I have two exact same pictures and they are both the same x:y size. But one of them is 105 KB and the other one is 109 KB. Which of them is of better quality?


Greater KB — bigger files — means more data, but doesn't necessarily mean more information. A file with a lot of sharp detail will be bigger than a blurry one, but also a file with lots of noise will be bigger than a clean one.

There's no way to tell anything about image quality from small differences in file size — and that's even if you have a solid, workable definition of "quality". There isn't a standard one; quality can mean different things to different people, and a given photo can be high in certain technical aspects and flawed in others. And a completely technically-flawed photo can be a complete success in other ways.

In your particular case, especially if the images are visually identical, it's likely that the difference isn't in the image at all, but in the metadata. Metadata is encoded information about the image, like the time it was taken, technical camera settings, color information, and so on. And in fact, as @flolilolilo points out, such a small difference could simply be a difference in encoding with no real change in included information at all.

  • Also, it could come down to compression efficiency: E.g. a better suited Huffman-table could be used. – flolilo Nov 26 '18 at 15:30
  • True — there might not even be more data! – mattdm Nov 26 '18 at 15:30

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