Thanks everybody for your replies. It's more clear for me now.
- Compression factor isn't linear and in IJG, and that factor doesn't 
represent a percentage.
- Photoshop converted its scale for making it more "intuitive", but it 
has nothing to do with the right IJG scale.

Thanks Michael for the link for the jpeg FAQs. It's very clear in the 
item 5:

 * Recent Apple software uses an 0-100 scale that has nothing to do with
    the IJG scale (their Q 50 is about the same as Q 80 on the IJG scale).

It's clear that photoshop uses that scale.

Tor wrote:

> It is just a number whose exact meaning you will have
> to check from the libjpeg sources in GIMP's case.
> It is just a coincidence that both programs in this case
> happen to use a scale from 1 to 100.

That's the problem: That coincidence.
The 1-100 scale is commonly used for percentages, and everybody tends to 
take it as that.
My problem isn't that I can't understand how jpeg works... My problem is 
that I'm used to take 1-100 scales as a percentage.
I'm pretty sure that I'm not the only one.

gg wrote:
> Maybe you should try adjusting the compression level on the camera, it 
> maybe a simple A,B or C choice and is probably not presented as "jpeg 
> quality".
The images I get from the camera are fine. My problem is that if adjust 
them, scale them and re-save them without explicitly change the quality 
setting they turn out really distorted.
Even though I understand (now) the difference between that scales, I'm 
still concerned about that quality loss in my images.
I repeat: I just open them from the camera and they look great (Nikon 
Coolpix 800), adjust levels, curves and color (they still look great), 
scale them down, and save them using CTRL+S. When I re-open them they 
are distorted.
I'm not talking about repetitive openings and saves.

> I dont use PS but my guess is that there is no real difference in the 
> implementation , simply in the way this parameter is presented to the 
> user.
That was my whole point. If the quality is presented as a scale between 
0 and 100 the user assumes that is the same 0-100% of photoshop and 
other programs.
That happened to me. People is always more comfortable with linear 
scales, they are easier to understand.
If the compression factor isn't linear, photoshop must have "linearized" 
it for translating that into a sort of porcentage of quality and make it 
easier to understand.
It goes against the IJG scale, but unfortunately this different scale is 
now more popular than the right one.

> Does that tie in with your experience?
Yes, it does.
I can see you understand my point.
We, the users, tend to assume things based on the information we get on 
screen.It should be considered as a useability factor, imo.
Developers see it from the technical perspective, but users don't. Users 
want to have things done.
Now, with all these replies I  can understand it, but it won't take too 
long to see a user complaining for the same thing.

It's very clear that Photoshop doesn't use the IJG scale, but it should. 
Users who came from PS are used to that incorrect scale, and find this 
I think it should be documented in the Gimp Manual and FAQs. It's not a 
minor issue.

Thanks everybody for your explainations!
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