saving from jpg to jpg file size increase. why?

Hi all, i need an input from you who are more knoledgeable than me in digital darkroom.

I received scan of old slides, the files are jpg, the image present a slight magenta cast so I adjusted it using photoshp (auto color) then

saved again as jpg selecting the max quality (12). I expected to see some image degradation but to my eyes ther is not a noticeable

difference after the retouching and saving (no artifacts, no loss of details) but what surprised me more is that the new file is bigger

(about 2x) than the original, how is it possible? Should'nt jpg compress the files (causing loss of image quality, may be not visible)

causing reduction of its size? Why it increase? Thank you in advance for your help.

Diego

<p>I'm not sure that a JPEG to JPEG resave would compress the data in the same way that a TIFF to JPEG would. It's also possible that the original JPEG was created with a quality setting less than 12, so when you saved it at 12, it would have been upsized.</p>

<p>It's because you <em>decreased</em> the compression by choosing "12'- hence a larger file.</p>

<p>At 12 setting it would take a long time, resaving on itself, to begin showing compression effects. Save at "3" and you'll see the effects very soon, but will have relatively tiny files.</p>

<p>When you "save for web" you further reduce the amount of information in the file.</p>

It's because you decreased the compression by choosing "12'- hence a larger file.

 

JDM an william, this imply that in the jpg file ther are data compressed but not lost? Does it really work this way? I'm not

so sure. Or I misunderstood your statement? Sorry I'm not very knoledgeable about digital photography

<p>Data are encoded into smaller packets in compression, so to speak, and are 'restored' when opened (and thence, decompressed). But some data are <em>lost</em> in compression--the degree of compression determines how much loss you are willing to accept.</p>

<p>Hence, jpeg is known as a "lossy" file format.</p>

<p>Also, some programs may add their own file 'data' into the mix, so size of a file may grow when re-saved in a different program. It's more likely, I think, that the original scans you got were saved at some medium setting and when you opened and saved them again at '12' setting you increased the size of the file. Unfortunately, while this would make the file bigger, it does not recover the lost information from the original scan compression.</p>

<p>Other variables such a bit-depth are also involved, depending on how the file was set up originally and what you have done to it in re-saving.</p>

<p> </p>

Thank you JDM I understand now (I know jpg is a lossy file format, that is because I was confused when i saw the

increased size), other variable are involved, that make sense.

Even if i do not recover the lost data I suppose the further loss due to the saving after color correction would be minimal

and I prefer to have those pictures with the proper colors so I will go for that route: open; color adjust, saving with the

maximum quality. Thank you JDM and William for your help.

Ciao!

Diego