Blaine Russom
If you're talking about a situation where the subject/camera distance doesn't change, but the focus is shifting (going out of focus) spontaneously, then there's apparently something wrong with your lens. The BMCC doesn't have any control over lens focus. Focus is manual; a lens function.
But if the subject/camera distance is changing then, as expected, you'll need to adjust focus periodically to keep the subject in focus. Use the camera's double-tap magnify feature on the touchscreen. Shooting with a relatively high aperture value will make it easier to hold focus, because the depth of field will be deeper.
A follow focus mechanism can make manual focussing easier, but it doesn't make it easy. Keeping a moving subject in focus is a skill; like other skills, it usually improves with practice.
A high quality external EVF or monitor can also be a huge help for focussing, but of course can be expensive.
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