focus setting in usb_cam

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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:

<include file="$(find usb_cam)/launch/usb_cam-test.launch">
    <arg name="video_device"     value="/dev/video1"/>
    <arg name="image_width"      value="1600"/>
    <arg name="image_height"     value="1200"/>
    <arg name="autofocus"        value="false" />
    <arg name="focus"            value="100" />
</include>

Also, if I use guvcview to change its focus, it does have effects and can keep the focus parameter in mind.