Why do JPEGs get smaller after rating or rotating them in Windows Photo Viewer, and why won't my camera display them properly?
Asked 9/6/2018
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When I rate or rotate JPEGs from my Nikon D5500 in Windows Photo Viewer on Windows 7, the files become about 10% smaller even though the pixel dimensions stay the same and visual comparison shows no obvious image changes. After copying those edited files back to an SD card, my camera either shows a blurred preview unless I zoom, or gives an error after rotation. EXIF data in the file properties appears unchanged. What is Windows changing in the JPEG, and does this mean the image quality is being degraded?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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It is possible that Windows re-encodes the picture as interlaced/progressive JPEG(*), more suitable for the web. These are usually slightly smaller than their "baseline" equivalents. Although quite old (20 years now) the "progressive" variant is often not supported by embedded software, such as LCD photoframes and... cameras, which would explain why the picture is not displayable in your camera. Pictures editors (Gimp et al.) usually can save as plain JPEG (even if "progressive" is often the default).
Several tools can report the baseline/progressive encoding option:
exiftool -EncodingProcess- the
filecommand in Unix/Linux identify -verbosein the ImageMagick toolbox (search "Interlace" in the output)
(*) in that format a low-res version (1 in 4 scan lines, then 1 in 2) of the complete image is available without needing to read the whole file, this was very useful in the 90s when you were downloading such images at modem speeds.
Originally by user75947. Source · Licensed CC BY-SA 4.0
user75947
7y ago
0
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Windows Photo Viewer may be changing the JPEG encoding rather than the visible image data. A likely cause is that it rewrites the file as progressive/interlaced JPEG instead of baseline JPEG, or alters the embedded preview thumbnail. That can reduce file size without obvious quality loss, but some cameras only reliably display baseline JPEGs or expect a camera-generated thumbnail, which explains the blurry preview or playback error.
For rotation, Windows Photo Viewer can do lossless JPEG rotation when the image dimensions fit JPEG block boundaries, so the main image may not be degraded at all. The smaller file size could come from changed encoding, thumbnail updates, or compression optimizations rather than loss of pixels or EXIF.
If you want to confirm what changed, compare the original and edited files with a tool like ExifTool and check the JPEG encoding process as well as all metadata groups. In short: the image may not be visibly degraded, but the file structure can still be changed in ways your camera does not like.
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