Increase the frequency at which images are updated in a subscriber

edit

I am using code which tracks the image of a robot. Specifically, the rospy-based code I am using is here.

Here's the relevant part of the code.

"""
The DataCollector class polls data from the rostopics periodically. It manages 
the messages that come from ros.
"""
class DataCollector:

    def __init__(self, 
             camera_left_topic="/endoscope/left/",
             camera_right_topic="/endoscope/right/",
             camera_info_str='camera_info',
             camera_im_str='image_rect_color'):

        self.left_image = None
        rospy.Subscriber(camera_left_topic + camera_im_str, Image,
                     self.left_image_callback, queue_size=1)

This segment defines the DataCollector class and subscribes to a "left image callback method" which continually updates self.left_image to be the image as seen by the robot's left camera:

def left_image_callback(self, msg):
    if rospy.is_shutdown():
        return

    self.left_image = self.bridge.imgmsg_to_cv2(msg, "rgb8")

I timed how often the left_image_callback method is called, and it updates at roughly intervals of 0.5 seconds. In other words, if the robot is moving, the self.left_image will only update every 0.5 seconds.

My question is simple: how do I increase the rate at which the image gets updated? Ideally I can get this to be 0.2 seconds, if possible. If it matters, I'm using ros indigo, and the robot is Intuitive Surgical da Vinci.