diff --git a/image_analysis/scale_merge/ScaleOnTheFly.cpp b/image_analysis/scale_merge/ScaleOnTheFly.cpp
index 3d431bd2..fda12d97 100644
--- a/image_analysis/scale_merge/ScaleOnTheFly.cpp
+++ b/image_analysis/scale_merge/ScaleOnTheFly.cpp
@@ -8,6 +8,12 @@
 #include <ceres/rotation.h>
 
 namespace {
+    // Robust loss scale (in sigma units) for the per-image scale fit: a few outlier reflections
+    // (zingers, overlaps, a mis-predicted spot) must not drag a frame's G/B into a bad optimum -
+    // that is the stochastic per-frame mis-scaling that elevates R-meas and collapses CC1/2 at low
+    // symmetry. Cauchy down-weights residuals beyond ~this many sigma without a hard cut.
+    constexpr double SCALE_ROBUST_K = 3.0;
+
     double SafeInv(double x, double fallback) {
         if (!std::isfinite(x) || x == 0.0)
             return fallback;
@@ -248,19 +254,19 @@ void ScaleOnTheFly::Scale(IntegrationOutcome &integration_outcome) const {
             case PartialityModel::Fixed: {
                 auto *cost = new ceres::AutoDiffCostFunction<IntensityFixedResidual, 1, 1, 1>(
                     new IntensityFixedResidual(r, Itrue, sigma, r.partiality));
-                problem.AddResidualBlock(cost, nullptr, &result.G, &result.B);
+                problem.AddResidualBlock(cost, new ceres::CauchyLoss(SCALE_ROBUST_K), &result.G, &result.B);
             }
             break;
             case PartialityModel::Unity: {
                 auto *cost = new ceres::AutoDiffCostFunction<IntensityFixedResidual, 1, 1, 1>(
                     new IntensityFixedResidual(r, Itrue, sigma, 1.0));
-                problem.AddResidualBlock(cost, nullptr, &result.G, &result.B);
+                problem.AddResidualBlock(cost, new ceres::CauchyLoss(SCALE_ROBUST_K), &result.G, &result.B);
             }
             break;
             case PartialityModel::Rotation: {
                 auto *cost = new ceres::AutoDiffCostFunction<ScalingRotationResidual, 1, 1, 1, 1, 1>(
                     new ScalingRotationResidual(r, Itrue, sigma));
-                problem.AddResidualBlock(cost, nullptr, &result.G, &result.B, &result.mos,
+                problem.AddResidualBlock(cost, new ceres::CauchyLoss(SCALE_ROBUST_K), &result.G, &result.B, &result.mos,
                                          &result.wedge);
             }
             break;