diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 5f375c39..0e3f1250 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -110,3 +110,6 @@
 
 - Pavel Campr
   Fixes and tests for Histograms of Oriented Gradients.
+
+- Joshua Warner
+  Multichannel random walker segmentation.
diff --git a/skimage/util/dtype.py b/skimage/util/dtype.py
index d2a0dd61..9f804406 100644
--- a/skimage/util/dtype.py
+++ b/skimage/util/dtype.py
@@ -265,8 +265,8 @@ def img_as_float(image, force_copy=False):
 
     Notes
     -----
-    The range of a floating point image is [0, 1].
-    Negative input values will be shifted to the positive domain.
+    The range of a floating point image is [0.0, 1.0] or [-1.0, 1.0] when
+    converting from unsigned or signed datatypes, respectively.
 
     """
     return convert(image, np.float64, force_copy)