diff --git a/sample/simple3DFilter.cpp b/sample/simple3DFilter.cpp
index 000c6b7..a774519 100644
--- a/sample/simple3DFilter.cpp
+++ b/sample/simple3DFilter.cpp
@@ -86,8 +86,7 @@ void computeInterpolatedField(MyTree *tree1, double boxsize, int Nres, double cx
 
 int main(int argc, char **argv)
 {
-
-  char *fname1, *fname2;
+  char *fname1, *outFile;
   double rLimit, boxsize, rLimit2, cx, cy, cz;
   int Nres;
 
@@ -99,6 +98,7 @@ int main(int argc, char **argv)
     { "CX", &cx, MINIARG_DOUBLE },
     { "CY", &cy, MINIARG_DOUBLE },
     { "CZ", &cz, MINIARG_DOUBLE },
+    { "OUTPUT FILE", &outFile, MINIARG_STRING },
     { 0, 0, MINIARG_NULL }
   };
 
@@ -106,7 +106,7 @@ int main(int argc, char **argv)
     return 1;
 
   H5::H5File in_f(fname1, 0);
-  H5::H5File out_f("fields.h5", H5F_ACC_TRUNC);
+  H5::H5File out_f(outFile, H5F_ACC_TRUNC);
   array_type v1_data;
   uint32_t N1_points, N2_points;