cv_bridge: reading distance from /camera/depth/image_raw

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Hi, I'm running an turtlebot simulation on gazebo. roslaunch turtlebot_gazebo myrobot.launch

I subscribed to the topic /camera/depth/image_rawto get the distance information using code

def get_distance(img):
       #calculate x and y
       bridge=CvBridge()
       cv_image = bridge.imgmsg_to_cv2(img, "passingthrough")
       print cv_image[x,y]

rospy.Subscriber('/camera/depth/image_raw',Image,get_distance)

according to the document,

depth/image_raw (sensor_msgs/Image)

Raw image from device. Contains uint16 depths in mm.

But I keep geting float numbers between 0 and 5.

Then I tried several other converting types besides "passthrough"

32FC1 gave me a complex number.

32SC1 gave me a large negative number.

16UC1 usually gave me 0.

16SC1 and 8UC1 gave me contant 5.

none of them makes sense, can anyone give me some idea? Thanks.