Webcams that don't support 640x480 require fiddling

[Tim Band]

If you attach a webcam that doesn't support 640x480, because all the import options have this resolution set, it doesn't work. The capture preview window remains black.
With my old Sony camcorder attached via USB, only 320x240 is supported:
tim@Equium:~$ v4l2-ctl --list-formats
ioctl: VIDIOC_ENUM_FMT
    Index : 0
    Type : Video Capture
    Pixel Format: 'MJPG' (compressed)
    Name : MJPEG

tim@Equium:~$ v4l2-ctl --list-framesizes=MJPG
ioctl: VIDIOC_ENUM_FRAMESIZES
    Size: Discrete 320x240

Of course, it's pretty easy to fix once you know all this, but it's not exactly noob-friendly.