Here's an example of why you need to set your camera to manual focus, even when using full-auto and VirtualDub. Everything is smooth up to near the end, where suddenly the camera has trouble picking the focal point. This causes some flashing, a small zoom effect, and mis-timings on the shutter.
Shot on a Canon EOS 50D with a 10mm lens.
Shutter speed, aperture, ISO: all auto
Post-processing: VirtualDub with MSU Deflicker plugin; also added 30% saturation to bring out the colours of the sunset, such as it was.