diff --git a/app/raster_utils.py b/app/raster_utils.py
index 2ed1ada8..e6083ef7 100644
--- a/app/raster_utils.py
+++ b/app/raster_utils.py
@@ -16,24 +16,27 @@ def export_raster_index(input, expression, output):
             nodata=-9999
         )
 
-        data = src.read().astype(np.float32)
+        bands_names = ["b{}".format(b) for b in tuple(sorted(set(re.findall(r"b(?P<bands>[0-9]{1,2})", expression))))]
+        rgb = expression.split(",")
+        indexes = tuple([int(b.replace("b", "")) for b in bands_names])
+
         alpha_index = None
         if has_alpha_band(src):
             try:
-                alpha_index = src.colorinterp.index(ColorInterp.alpha)
+                alpha_index = src.colorinterp.index(ColorInterp.alpha) + 1
+                indexes += (alpha_index, )
             except ValueError:
                 pass
 
-        bands_names = ["b{}".format(b) for b in tuple(sorted(set(re.findall(r"b(?P<bands>[0-9]{1,2})", expression))))]
-        rgb = expression.split(",")
-
+        data = src.read(indexes=indexes, out_dtype=np.float32)
         arr = dict(zip(bands_names, data))
         arr = np.array([np.nan_to_num(ne.evaluate(bloc.strip(), local_dict=arr)) for bloc in rgb])
 
         # Set nodata values
         index_band = arr[0]
         if alpha_index is not None:
-            index_band[data[alpha_index] == 0] = -9999
+            # -1 is the last band = alpha
+            index_band[data[-1] == 0] = -9999
 
         # Remove infinity values
         index_band[index_band>1e+30] = -9999