z buffer , anisotropic ??

when im looking out of the cockpit on the grid the advertising boards are clear for a few metres then blur out. similarly on the cars, the logos are blurred when viewed from an angle but sharpen up as the car turns.

is this something to do with the z buffer or anisotropic filtering. Im sure it didnt do it before, its just since i started tweaking.

Any suggestions?

MipMapping this one is called I think.

Neil is right Andy, MipMap detail should be around -1.0