diff --git a/src/geomtools/powder/powder.py b/src/geomtools/powder/powder.py
index af7cb67eb87a8f6880ea42a50fdd1bbb82f9504e..824ae792eaf40747f772946b4aea4dbf5f085139 100644
--- a/src/geomtools/powder/powder.py
+++ b/src/geomtools/powder/powder.py
@@ -9,7 +9,7 @@ from .refine import fit_rings, refine_clen
 
 class PowderDiffraction:
     def __init__(self, geom, peaks, clen, wave_length,
-                 min_area=30, sigma=7, snr=1, border_mask=None,
+                 min_area=30, sigma=1.8, snr=1, border_mask=None,
                  make_shadow_mask=True):
         self.geom = geom
         self.peaks = peaks
@@ -34,8 +34,9 @@ class PowderDiffraction:
                 threshold = threshold[slice_thr]
             self.shadow = make_shadow_mask(img, ~mask, threshold)
 
+        sig = self.sigma * 1e-3 / self.geom.pixel_size
         fg = threshold_gaussian(
-            self.geom, img, mask, self.shadow, self.sigma, self.snr)
+            self.geom, img, mask, self.shadow, sig, self.snr)
 
         fg = fg & self.border_mask