An ophthalmic surgical laser system for modifying an intraocular lens implanted in a patient's eye, the system comprising: a laser light source configured to generate a laser beam; an optical delivery system configured to deliver the laser beam to the IOL, including a scanner system configured to scan the laser beam within the IOL; a video camera configured to acquire an image of a field of view containing the IOL; and a controller configured to control the laser light source and the scanner system to deliver the laser beam in the IOL to modify a refractive property of the IOL, characterized in that the controller is programmed to simultaneously monitor a position of the IOL within the eye, including: to operate the video camera to acquire an image of a field of view containing the IOL; to perform a contrast analysis on the image to locate a high contrast fiducial of the IOL; to obtain a reference area of interest from the image, the reference AOI being centered around the fiducial and containing a predefined number of video camera scan line segments with a predefined number of pixels per scan line segment; to operate the video camera to obtain multiple successive AOIs from successive images, each AOI containing the predefined number of video camera scan line segments with the predefined number of pixels per scan line segment; and to analyze each of the multiple successive AOIs to determine a direction and magnitude of movement of the fiducial; and wherein the controller is further configured to control delivery of the laser beam into the IOL based on the direction and/or magnitude of movement of the fiducial. a laser light source configured to generate a laser beam; an optical delivery system configured to deliver the laser beam to the IOL, including a scanner system configured to scan the laser beam within the IOL; a video camera configured to acquire an image of a field of view containing the IOL; and a controller configured to control the laser light source and the scanner system to deliver the laser beam in the IOL to modify a refractive property of the IOL, characterized in that the controller is programmed to simultaneously monitor a position of the IOL within the eye, including: to operate the video camera to acquire an image of a field of view containing the IOL; to perform a contrast analysis on the image to locate a high contrast fiducial of the IOL; to obtain a reference area of interest from the image, the reference AOI being centered around the fiducial and containing a predefined number of video camera scan line segments with a predefined number of pixels per scan line segment; to operate the video camera to obtain multiple successive AOIs from successive images, each AOI containing the predefined number of video camera scan line segments with the predefined number of pixels per scan line segment; and to analyze each of the multiple successive AOIs to determine a direction and magnitude of movement of the fiducial; and wherein the controller is further configured to control delivery of the laser beam into the IOL based on the direction and/or magnitude of movement of the fiducial. to operate the video camera to acquire an image of a field of view containing the IOL; to perform a contrast analysis on the image to locate a high contrast fiducial of the IOL; to obtain a reference area of interest from the image, the reference AOI being centered around the fiducial and containing a predefined number of video camera scan line segments with a predefined number of pixels per scan line segment; to operate the video camera to obtain multiple successive AOIs from successive images, each AOI containing the predefined number of video camera scan line segments with the predefined number of pixels per scan line segment; and to analyze each of the multiple successive AOIs to determine a direction and magnitude of movement of the fiducial; and wherein the controller is further configured to control delivery of the laser beam into the IOL based on the direction and/or magnitude of movement of the fiducial.