diff -r 3a6b1186745f Lib/tkinter/__init__.py
--- a/Lib/tkinter/__init__.py	Fri Jan 01 23:27:38 2016 -0800
+++ b/Lib/tkinter/__init__.py	Sat Jan 02 13:19:45 2016 +0200
@@ -3407,7 +3407,8 @@ class PhotoImage(Image):
         return destImage
     def zoom(self,x,y=''):
         """Return a new PhotoImage with the same image as this widget
-        but zoom it with X and Y."""
+        but zoom it with a factor of x in the X direction and y in the Y
+        direction.  If y is not given, the default value is the same as x."""
         destImage = PhotoImage(master=self.tk)
         if y=='': y=x
         self.tk.call(destImage, 'copy', self.name, '-zoom',x,y)