ROS C++: nh.subscribe vs message_filters::Subscriber<> , nh.subscribe works well but message_filters::Subscriber<> does not work

I'm tring to learn ROS topic, but I met a problem that really confused me.

Here is the origin code. It works well, I can receive the image and pass it to CamImgcb.

mSubCam = mNh.subscribe<sensor_msgs::Image>(TopicNameCamSub,10,boost::bind(&ClientHandler::CamImgCb,this,_1));

But when I change it to this code below, CamImgcb can not receive anything, the whole program is stuck to wait for the image to come.

message_filters::Subscriber<sensor_msgs::Image> rgb_sub(mNh, TopicNameCamSub, 10);
rgb_sub.registerCallback(boost::bind(&ClientHandler::CamImgCb,this,_1));

My question is that aren't those two codes means the exact same thing? Why is there a difference between them? I just can't figure it out.

Does anyone know what is the problem? Please help me and thank you so much!