As with all rules in creative ventures, they are able to be broken in some circumstances. But understanding their importance (how and why they work) can be useful as a starting point. The following briefly covers some very basic established norms in film and video shooting.
ANGLE OF THE CAMERA IN RELATION TO SCREEN SUBJECTS
The height and angle of the camera can give emphasis to a subjects dominance, or passivity (also depending on setting, performance and atmosphere). Shooting from a low angle, looking up at a subject can give them a powerful and even threatening appearance (Diagram 1/A). Placing the camera at a high angle, looking down can give the opposite effect, tending to make the subject passive and threatened (Diagram 1/B).
Diagram 1/A (Low angle shot)
Diagram 1/B (High angle shot)
CROSSING THE LINE (180 DEGREES)
NOT “crossing the line” is a basic rule of thumb in filming (especially during dialogue). To start, imagine there is a line drawn down the centre, between two people, the centre of the line being their noses. You want to keep the camera/s on the same side of the line (Diagram 2/A). To cross the line would be visually confusing (Diagram 3/A). Note: the edited shot views for both examples (2/B & 3/B).
Diagram 2/A. (Not crossing the line)
Diagram 2/B. (Final edited effect)
Diagram 3/A. (Crossing the line)
Diagram 3/B. (Final edited effect)
The 180 degree line applies to many other circumstances when filming, but the ultimate purpose is to be able to edit between shots without any confusing, or distracting results. The line although present in theory, does not have to be the centre of a performers face (such as Diagram 4/A & 4/B below). It also applies within sets and places, with objects and other subjects.