nontroppo wrote:OK makes sense! Exiftool doesn't show what the JPEG compression is either, and what I gleaned online is this is not part of the JFIF spec to store this info at all. It probably doesn't matter too much anyway as JPEG is lossy, so whenever a 75% JPEG is saved again at 75%, it will always gradually become worse AFAIK.
That's a good point. What's frustrating, though, is that if you save freshly-created JPG data in Cocoa with no compression, it ends up bigger - a 60kb file altered only to have its dpi changed becomes 120kb. Otherwise I could save without compression, but as it is I have to put an option in the Preferences.