diff --git a/src/iiiftilesource.js b/src/iiiftilesource.js
index 0af45c8b..c80a3f2d 100644
--- a/src/iiiftilesource.js
+++ b/src/iiiftilesource.js
@@ -127,7 +127,8 @@ $.IIIFTileSource = function( options ){
         if (!this.scale_factors) {
             options.maxLevel = Number(Math.ceil(Math.log(Math.max(this.width, this.height), 2)));
         } else {
-            options.maxLevel = Math.floor(Math.pow(Math.max.apply(null, this.scale_factors), 0.5));
+            var maxScaleFactor = Math.max.apply(null, this.scale_factors);
+            options.maxLevel = Math.round(Math.log(maxScaleFactor) * Math.LOG2E);
         }
     }