Setting VideoFps in ConversionOptions does nothing

[Alexander-0x80]

Hey , i am trying to reduce fps of a video with this code :

    var conversionOpt = new ConversionOptions
    {
        VideoSize = VideoSize.Qvga,
        VideoFps = 2,
    };

     engine.Convert(videoIn, videoOut, conversionOpt);

Changing VideoFps does not not affect the video .