the Defocus (with use Z buffer checked) works perfectly with a render layer node (tested on one frame per time).
however when i rendered the whole sequence as an OpenEXR (Multilayer or not) and imported to another blend file it works no more.
I don't see a difference between the too Z passes. Is this a normal thing ? Does saving the Z pass as another format will solve this ? i want to know before i re-render the sequence.