A bit offtopic, but I'm constantly annoyed by applications using 'x' and 'z' for related operations like zoom in and out in this case. The reason is, German keyboards use the QWERTZ layout and as you can tell from the name, the 'z' key is in the upper row, right in the middle. Maybe use 'w' and 's' instead? That's the default in first-person-type games. Actually, never mind, that doesn't work for the French who have…
Since mouse dragging works fine, I expected the scroll wheel to control zooming. In fact, that was the first thing I tried before reading the instructions.