For openSUSE users @Marcus_Meissner already submitted a small script in the gphoto2-webcam package. For Tumbleweed it is already part of the distribution. For older distributions see https://software.opensuse.org//download.html?project=graphics&package=gphoto2-webcam
Of course it is worth mentioning … if you have an UVC/UAC compliant HDMI grabber and hook up your camera via HDMI to that, it will show up as a video4linux device directly and you don’t even need gphoto2+ffmpeg.
That is really nice but we also have to keep that in mind that using a dslr as our webcam needs a very strong internet connection for smooth buffer less video calling experience. Also form the other side the person I am talking with also has to have a very strong internet to receive the video quality I am offering. I think with average internet connection is will not be possible. Is it?.