The "Zoom" control sets the scale of the map you are viewing. The scale is actually determined by the type of  camera lens and altitude in the case of Aerial Photography, the detail set for scanning in the case of Topo Maps, and the desired enlargement in the case of Tiger maps.

 

The available scales are generally measured in "Meters per pixel", which effectively represents the level of detail visible and the size the picture covers on the ground. The tiger map is different, being continuously variable. The following shows the scales available:

 

 

Map Scales (Zoom)

Map Type	1 Meters/pixel	2 Meters/pixel	4 Meters/pixel	8 Meters/pixel
Aerial	x	x	x	x
Topo		x	x	x
Tiger	Varies continuously