Why does Microsoft Photo Viewer reduce file size after rotating a jpeg photo or giving it a star rating (Windows 7)?

Using Microsoft Photo Viewer to classify my jpeg photos after uploading them from my camera, I have noticed this behavior: if I rotate an image (say 90 degrees), file size goes down. For example, it brings a 14 MB jpeg to 13 MB. The same happens if, rather than rotating the image, I give it a star rating (say 2 stars) and then save. If I compare a star-rated photo with the original, however, there does not seem to be any difference at all.

So the question is, why did that 1 MB disappear? Did the photo lose any information or image quality? It does not seem to be the case, but then why the file size reduction?

Please find below a sample original photo and its star-rated version. Original is 3.7 MB, star-rated is 3.2 MB, although they seem identical (including meta-data).

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JimWynne

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Jpeg is a "lossy" format--every time a file is saved, it gets compressed. This will even out sooner or later as the compression limit of the format is reached, but a small amount of loss of resolution is to be expected.

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paolobinetti

Hi Jim, thank you for your answer, but this does not seem to be my case. Indeed, I have tried cropping both the above images at the same location, then compared the files, and they seem strictly identical. So I do not seem to be losing anything, except maybe in the first save, but what it is I lose, I can't see ...

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JimWynne

Volunteer Moderator

Hi Jim, thank you for your answer, but this does not seem to be my case. Indeed, I have tried cropping both the above images at the same location, then compared the files, and they seem strictly identical. So I do not seem to be losing anything, except maybe in the first save, but what it is I lose, I can't see ...

You wouldn't necessarily be able to see the difference unless the images were blown up. They would appear the same when viewing them on a computer screen, but if you were to print them to a large format you would see a difference.

Why does Microsoft Photo Viewer reduce file size after rotating a jpeg photo or giving it a star rating (Windows 7)?

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