Many people, including experts, have told me that EVERY saving to a jpeg adds loss of quality, even adding IPTC info, etc. There is so much misinformation about this, I'd like to ask those who really know the details about jpeg algorithms and how they work.

From what I have researched and tested,if there is a small change or no change in the image (say you added IPTC info)there is little or no degradation. If one saves at the same setting. It seems that saving to a higher setting may actually DEGRADE the image. For example, if I save a file at a 7 setting, open it up, and save it again at 7, there is no additional jpegging, and the quality is retained. If however, I set it to 12, the file balloons to about 300% the original size, and I assume the file is completely re jpegged, adding more loss. So, if one does this, not only are you adding a huge amount to the file size, you are degrading the image further, even with a higher setting.

Can anyone clarify this 100%?

Also, if one were to say, change a small portion of the image (say correct the red eye), and save to the same previous setting, does the new round of jpegging just happen in the changed pixels, or to the entire image?

Related to this, is how does one make sure that the current saving is on the same level as the previous one? Every time I save, there doesn't seem to be any rhyme or reason as to the level of compression that is the default setting on the window. For example, I could be saving all day to a level 8, and then for no reason, 10 or 11 will be the default. Sometimes even the "progressive" option is chosen, rather than baseline, even though I never ever chose progressive. Or, I could be saving a whole bunch of files, and it won't ask me for a setting, I assume the previous setting is being used. HOw does one confirm the same level compression is being used, and how does one set this without the program seeming to choose random settings???

Many thanks in advance to all that answer. Any good, reference websites to explain this whole thing would also be most welcome.

Stuart

John_FR_Powers

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Jpeg settings, defaults, when saving and quality issues