I solved the problem like this thanks to a french mate :

Go there documents\My Games\FINAL FANTASY XIV - A Realm Reborn and edit FFXIV.cfg like this :

LodType 0

...

ShadowLOD 0

...

LodType_DX11 0

...

ShadowLOD_DX11 0
like this, you'll have high rez shadows and objects LOD off