iviewer_afterZoom_patch.js

       //This will patch the jquery.iviewer plugin to accept a post-zoom callback function
       //http://github.com/can3p/iviewer/issues/issue/1
       /**
        * event is triggered when zoom value is changed
        * @param int new zoom value
        * @return boolean if false zoom action is aborted
        **/
        onZoom: null,

        //begin patch
        /**
         * event is triggered AFTER the image is zoomed
         * @param int new zoom value
         * @return void
         **/
         afterZoom: null,
        //end
        ...

       /**
        * set image scale to the new_zoom
        * @param new_zoom image scale in % 
        **/
        set_zoom: function(new_zoom)
        {
            if(this.settings.onZoom && this.settings.onZoom.call(this, new_zoom) == false)
            {
                return;
            }
            
            ......

            this.current_zoom = new_zoom;
            
            //begin patch
            if($.isFunction(this.settings.afterZoom))
            {
            	this.settings.afterZoom.call(this, new_zoom);
            }
            //end
            
            this.update_status();
        },