If you still don't have enough light for correct exposure of your video despite a low f-number, you need the last value of this article: the ISO value. In digital cameras, the ISO regulates the sensor's light sensitivity and thus has a direct influence on the brightness of your image. You will also get a brighter picture if you set a high ISO.
In this way, the ISO can help you achieve correct exposure, even in rather dark scenarios when a low f-number is insufficient. Of course, it would be nice if you could adjust the ISO at will to get a nice picture even in the darkest situations. Unfortunately, using a high ISO also entails a loss of quality, similar to trying to lighten videos or images that are too dark in post-processing. This then manifests itself in image noise...
