An optical based system and method for monitoring turbine engine blade deflection during engine operation. An optical camera is coupled the exterior of an engine inspection port, so that its field of view captures images of a rotating turbine blade, such as the blade tip. The camera's image capture or sampling rate matches blade rotation speed at the same rotational position, so that successive temporal images of one or more blades show relative movement of the blade tip within the image field of view. The captured successive images are directed to a blade deflection monitoring system (BDMS) controller. The controller correlates change in a blade's captured image position within the camera field of view between successive temporal images with blade deflection. The BDMS alarms or trips engine operation if the monitored blade deflection falls outside permissible operation parameters.