diff --git a/cpp/Cal3_S2.h b/cpp/Cal3_S2.h
index ef16e2932..04fe70d0e 100644
--- a/cpp/Cal3_S2.h
+++ b/cpp/Cal3_S2.h
@@ -50,6 +50,13 @@ namespace gtsam {
 			return Cal3_S2(fx_ + d(0), fy_ + d(1), s_ + d(2), u0_ + d(3), v0_ + d(4));
 		}
 
+		/**
+		 * return the principal point
+		 */
+		Point2 principalPoint() const {
+			return Point2(u0_,v0_);
+		}
+
 		/**
 		 * return vectorized form (column-wise)
 		 */