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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?

Ryan C. Thompson
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  • Related: [How to perform a lossless crop of a JPEG image?](http://superuser.com/questions/319899/os-x-how-to-perform-a-lossless-crop-of-a-jpeg-image) – Denilson Sá Maia Mar 13 '12 at 03:19

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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.

Dario Solera
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  • +1 for the 100% quality suggestion, but -1 for the "pretty sure", so no change in vote. –  Jan 24 '11 at 05:07
  • 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 C. Thompson Jan 25 '11 at 04:25
  • 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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    This answer is incorrect. You can crop and save a JPEG without recompressing, as long as you crop along block borders. There are several programs out there that support JPEG lossless cropping. – Matthew S Feb 20 '22 at 10:35