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When I'm saving a JPG file, Paint Shop Pro (Windows) and Gimp (Linux) show a pop-up to choose the JPG compression level. The apps have something like 85 as default value, and I think reducing the image quality in exchange for a smaller file size made sense bank in 1996 or perhaps even in 2006.

What would a sensible compression level be today?

This is not about high-end lossless photography, nor about small graphic files for web sites — I'm editing vacation photos made with a regular digital compact camera. Given that the photo files are already JPG format, I want to keep as much quality as possible without switching file format (because all the apps work with JPG). The file size is 2-5MB per photo and I don't care if the file sizes are 10% bigger or smaller - disk space is not a concern, but picture quality is (although not professional-level).

My understanding is that saving a JPG file with "85" reduces the quality, while "100" retains the quality of the original (which doesn't mean it's perfect either).

Why should I choose the default value "85", or why should I choose "100" instead?

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You also need to optimise for sensible resolution - no point keeping full 12MP pix when you're going to view it on a screen that's sub megapixel resolution. Thats a better optimisation than quality –  Journeyman Geek Jun 26 '12 at 9:50

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I don't think it's correct that 100 keeps the quality of the original. If you are re-compressing a compressed JPEG, you'll always be losing some quality. The reason that 85 is the default is that the perceived quality increases in an approximately linear way to the quality, but the file size grows exponentially, as you'll see from this graph. That means that to go from 85 to 100 in quality, the file size has doubled, but the "quality" has only increased by 17%.

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