thanks for your answer Ashley.
I never mix dt and every seconds.
My pb is that :
I scroll to my heros.x, or put a scroll behaviour on it, whatever, and my hero moves with dt (+600*dt) to the right, for example.
As long as my fps is 60, no pb, but if it goes down, even just a few fps, my camera "jumps".
So i tried to scroll using dt, using everyseconds, etc..., to have my camera moving independently from the fps.
I don't know if it's normal, but i thought that having everything moving with dt, even the scrolling, would erase the fact that every framerate drop is visible, but maybe it's just impossible to hide this kind of things, I don't know.
That's why too, and i know you're not agree, maybe it would be nice to have an option to chose to render the games at 30 or 60 fps.
I mean, i would prefer to have my game running (at the same speed using dt) with a perfect 30 fps and no framerate drops who make the camera jump, than being between 60 and 55 and having this camera jumps, which are very annoying me.
It would be very usefull for old mobile devices : still having my character moving at 600*dt, but on a stable 30 fps. Not sure if i'm clear, but in my case, to make my game working perfectly on an ipod4, it would be great. i succeded on that device to have a nice game and to stay between 55-60 fps, but i just try to get ride of the jump of the camera :), if it's possible.
Now, maybe i understand nothing in how framerate work, but for what i understand, it would nice, again, for old devices, to give a kind of "security" part concerning the framerate.