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In Lightroom CC, I'm working with *.jpg files imported from a cell phone (Samsung Gallaxy S9), not RAW files.

I've made changes inside Lightroom and go to export my images.

(Q) What quality should I use when exporting to maintain the same quality as the original jpeg image?

My desire is to not introduce artifacts or decrease the quality nor make the file significantly larger.

I would have thought Quality = 100 to get the best quality, but in some quick tests the file size grew instead of remaining roughly the same size as the original. I'm not sure what this means if anything.

In the tests that I did the changes were simply crops so they should have resulted in smaller files than the original.

I searched for an answers and found these links helpful:

  • Does JPEG-to-JPEG export Lightroom reapply compression? - Asks if an original image is Quality 80, and it's changed and re-exported, should the 2nd export be "Quality 100" or "Quality 80".

  • Google Search for lightroom exporting jpg images what quality should I use? - Interesting article that explains lighroom export's quality works in bands where 93 to 100% were identical, 85-92% were identical. So interesting article, but it did not answer my question.

  • Google search for Does jpeg file store the quality used when compressing it? but nothing useful showed up. If the JPEG standard stored the compression technique and values used to generate the image than it might be possible to lookup the original compression used and use the same value when creating the exported jpeg file.

  • Google search What quality should I use for JPEG? says: In general, quality 90 to 100% and higher are high quality, 80-90% are medium quality and 70-80% is low quality. Makes complete sense (not!).

In my case, I'm dealing with images taken from a cell phone so I'm not sure of the original "Quality" of the JPEG image.

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    "nor make the file significantly larger". Depending on the extents of your edit, I'm not sure this is doable together with "not introduce artifacts or decrease the quality". – Lasse V. Karlsen Dec 31 '19 at 23:15
  • You'll just have to experiment with the quality settings to find the one that produces the smallest file that avoids introducing artifacts you find too severe. – L. Scott Johnson Jan 2 at 14:58
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    Any compression with JPEG is lossy, you can only strive for acceptable loss. Alternatively you can save using a lossless compression, both PNG and TIF are widely supported. – user10216038 Jan 2 at 21:53
  • @LasseV.Karlsen, I realize that I should have stated something like that. I've updated the question to include that hugely important point. When I experimented, I simply cropped images which should have resulted in files smaller than the original. I realize if I enhanced the colors, or did other things to change the image so that it compressed differently, then I'm really comparing apples to oranges (not a fair comparison). – PatS Jan 3 at 12:36
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You can't

In order to apply any modifications to a JPG file, it needs to be fully decoded into plain image data first(1) - in memory the data is much bigger, as it's necessary to store all RGB-Bytes directly. If you save the modified image it's just like doing a completely new JPG compression on the data. This will cause additional artifacts. Since this new compression step has it's own parameters, a file size increase comparing to the original can very likely occur.

The only thing you can do other than adjusting the settings by trial-and-error is to use the file size parameter during export to define an upper bound for the image size.

(1) There are some possibilites to loslessly rotate or crop a JPG, but this most likely doesn't apply for this answer.

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