Sony flashes are named after their guide number in meters at maximum zoom.
F36 = 36 meters at ISO100 zoomed to 85mm maximum
F56 = 56 meters at ISO100 zoomed to 85mm maximum
F42 = 42 meters at ISO100 zoomed to 105mm maximum
F58 = 58 meters at ISO100 zoomed to 105mm maximum
Then, you need to divide it by your F-stop.
So if your aperture is F5.6, then the F56 at F5.6 will only reach a maximum of 10 meters at ISO100, 85mm zoom.
You can double the flash distance by increasing ISO by 2 stops - so, at ISO400, the F56 at F5.6 will reach 20 meters at 85mm zoom.
chiggy, yes (though, it's not quite as useful other than it being a lot higher than your pop-up flash.) That said direct flash is the way to go over large distances.
Radeon: no, Sony's guide numbers are measured in meters.
http://www.mhohner.de/sony-minolta/flashes.php