The theory is pretty simple: if you place your subject along one of the four intersections in the nine-square grid, the viewer’s eye is supposedly more easily drawn to that subject, since it’s argued that that’s where the eye drifts anyway, rather than straight to the centre.
Of course, while photography and cinematography are both highly technical, that still doesn’t exactly make them a science, as subjectivity and creative freedom are both critical for the creation of vibrant and unique work, so this rule is more accurately called a guideline, a suggestion, or, for more hidebound photographers or videographers, an insistence.
With that said, there are some pretty strong arguments in its favour. Proponents argue that when the eye is drawn straight to the centre of an image, it then has nowhere else to go, having taken in the primary visual aim of the image. By shifting the subject to align with one of the four intersections, the subject is placed more in the context of an image while remaining the focal point, while subconsciously encouraging the viewer to explore the image beyond the subject itself.
The rule of thirds grid, with the focal axes circled.
The trouble with the rule of thirds is that some subjects are too big or imprecise to place along those four points, and if it comes close to, but not quite on, one of the four intersections, it can be argued that it’s more of a coincidence of thirds, rather than a rule.
The rule of thirds plays along quite well with natural points of focus like human eyes, as in image like this from Blade Runner (1982) . We get a subconscious signals that tells us Harrison Ford is the central character, but he’s within a broader story, and not in control, but navigating it as best he can
Harrison Ford in Bladerunner (1982)
Jack Nicholson in Chinatown 1974
But then, consider the above image. The rule doesn’t work here. Why not?
Well, because Jack Nicholson’s eyes, already a focal point, are in the centre of the image. That’s because at this point in the story, we’re meant to be imagining what’s going through Jake Gittes’ mind and what that looks like in response to his surroundings. The background characters are out of focus because their mere presence is more than enough to inform the scene.
So the rule of thirds is a useful tool, and it can help set up some fantastic shots. It may not be a rule, and it can easily be abandoned in favour of a layout that more directly serves the visual language of a piece of work, but “rough guide of thirds” doesn’t exactly roll off the tongue.