diff --git a/builtin/log.c b/builtin/log.c
index 77a7bba543..a7791c003c 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -599,7 +599,10 @@ static int show_tree_object(const struct object_id *oid,
 static void show_setup_revisions_tweak(struct rev_info *rev,
 				       struct setup_revision_opt *opt)
 {
-	diff_merges_default_to_dense_combined(rev);
+	if (rev->first_parent_only)
+		diff_merges_default_to_enable(rev);
+	else
+		diff_merges_default_to_dense_combined(rev);
 	if (!rev->diffopt.output_format)
 		rev->diffopt.output_format = DIFF_FORMAT_PATCH;
 }
@@ -724,7 +727,8 @@ static void log_setup_revisions_tweak(struct rev_info *rev,
 	if (!rev->diffopt.output_format && rev->combine_merges)
 		rev->diffopt.output_format = DIFF_FORMAT_PATCH;
 
-	diff_merges_first_parent_defaults_to_enable(rev);
+	if (rev->first_parent_only)
+		diff_merges_default_to_enable(rev);
 }
 
 int cmd_log(int argc, const char **argv, const char *prefix)
diff --git a/diff-merges.c b/diff-merges.c
index 9286dbbd4d..a3bafa92f3 100644
--- a/diff-merges.c
+++ b/diff-merges.c
@@ -56,19 +56,17 @@ void diff_merges_setup_revs(struct rev_info *revs)
 		die("--combined-all-paths makes no sense without -c or --cc");
 }
 
-void diff_merges_first_parent_defaults_to_enable(struct rev_info *revs)
+void diff_merges_default_to_enable(struct rev_info *revs)
 {
-	if (revs->first_parent_only && revs->ignore_merges < 0)
+	if (revs->ignore_merges < 0)		/* No -m */
 		revs->ignore_merges = 0;
 }
 
 void diff_merges_default_to_dense_combined(struct rev_info *revs)
 {
-	if (revs->ignore_merges < 0) {
-		/* There was no "-m" variant on the command line */
+	if (revs->ignore_merges < 0) {		/* No -m */
 		revs->ignore_merges = 0;
-		if (!revs->first_parent_only && !revs->combine_merges) {
-			/* No "--first-parent", "-c", or "--cc" */
+		if (!revs->combine_merges) {	/* No -c/--cc" */
 			revs->combine_merges = 1;
 			revs->dense_combined_merges = 1;
 		}
diff --git a/diff-merges.h b/diff-merges.h
index 4ced909b79..6937d46219 100644
--- a/diff-merges.h
+++ b/diff-merges.h
@@ -17,7 +17,7 @@ void diff_merges_setup_revs(struct rev_info *revs);
 
 void diff_merges_default_to_dense_combined(struct rev_info *revs);
 
-void diff_merges_first_parent_defaults_to_enable(struct rev_info *revs);
+void diff_merges_default_to_enable(struct rev_info *revs);
 
 
 #endif