diff --git a/field.py b/field.py
index 4a30082..084aa5e 100644
--- a/field.py
+++ b/field.py
@@ -379,18 +379,17 @@ class Field3d:
             if abs(rel_shift[axis])<self.eps_collapse:
                 continue
             elif rel_shift[axis]>0:
-                weights = (1.0-rel_shift[axis],rel_shift[axis])
+                w = rel_shift[axis] if rel_shift[axis]<=1.0 else 1.0
+                weights = (1.0-w,w)
                 data = ndimage.correlate1d(data,weights,axis=axis,mode='constant',cval=numpy.nan,origin=-1)
-                origin[axis] += rel_shift[axis]*self.spacing[axis]
+                origin[axis] += w*self.spacing[axis]
                 if only_keep_interior:
                     sl[axis] = slice(0,-1)
             else:
-                # It has been asserted that rel_shift is within range, so map it on boundaries if necessary
-                if rel_shift[axis]>1.0: rel_shift[axis]=1.0
-                if rel_shift[axis]<-1.0: rel_shift[axis]=-1.0
-                weights = (-rel_shift[axis],1.0+rel_shift[axis])
+                w = rel_shift[axis] if rel_shift[axis]>=-1.0 else -1.0
+                weights = (-w,1.0+w)
                 data = ndimage.correlate1d(data,weights,axis=axis,mode='constant',cval=numpy.nan,origin=0)
-                origin[axis] += rel_shift[axis]*self.spacing[axis]
+                origin[axis] += w*self.spacing[axis]
                 if only_keep_interior:
                     sl[axis] = slice(1,None)
                     origin[axis] += self.spacing[axis]