diff --git a/skimage/filter/rank/_crank16.pyx b/skimage/filter/rank/_crank16.pyx
index 667c179d..232d6812 100644
--- a/skimage/filter/rank/_crank16.pyx
+++ b/skimage/filter/rank/_crank16.pyx
@@ -4,7 +4,7 @@
 #cython: wraparound=False
 
 cimport numpy as cnp
-from libc.math cimport log2
+from libc.math cimport log
 from skimage.filter.rank._core16 cimport _core16
 
 
@@ -276,7 +276,7 @@ cdef inline dtype_t kernel_entropy(Py_ssize_t * histo, float pop,
         for i in range(maxbin):
             p = histo[i] / pop
             if p > 0:
-                e -= p * log(p) / 0.30102999566398119521373889472449
+                e -= p * log(p) / 0.6931471805599453
 
         return <dtype_t>e * 1000
     else:
diff --git a/skimage/filter/rank/_crank8.pyx b/skimage/filter/rank/_crank8.pyx
index 3e06f319..cb7febac 100644
--- a/skimage/filter/rank/_crank8.pyx
+++ b/skimage/filter/rank/_crank8.pyx
@@ -4,7 +4,7 @@
 #cython: wraparound=False
 
 cimport numpy as cnp
-from libc.math cimport log2
+from libc.math cimport log
 from skimage.filter.rank._core8 cimport _core8
 
 
@@ -281,7 +281,7 @@ cdef inline dtype_t kernel_entropy(Py_ssize_t * histo, float pop,
         for i in range(256):
             p = histo[i] / pop
             if p > 0:
-                e -= p * log(p) / 0.30102999566398119521373889472449
+                e -= p * log(p) / 0.6931471805599453
 
         return <dtype_t>e * 10
     else: