code: function(res) {
          var start = this.getUpperLeftTileCoord(res);
          var end = this.getLowerRightTileCoord(res);
  
          var numTileCols = (end.x - start.x) + 1;
          var numTileRows = (end.y - start.y) + 1;
  
          var minX = this.tileOrigin.lon + (start.x * this.tileSize.w * res);
          var maxX = minX + (numTileCols * this.tileSize.w * res);
          
          var maxY = this.tileOrigin.lat - (start.y * this.tileSize.h * res);
          var minY = maxY - (numTileRows * this.tileSize.h * res);
          return new OpenLayers.Bounds(minX, minY, maxX, maxY);
      }
docs: Method: getMaxExtentForResolution
  Since the max extent of a set of tiles can change from zoom level
  to zoom level, we need to be able to calculate that max extent
  for a given resolution.
  
  Parameters:
  res - {Float} The resolution for which to compute the extent.
  
  Returns:
  {<OpenLayers.Bounds>} The extent for this resolution