focus setting in usb_cam

When I try to use `usb_cam` to control uvc camera, setting the "focus" parameter in the launch file, it seems to have no effect on my image result. But it does have effects when I change the "image_height" or "image_width" parameters. why did it happen? Here is my lanuch file code: Also, if I use `guvcview` to change its focus, it does have effects and can keep the focus parameter in mind.