diff --git a/image_analysis/scale_merge/ScaleAndMerge.cpp b/image_analysis/scale_merge/ScaleAndMerge.cpp
index 14b129cb..a45264bc 100644
--- a/image_analysis/scale_merge/ScaleAndMerge.cpp
+++ b/image_analysis/scale_merge/ScaleAndMerge.cpp
@@ -388,6 +388,7 @@ namespace {
         options.max_num_iterations = opt.max_num_iterations;
         options.max_solver_time_in_seconds = opt.max_solver_time_s;
         options.num_threads = static_cast<int>(hw);
+        options.function_tolerance = 1e-4;
 
         ceres::Solver::Summary summary;
         ceres::Solve(options, &problem, &summary);