/* tested on the iPad and iPhone */
function getZoomFactor() {
  var deviceWidth, landscape = Math.abs(window.orientation) == 90
  // workaround for strange screen.height on the iPhone (v3.1.3)
  if (window.screen.width == 320) deviceWidth = landscape ? 480 : 320
  else deviceWidth = window.screen[landscape ? "height" : "width"]
  return deviceWidth / window.innerWidth
}