Xevi soler

How do I lock X-E1 exposure in video mode? It's stuck on auto, and the only thing it can be adjusting is ISO, as the shutter is locked at 24fps, and I'm using a vintage Nikon lens - so manual f-stop. I know its not designed to shoot video, but I find it hard to believe a camera of this quality that offers video at full HD 24fps, would not allow an exposure lock function. Without exposure control, the video mode is useless.