diff --git a/image_analysis/geom_refinement/XtalOptimizer.cpp b/image_analysis/geom_refinement/XtalOptimizer.cpp
index b98a3b0a..96f6ded1 100644
--- a/image_analysis/geom_refinement/XtalOptimizer.cpp
+++ b/image_analysis/geom_refinement/XtalOptimizer.cpp
@@ -458,7 +458,8 @@ bool XtalOptimizerInternal(XtalOptimizerData &data,
         if (problem.NumResidualBlocks() < data.min_spots)
             return false;
 
-        problem.SetParameterBlockConstant(&distance_mm);
+        if (!data.refine_distance_mm)
+            problem.SetParameterBlockConstant(&distance_mm);
 
         if (!data.refine_beam_center) {
             problem.SetParameterBlockConstant(&beam_x);
@@ -503,6 +504,9 @@ bool XtalOptimizerInternal(XtalOptimizerData &data,
             data.geom.BeamX_pxl(beam_x).BeamY_pxl(beam_y);
         }
 
+        if (data.refine_distance_mm)
+            data.geom.DetectorDistance_mm(distance_mm);
+
         if (data.crystal_system == gemmi::CrystalSystem::Orthorhombic)
             data.latt = AngleAxisAndLengthsToLattice(latt_vec0, latt_vec1, false);
         else if (data.crystal_system == gemmi::CrystalSystem::Tetragonal) {
diff --git a/image_analysis/geom_refinement/XtalOptimizer.h b/image_analysis/geom_refinement/XtalOptimizer.h
index 9bd0dabe..bac1f99c 100644
--- a/image_analysis/geom_refinement/XtalOptimizer.h
+++ b/image_analysis/geom_refinement/XtalOptimizer.h
@@ -23,6 +23,7 @@ struct XtalOptimizerData {
     float max_angle_deg = 120.0f;
 
     bool refine_beam_center = true;
+    bool refine_distance_mm = false;
 
     // output
     std::optional<double> beam_corr_x;