Somebody asked in the Zee twins thread about their advent scene being "out of focus". I'm replying here because I don't think it's a Zee twins issue, just a general challenge for every studio making 2+ girl scenes.
Full disclosure: I haven't watched the full Zee twins scene, only the trailer. I'm not criticising the scene, which is the best seller for December. I don't know how much of a problem the focus is in this scene, or not. I'm just rambling on in general about focus and using this scene trailer as an example.
It looks like this scene was shot with a shallow depth of field, meaning foreground and background cannot both be in focus at the same time.
Sometimes this is used for cinematic effect, e.g. when the girls are talking to santa in a close-up headshot, either the girls or santa are pulled into in focus. In cinema, this effect is used to draw the viewer's attention onto whichever actor the director wants us to watch at that moment.
Sometimes it can be a problem, e.g when panning across between 2 girls, the camera loses focus because there is nothing in the centre of the frame for it to focus on, so it "hunts" until it latches onto something random in frame which has a sharply defined edge to focus on.
In one shot, you can see the camera has focussed on one of the girls' fishnet stocking because that's a nice sharp-edged shape for the automatic focus software to lock on to. But the stockings are in foreground and the rest of her is further back and drifts out of focus.
In another shot where the girls faces aren't in he centre of the frame, the camera has locked onto the tattoos on the girl's arm in foreground, and left their faces out of focus.
AFAIK there are only 2 ways to fix this;
(1) With experience and planning, the camera(wo)man can manually control the focus for each shot, to concentrate on what they want (e.g. the santa headshots). To keep both girls in focus at the same time, you either keep them really close together, or at least both the same distance from the camera. But, the downside with this approach is everything has be super well planned which can mean loss of spontaneity, slow progress and lots of stop-start.
(2) If you want to use autofocus and "go with the flow" of the action (i.e. just shoot whatever is happening in front of you even if it's unplanned), then you need a wider depth of field to maximise the working area which will stay in focus. But, the downside with this approach is that any distracting backgrounds also stay in focus.
How do you get a wider depth of field? Either increase the light levels (sunlight does this nicely, if you have any), or use a wider-angle lens (but watch for distortions).
Everything is a trade-off. There is no perfect technique, or camera, or lens, or lighting, or autofocus mechanism. Again, it's not criticism, just trying to explain what I see. I don't think the technical stuff is easy, especially on set with a lot of moving parts. Personally, I think the main selling point of the Zee twins advent scene is the fun and spontaneity, not the technical stuff.
