Pulling Focus on HD
Generally you concentrate on the vf and as soon as you see the subject start to go soft you pull focus and then again and so on.
This technique works if the audience see less resolution than we do in the viewfinder. So it has worked over the years on video viewed via standard def transmission.
It *doesn't* work if the audience see more resolution on screen than we do in the viewfinder. This is the case with current viewfinders on HD.
The *Apparent depth of field*, my term:) increases with lower resolution displays.
You could say the actual limits of the depth of field are only visible on the highest quality display device for that meduim/format.
The higher the resolution of the capture and display medium the more the viewer can discern what is and what isn't sharp. A slightly soft shot on HD looks fine on a 9 inch monitor but is clearly soft on a 20 inch HD monitor.
For the moment the highest res display of HD for an audience is a good transfer to 35mm film closely followed by a full res HD monitor.
If the shot is being transferred to film don't rely on pulling focus on a moving subject by the above quoted method.
When working with a HD version of their favorite standard def lens DPs will be surprised on how more obvious the depth of field is... the apparent depth of field is less on HD when viewed on a high res monitor. Same lens design, same T stop so same depth of field you will say... in theory yes in practice no.
HD brings us into the difficult situation where the viewer, if watching on a high res display, will see more than the operator....ouch!