Ok, i've come to my limitation of expression skills.
I've setuped a 3D composition, with various elements at various depths. One of this element is a Sun. I setuped a camera to do a pan inside my 3D world.
So far, so good. None of my elements are animated.
Now i'd like to link a lens flare on my sun, which is a 2D layer. I've tried with the "toWorld" and "toComp" expression, but i don't get any results as my 3D layer "Sun" is not moving. Only my camera (and for some reasons i mustn't turn my Lense flare into a 3D layer).
But i can't figure out how to calculate the position of my "Sun" layer inside my comp with an offset from the camera position.
I'm sure it's possible, but i'm so bad when it comes to heavy math.