When manually zooming the 580EX II, it will cover the field of view of the displayed focal length on a 35mm camera (24mmx36mm sensor). In other words, there is no accounting for the camera's crop factor.
When set to autozoom, it will automatically adjust for the camera's crop factor by default. This can be disabled by custom function.
Note: Manual zoom and autozoom options are always available, whether the flash is in manual or E-TTL mode.
So if, for example, you were using a 70-200 lens on a 1.6x camera, you could manually zoom the flash to 105mm and it would still cover the FOV when you zoomed the lens to 70mm. With the same lens on the same camera in autozoom mode, the flash head would never move. The display on the flash will change from 70mm to 105mm when you zoom the lens but in reality, the flash isn't adjusting anything, since it's maxed out all the time.