OK. Yup the wide open button works in movie mode. However it is quite tricky to use when handholding it when shooting video. I can press and hold the button and turn the focus ring easily with my left hand. Otherwise it would be a good feature to have, to be able to quickly MF in wide open to get the most accurate focus. I suppose you can use the wide open button to quickly AF if the lighting is too dark. But it is not very helpful as the EF lenses focus so slow it would take too long to let it just AF during a video shoot.