only 3 divides are necessary, not 6.

src/math/Matrix4.js

@@ -903,17 +903,17 @@ THREE.Matrix4.prototype = {
 	makeOrthographic: function ( left, right, top, bottom, near, far ) {
 
 		var te = this.elements;
-		var w = right - left;
-		var h = top - bottom;
-		var p = far - near;
+		var w = 1.0 / ( right - left );
+		var h = 1.0 / ( top - bottom );
+		var p = 1.0 / ( far - near );
 
-		var x = ( right + left ) / w;
-		var y = ( top + bottom ) / h;
-		var z = ( far + near ) / p;
+		var x = ( right + left ) * w;
+		var y = ( top + bottom ) * h;
+		var z = ( far + near ) * p;
 
-		te[ 0 ] = 2 / w;	te[ 4 ] = 0;	te[ 8 ] = 0;	te[ 12 ] = - x;
-		te[ 1 ] = 0;	te[ 5 ] = 2 / h;	te[ 9 ] = 0;	te[ 13 ] = - y;
-		te[ 2 ] = 0;	te[ 6 ] = 0;	te[ 10 ] = - 2 / p;	te[ 14 ] = - z;
+		te[ 0 ] = 2 * w;	te[ 4 ] = 0;	te[ 8 ] = 0;	te[ 12 ] = - x;
+		te[ 1 ] = 0;	te[ 5 ] = 2 * h;	te[ 9 ] = 0;	te[ 13 ] = - y;
+		te[ 2 ] = 0;	te[ 6 ] = 0;	te[ 10 ] = - 2 * p;	te[ 14 ] = - z;
 		te[ 3 ] = 0;	te[ 7 ] = 0;	te[ 11 ] = 0;	te[ 15 ] = 1;
 
 		return this;