When I got the F5 my flash pictures are always underexpsoed. I sent the camera back to Nikon twice to have it calibrated but same problem. I had to do a 1 stop + compensation and this is very inconvienient because I have to remove the compensation when shoot without flash.
I recently read a book about Nikon flash (forgot the tittle) and in most example the author had to use + compensation. On the everage about 2/3 stop. So I am confident that Nikon intentionally made the TTL system underexpose.