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If I crop a JPEG photo in iPhoto and then export it as JPEG, is the exported version re-compressed a second time, or does iPhoto somehow crop without re-encoding, thus avoiding further image degradation?

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Every time you save a JPEG file, you have to re-compress it. It's likely that if you save it at 100% quality, you won't be able to see any quality difference from the original. This is because the first compression already removed all parts that are invisible to the human eye. Another compression might remove something more, but I doubt it will be noticeable. So, I don't have a real answer to your question, but I'm pretty sure that you won't notice a second compression.

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+1 for the 100% quality suggestion, but -1 for the "pretty sure", so no change in vote. – Randolph West Jan 24 '11 at 5:07
So you're saying that if an image has already been JPEG-compressed, then recompressing at the same quality level will not lose any image quality whatsoever? Or are you saying that there may be some loss of quality, but it won't visibly reduce the image quality? – Ryan Thompson Jan 25 '11 at 4:25
I'm saying that there may be some loss of quality, but it won't be visible. Yet, YMMV. – Dario Solera Jan 25 '11 at 10:29
This is my experience also. I saved a raw image as 90% JPEG, and there was noticeable (when looking at the pixel-level) quality loss. Then I repeated 20 times saving the previous JPEG as a new 90% JPEG. The final JPEG had extremely little change compared to the original JPEG, and far less quality loss than saving as JPEG in the first place. So to conclude, if you start saving as JPEG anyway, then don't worry about resaving in the same quality level. – Peter Oct 15 '11 at 21:56
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