Benefits from editing images as a TIFF, and then converting this over to a JPEG?

[PeterAit]

Saving a JPEG over and over again doesn't alter quality as long the compression rate is kept. I guess the algorithm just saves back the same RGB numbers again and again.

I don't believe this is true. I ran a test where I opened a TIFF, saved it as JPEG with Photoshop quality 8. I then closed the file, re-opened the just-saved jpg, "saved as" another jpeg with the same quality and new name, and repeated 4 times. The file sizes were:

Iteration 1: 1695 kb
              2: 1670
              3: 1668
              4: 1669

The slight size increase at iteration 4 is a bit of a mystery, but you can see the file size goes down with each re-saving of the file (no changes were made to the image). Something is being lost.

[David Sutton]

I don't believe this is true. I ran a test where I opened a TIFF, saved it as JPEG with Photoshop quality 8. I then closed the file, re-opened the just-saved jpg, "saved as" another jpeg with the same quality and new name, and repeated 4 times. The file sizes were:

Iteration 1: 1695 kb
              2: 1670
              3: 1668
              4: 1669

The slight size increase at iteration 4 is a bit of a mystery, but you can see the file size goes down with each re-saving of the file (no changes were made to the image). Something is being lost.

I'm not convinced the difference in the numbers are big enough to indicate jpeg compression is kicking in. It could be due to the file name changing or some other factor.
I just ran the same test with a tiff. Opening and saving with an added underscore and number. The file sizes were:
1: 63,115,564 bytes
2: 63,116,452 bytes
3: 63,117,312 bytes
David

[digitaldog]

I ran a test where I opened a TIFF, saved it as JPEG with Photoshop quality 8. I then closed the file, re-opened the just-saved jpg, "saved as" another jpeg with the same quality and new name, and repeated 4 times.

I did the same test. But I used Calculations (Subtract) on JPEG 1 vs. JPEG 4 and they are identical.
If we alter the image then re-save, then I'd expect to see differences. But a Save As... multiple JPEG produces the same JPEG on this end.

JPEG 1 is 613KB while JPEG 4 is 608KB so the Mac Finder isn't a good indicator of the data differences.