answers
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On a Canon Rebel T5, when shooting RAW+JPEG, I noticed the JPEG preview extracted from the RAW file is much smaller than the separately saved high-quality JPEG, even though they look almost identical at 100% view. The embedded preview also contains much less EXIF metadata, but not enough to explain the size difference.
Why can two images that look so similar have such different file sizes? Is the embedded preview using different JPEG compression or quality settings than the camera’s full JPEG output?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
5
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.
Originally by user47528. Source · Licensed CC BY-SA 4.0
user47528
10y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. The most likely reason is that Canon stores the embedded RAW preview as a more heavily compressed JPEG than the separately saved JPEG.
From the community tests, the full JPEG was saved around JPEG quality 98, while the embedded preview was around quality 81. That difference can reduce file size a lot even when the images still look nearly identical in normal viewing or even at 100%.
JPEG size is strongly affected by compression level, and visual differences between high and slightly lower quality settings can be subtle. But if you inspect carefully, the lower-quality preview may show compression artifacts such as blotchiness or colored artifacts in detailed areas.
The smaller amount of EXIF data in the preview may contribute a little, but it would not explain a multi-megabyte difference by itself.
So the short answer is: the embedded preview is smaller mainly because it is encoded as a lower-quality JPEG, not because the image content is dramatically different.
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