How far can Google Earth zoom in?

The focal length of google Earth is 31.2 mm.

The focal length f for a given field of view (FOV) is calculated as follows:

f = x/(2*tan(FOV/2))

Usually, x is the diagonal length of the picture and the FOV in Photography is called the angle of view (also of the diagonal)

The formula is also valid for the horizontal or the vertical length of the picture.

The horizontal FOV in google Earth is 60°. For the horizontal length of the film I assumed the usual 36 mm of ordinary film in photo cameras. With these values, the formula gives 31.2 mm.

google Earth makes available the horizontal and vertical field of view in degrees via a NetworkLink. Click related link to launch dynamically generated KML that dumps all client properties of google Earth client including the field of view. The horizontal FOV is fixed at 60° but the vertical changes with the aspect ratio of the client window.