diff --git a/field.py b/field.py
index 453817e..a14c863 100644
--- a/field.py
+++ b/field.py
@@ -1,9 +1,13 @@
 import numpy as np
 class Field3d:
-    def __init__(self,data,origin,spacing):
+    def __init__(self,data,origin,spacing,deep=False):
         assert len(origin)==3,  "'origin' must be of length 3"
         assert len(spacing)==3, "'spacing' must be of length 3"
-        self.data      = np.array(data)
+        assert isinstance(data,np.ndarray), "'data' must be numpy.ndarray."
+        if deep:
+            self.data = data.copy()
+        else:
+            self.data = data
         self.origin    = tuple([float(x) for x in origin])
         self.spacing   = tuple([float(x) for x in spacing])
         self.eps_collapse = 1e-8
@@ -187,16 +191,28 @@ class Field3d:
         self.data[ib:ib+nx,jb:jb+ny,kb:kb+nz] = subfield.data[:,:,:]
         return
 
-    def extract_subfield(self,idx_origin,dim,stride=(1,1,1)):
-        assert all(idx_origin[ii]>=0 and idx_origin[ii]<self.dim(axis=ii) for ii in range(3)),\
-                "'origin' is out-of-bounds."
-        assert all(idx_origin[ii]+stride[ii]*(dim[ii]-1)<self.dim(axis=ii) for ii in range(3)),\
-                "endpoint is out-of-bounds."
-        sl = tuple(slice(idx_origin[ii],idx_origin[ii]+stride[ii]*dim[ii],stride[ii]) for ii in range(3))
+    def extract_subfield(self,idx_origin,dim,stride=(1,1,1),deep=False,strict_bounds=True):
+        if strict_bounds:
+            assert all(idx_origin[ii]>=0 and idx_origin[ii]<self.dim(axis=ii) for ii in range(3)),\
+                    "'origin' is out-of-bounds."
+            assert all(idx_origin[ii]+stride[ii]*(dim[ii]-1)<self.dim(axis=ii) for ii in range(3)),\
+                    "endpoint is out-of-bounds."
+        else:
+            for ii in range(3):
+                if idx_origin[ii]<0:
+                    dim[ii] += idx_origin[ii]
+                    idx_origin[ii] = 0
+                if idx_origin[ii]+stride[ii]*(dim[ii]-1)>=self.dim(axis=ii):
+                    dim[ii] = (self.dim(axis=ii)-idx_origin[ii])//stride[ii]
+                if dim[ii]<=0:
+                    return None
+        sl = tuple(slice(idx_origin[ii],
+                         idx_origin[ii]+stride[ii]*dim[ii],
+                         stride[ii]) for ii in range(3))
         origin  = self.coordinate(idx_origin)
         spacing = tuple(self.spacing[ii]*stride[ii] for ii in range(3))
-        data = self.data[sl].copy()
-        return Field3d(data,origin,spacing)
+        data = self.data[sl]
+        return Field3d(data,origin,spacing,deep=deep)
 
     def coordinate(self,idx,axis=None):
         if axis is None:
diff --git a/parallel.py b/parallel.py
index 5a50dbf..a213e15 100644
--- a/parallel.py
+++ b/parallel.py
@@ -728,7 +728,8 @@ class PPP:
         return self._subfield(key,stride,(0,0,0))
 
     def _subfield(self,key,stride,num_ghost,no_lower_ghost=(False,False,False),
-                                            no_upper_ghost=(False,False,False)):
+                                            no_upper_ghost=(False,False,False),
+                                            deep=True):
         '''Returns the field with a stride applied.'''
         stride    = np.array(stride,dtype=int)
         num_ghost = np.array(num_ghost,dtype=int)
@@ -765,7 +766,7 @@ class PPP:
         assert all(idx_origin>=0)
         assert all(idx_endpoint<np.array(self.field[key].dim()))
         # Return the subfield
-        return self.field[key].extract_subfield(idx_origin,num_points,stride=stride)
+        return self.field[key].extract_subfield(idx_origin,num_points,stride=stride,deep=deep)
 
     def _merge_at_root(self,key,stride=(1,1,1)):
         '''Returns the entire field gathered from all processors with a