JPG Quality tests - Q4 - Q12

I was reading a thread about JPG vs TIFF for printing, and I wondered how bad JPG compression really was, so I did some tests to test it. In practice I print 30x20" prints from Q12 JPGs regularly and they look great. My workflow for this test was:

1) Open RAW file (20D), resize to 800px wide.
2) Save as Q12 JPG, close file. Open file in photoshop, save as Q12 again. Repeat 12 times.
3) Do step 2 for Q10, Q8, Q6, and Q4.

The resultant images can be found linked below. Total file size is 1MB.

Original, resized, saved Q12
Original saved as Q12 a dozen times
Original saved as Q10 a dozen times
Original saved as Q8 a dozen times
Original saved as Q6 a dozen times
Original saved as Q4 a dozen times

Conclusions:
- Q12 is pretty much lossless.
- Q10 is close to lossless.
- Q8 you can see artifacts especially around the lights on the left.
- Q6 compression artifacts.
- Q4 bad compression artifacts.

what about if it is saved 144 times at q12?

Dunno, try it and tell us. I'd never do that, or even do it twice, so I don't care enough to try it.

Tim, really appreciate the effort you went into this! I have always wondered about how many generations would manifest themselves at the different JPG qualities, but not interest enough to run the tests myself. The winter weather must have you trapped inside and bored in Wellington!

Years ago I was involved with medical imaging, and radiologist ran tests to see how much compression before it was discernable in the first generation JPG file, vs. Xray film as the gold standard. I don't recall exactly, but results were similar in 'medium' was about the point it became discernable and not have impact on diagnostic content.

Thanks very much for posting these results, it's nice to know this kind of stuff

I was at home after a few drinks, before I went out again, it helped keep me amused for 15 minutes

Thank you very much Tim for this information

Tim, are you sure you've labelled the pictures correctly? I'm using tabbed browsing in Firefox, so I can flick between two pictures very quickly to compare them. To my eyes the one you've labelled Q10-12 is by far the worst in terms of artefacts. Q12-12 is the next worst. It's hard to tell the difference between the rest...

I just looked at them in the same way, in firefox, tabbed, and my labeling seems accurate.

I always wondered about this. Glad someone has finally done a scientific report on it.

Servod wrote in post #1945054
=Servo'd;1945054]I always wondered about this. Glad someone has finally done a scientific report on it.

Without taking away from Tims efforts - who did the test at multiple quality settings - I wonder about your word finally, as I did this, HERE is the thread. Scroll about 2/3 the way down to see my results.

Rad

I realize this is an old thread, but I came here to learn something and the links at the top are no longer good ...oh well

Yeah sorry about that I had to clean up my web server and those image files are gone. You'll have to rely on my conclusions unfortunately, or try it yourself

no problem Tim...I trust your word and I appreciate your efforts nonetheless. I did however follow the link in Radtech's post and found his photos showing the degradation. I saw what I needed to see