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When saving JPG images with Pixelmator or Photoshop, I select a quality from the scale 1–100 that gives optimum file-size.

Later on, I sometimes take these images and crop them a little more (not always possible to find the originals), but when re-saving I can no longer know what quality I saved them with.*

Are there any Mac programs that will reveal the quality that the JPG was saved at?

* Perhaps this is not the right way of going about it, because it seems that, for example, if I'm already saving a file that was saved lossy at 59 quality previously, and then go ahead and I save it again (even at 59 or 99), it will loose even more quality? Correct?

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For the side question, yes, saving a jpg as a jpg compresses it again, losing more quality each time. Try to always keep the original and try to never save a jpg as a jpg. Topical side note: since each time a Facebook image is uploaded, it gets jpg-compressed, it's possible to guesstimate how many generations of friends any one of those red/pink equals sign icons has been through by looking at the jpg compression; as a way of estimating the spread of a viral campaign. –  user568458 Apr 3 '13 at 9:51
You can't. Once you save out the JPG, it's now a new 'original' image. It has no 'memory' of the original to figure out how much it was compressed. –  DA01 Apr 3 '13 at 21:44
I'm still trying to figure out how that information would benefit you? You can see just by looking at the image if it has good enough quality. –  Joonas Apr 3 '13 at 22:46

1 Answer 1

When saving images as .jpeg you always lose information. The dialog basically asks you how much information you would like to lose in favor of smaller size on disk (1 = most loss, 100 = least loss). There is no way to tell what you originally selected and the only use would be to have a history of your workflow because this loss is irrecoverably applied to the image when saving.

Your assumption is right; every time you open the compressed image and save it again (with anything else but setting the quality value to 100) you will lose more information.

Because of this I would encourage you to archive all the original pictures in full size and a lossless format.

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