When shooting film it would be a bad idea to try and pull focus off of a monitor. Most film camera's will have a standard definition video tap and even if they have an HD tap, it still wouldn't be advisable to pull focus off a monitor. The video tap is best used for giving a director an idea of the shot and for the AC to judge what's in frame or camera information if you have it set to display that.
Video taps are essentially small camera's pointed at the ground glass, they can appear very grainy and dark. Sometimes everything could look in focus and sometimes the image will be so soft in general that everything could appear out of focus. For best results with film, the AC should measure during the rehearsal and pull focus off those marks. What could appear in focus on a video tap image could be completely different from what will be on the film.
However, if you do go incredibly soft on a shot it will be apparent on even the lowest quality video tap and you'll know you need another take.
Edited by evanwalsh, 02 May 2016 - 09:28 PM.