Matt,
I follow what you are saying, but still don't think it is right.
I just pulled up a 1,425KB jpeg and re-saved it in Paint Shop Pro.
I purposely used the minimum compression I could and got a 1,814KB jpeg.
So I made this file BIGGER by not compressing some of the picture.
My arguement is, until I compress this new picture to a 1,425 jpeg,
I really don't have anything to loose! Saving this file again and again,
without any changes will result in nothing different... a 1,814KB jpeg and the
same zeros and ones in the final picture. So I'm happy with taking big jpegs
from my digital P&S and not waisting my time with the bigger Tiff's.
Regards, Bob S.
If you use less compression in the second save vs. the first save, you will get a bigger file. Say you have a 100 x 100 pixel image compressed as a jpeg with 75% quality. When you open the file, all 10,000 pixels are recreated from the compressed data. If you then re-save at 100% quality, it will go through those 10,000 pixels and re-compress them, but at the new setting. For example, I took an 18.6MB LZW-compressed TIFF, saved it as a JPEG from photoshop with a quality of 12/12 and got an 8.5MB JPEG file. I then went back to the original TIFF and saved it as a JPEG with 8/12 quality and I got a 1.2MB file. If I take this 1.2MB JPEG and save it with a quality level of 12/12 and I end up with a 3.1MB file. This makes sense, because the same number of pixels are subjected to a lesser degree of compression. The file saved at 8/12 then 12/12 is smaller than the file saved 12/12 right from the TIFF because there was less info there to work with. I imagine there is a more accurate way to describe this, but I think this is as clear as I can make it concisely.
As I noted before, if you keep saving a file over and over again with the same quality levels it will slowly lose information because of the way JPEG works, it only becomes noticeable with a large number of saves or with a high compression level. This is why I think it is a perfectly fine method to archive the original JPEG as long as you keep going back to it without re-saving. I agree that the quality difference between a TIFF and a maximum quality JPEG is negligible. I just don't trust myself to remember not to re-save a file if I go make slight changes later so I don't mind using the extra space for TIFFs.
-Matt
