code: function( viewportZoom ) {
          var ratio = this._scaleSpring.current.value *
                  this.viewport._containerInnerSize.x / this.source.dimensions.x;
          return ratio * viewportZoom;
      }
docs: Convert a viewport zoom to an image zoom.
  Image zoom: ratio of the original image size to displayed image size.
  1 means original image size, 0.5 half size...
  Viewport zoom: ratio of the displayed image's width to viewport's width.
  1 means identical width, 2 means image's width is twice the viewport's width...
  @function
  @param {Number} viewportZoom The viewport zoom
  @returns {Number} imageZoom The image zoom