diff --git a/doc/source/user_guide/numpy_images.txt b/doc/source/user_guide/numpy_images.txt
index e11616d8..d494ccdc 100644
--- a/doc/source/user_guide/numpy_images.txt
+++ b/doc/source/user_guide/numpy_images.txt
@@ -74,9 +74,11 @@ of same shape as the image (or a shape broadcastable to the image shape).
 This can be useful to define a region of interest, such as a
 disk: ::
 
-    >>> l_r, l_c = camera.shape
-    >>> R, C = np.ogrid[:l_r, :l_c]
-    >>> outer_disk_mask = (R - l_r / 2)**2 + (C - l_c / 2)**2 < (l_r / 2)**2
+    >>> nrows, ncols = camera.shape
+    >>> row, col = np.ogrid[:nrows, :ncols]
+    >>> cnt_row, cnt_col = nrows / 2, ncols / 2
+    >>> outer_disk_mask = ((row - cnt_row)**2 + (col - cnt_col)**2 <
+    ...                    (nrows / 2)**2)
     >>> camera[outer_disk_mask] = 0
 
 .. image:: ../../_images/plot_camera_numpy_1.png
@@ -85,7 +87,7 @@ disk: ::
 
 Boolean arithmetic can be used to define more complex masks: ::
 
-    >>> lower_half = R > l_r / 2
+    >>> lower_half = row > cnt_row
     >>> lower_half_disk = np.logical_and(lower_half, outer_disk_mask)
     >>> camera = data.camera()
     >>> camera[lower_half_disk] = 0