diff --git a/map2map/data/fields.py b/map2map/data/fields.py
index 4369afe..c610a3a 100644
--- a/map2map/data/fields.py
+++ b/map2map/data/fields.py
@@ -139,11 +139,11 @@ class FieldDataset(Dataset):
             if shift is not None:
                 anchor[d] += torch.randint(int(shift), (1,))
 
-        in_fields = crop(in_fields, anchor, self.crop, self.in_pad, self.size)
-        tgt_fields = crop(tgt_fields, anchor * self.scale_factor,
-                          self.crop * self.scale_factor,
-                          self.tgt_pad,
-                          self.size * self.scale_factor)
+        crop(in_fields, anchor, self.crop, self.in_pad, self.size)
+        crop(tgt_fields, anchor * self.scale_factor,
+             self.crop * self.scale_factor,
+             self.tgt_pad,
+             self.size * self.scale_factor)
 
         in_fields = [torch.from_numpy(f).to(torch.float32) for f in in_fields]
         tgt_fields = [torch.from_numpy(f).to(torch.float32) for f in tgt_fields]
@@ -158,11 +158,11 @@ class FieldDataset(Dataset):
                 norm(x)
 
         if self.augment:
-            in_fields, flip_axes = flip(in_fields, None, self.ndim)
-            tgt_fields, flip_axes = flip(tgt_fields, flip_axes, self.ndim)
+            flip_axes = flip(in_fields, None, self.ndim)
+            flip_axes = flip(tgt_fields, flip_axes, self.ndim)
 
-            in_fields, perm_axes = perm(in_fields, None, self.ndim)
-            tgt_fields, perm_axes = perm(tgt_fields, perm_axes, self.ndim)
+            perm_axes = perm(in_fields, None, self.ndim)
+            perm_axes = perm(tgt_fields, perm_axes, self.ndim)
 
         if self.aug_add is not None:
             add_fac = add(in_fields, None, self.aug_add)
@@ -180,22 +180,22 @@ class FieldDataset(Dataset):
 
 def crop(fields, anchor, crop, pad, size):
     ndim = len(size)
-    assert all(len(x) == ndim for x in [anchor, crop, pad, size]), 'inconsistent ndim'
+    assert all(len(x) == ndim for x in [anchor, crop, pad]), 'ndim mismatch'
 
-    new_fields = []
-    for x in fields:
-        ind = [slice(None)]
-        for d, (a, c, (p0, p1), s) in enumerate(zip(anchor, crop, pad, size)):
-            i = np.arange(a - p0, a + c + p1)
-            i %= s
-            i = i.reshape((-1,) + (1,) * (ndim - d - 1))
-            ind.append(i)
+    ind = [slice(None)]
+    for d, (a, c, (p0, p1), s) in enumerate(zip(anchor, crop, pad, size)):
+        i = np.arange(a - p0, a + c + p1)
+        i %= s
+        i = i.reshape((-1,) + (1,) * (ndim - d - 1))
+        ind.append(i)
+    ind = tuple(ind)
 
-        x = x[tuple(ind)]
+    for i, x in enumerate(fields):
+        x = x[ind]
 
-        new_fields.append(x)
+        fields[i] = x
 
-    return new_fields
+    return ind
 
 
 def flip(fields, axes, ndim):
@@ -205,17 +205,16 @@ def flip(fields, axes, ndim):
         axes = torch.randint(2, (ndim,), dtype=torch.bool)
         axes = torch.arange(ndim)[axes]
 
-    new_fields = []
-    for x in fields:
+    for i, x in enumerate(fields):
         if x.shape[0] == ndim:  # flip vector components
             x[axes] = - x[axes]
 
         shifted_axes = (1 + axes).tolist()
         x = torch.flip(x, shifted_axes)
 
-        new_fields.append(x)
+        fields[i] = x
 
-    return new_fields, axes
+    return axes
 
 
 def perm(fields, axes, ndim):
@@ -224,17 +223,16 @@ def perm(fields, axes, ndim):
     if axes is None:
         axes = torch.randperm(ndim)
 
-    new_fields = []
-    for x in fields:
+    for i, x in enumerate(fields):
         if x.shape[0] == ndim:  # permutate vector components
             x = x[axes]
 
         shifted_axes = [0] + (1 + axes).tolist()
         x = x.permute(shifted_axes)
 
-        new_fields.append(x)
+        fields[i] = x
 
-    return new_fields, axes
+    return axes
 
 
 def add(fields, fac, std):