A detailed description of exactly what you are doing would help, so that someone can try to reproduce your issue.
However, I suspect that what you are seeing just might be the normal operation of the camera. Depening upon the focus mode that you are using, it may not automatically refocus on moving subjects. If you are using "One Shot" focus mode, then the camera will not dynamically adjust focus as the video records.
Most videographers prefer to manually focus their lenses, anyway, and not rely on the camera to focus.
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"Fooling computers since 1972."
