usb_cam camera frame rate, and rostopic

Hello, I am using usb_cam module to get image from my usb logitech c910. I am using the following usb_cam.launch file: When I launch roslaunch usb_cam usb_cam.launch it indeed works and reports "Starting 'head_camera' (/dev/video0) at 800x600 via mmap (mjpeg) at 50 FPS" However, when I use `rostopic hz --window=1000 /usb_cam/image_raw` it will still report 24hz like: subscribed to [/usb_cam/image_raw] average rate: 23.950 min: 0.037s max: 0.047s std dev: 0.00267s window: 23 average rate: 23.994 min: 0.037s max: 0.047s std dev: 0.00233s window: 47 How can I make sure that the image is pipelined at 50Hz? Best regards, C.