The axis of action is a term used in filmmaking, in the context of continuity editing. Briefly, continuity editing is a style of film editing that strives to maintain clear spatial relationships between objects in the same scene, across different shots. The axis of action is a tool used during filming to create this spatial clarity. In most films, it is assumed that the action in a scene occurs along an apparent and predictable line. This imaginary and invisible line is called the axis of action, and is also known as the center line or 180 degree line.
When filmmakers go to shoot a scene, they plan almost all of their camera placements to respect the axis of action. That is, the camera will always be placed on one side of the axis of action, and will never cross the line. This particular style of camera placement is known variously as the 180 degree system or the 180 degree rule.
An annotated diagram is the easiest and clearest way to understand the ideas behind the axis of action and the 180 degree system (sorry for the bad ASCII art). In the scene illustrated below, two people, A and B, are simply facing each other.
[6]
^
>[7]
_ _
/ \ / \
-------|A )-------------------( B|--------
\_/ \_/
[4]< >[5]
v v
[2] [3]
v
[1]
Overhead: Here we see an overhead view of the situation. Person A and B, the two labeled lozenges, are facing each other. The dashed line drawn through them is the axis of action. The bracketed numbers represent camera placements. The convention here is that cameras 1 through 5, which are on the bottom, respect the axis of action. When shots taken from these cameras are edited together, they will form a clear and continuous space. Cameras 6 and 7 cross the axis of action, and shots taken from those positions will violate spatial continuity.
The 180 degree system derives its name from the fact that cameras obeying the system are positioned in a half-circle around the action, and that a half-circle has 180 degrees. The cameras obeying the system in this diagram are cameras 1 through 5, and form a rough half-circle. Cameras 6 and 7 violate the system, and occupy the "wrong" half-circle.
(A note on cameras 4, 5 and 7: Since ASCII is so limited, I couldn't angle these cameras correctly, so you'll have to do it for me. Imagine cameras 4 and 7 are rotated 45 degrees counterclockwise, and camera 5 is rotated 45 degrees clockwise.)
+------------------------------------+
| |
| __ __ |
| / \ / \ |
| | o o | |
| |@ \ / @| |
| \__- -__/ |
| / \ / \ |
| | A \ / B | |
+------------------------------------+
Camera 1: This shot shows the view from camera 1. This shot could be used as an establishing shot, which introduces the on-screen orientation of the people in the scene. Here, spectators would expect that person A is on the left side of the screen and is facing right, while person B is on the right side of the screen and is facing left. Future shots that respect the axis of action should maintain these orientations.
+------------------------------------+
| |
| __ |
| / \ |
| | o |
| |@ \ |
| \__- |
| / \ |
| | A \ |
+------------------------------------+
Camera 2: This is the view from camera 2. Note that camera 2 is located on the same side of the axis of action as camera 1, and that the orientation of person A is preserved. Here, person A is still facing towards the right, and there is an assumption that person B is off-screen to the right.
+------------------------------------+
| |
| __ |
| / \ |
| o | |
| / @| |
| -__/ |
| / \ |
| / B | |
+------------------------------------+
Camera 3: This is the view from camera 3. Camera 3 is on the same side of the axis of action as cameras 1 and 2. Functionally, this shot is identical to the shot from camera 2, except that the subject in the frame is person B. As spectators, we expect that person A is off-screen to the left. The shot from camera 1 showed the two people looking at each other, and we expect that person B is looking off-screen to person A. This particular assumption is the basis of another film editing technique, the eyeline match.
+------------------------------------+
| |
| ___ ___ |
| / \ / \ |
| | | o o | |
| | @| |/ | |
| \___/ \o__/ |
| / \ / \ |
| | A \ / B | |
+------------------------------------+
Camera 4: This is the view from camera 4 (remember to imagine camera 4 rotated 45 degrees counterclockwise in the diagram). This shot shows the back of person A's head and a three-quarters frontal view of person B. Notice that the spatial orientation from camera 1 is preserved here; that is, person A is on the left side of the screen and person B is on the right side. This sort of camera angle--over the shoulder of the listener, looking at the speaker--is typical for dialogue scenes.
+------------------------------------+
| |
| ___ ___ |
| / \ / \ |
| | o o | | |
| @ \| |@ | |
| \__o/ \___/ |
| / \ / \ |
| | A \ / B | |
+------------------------------------+
Camera 5: This view from camera 5 (remember to rotate the camera in the diagram 45 degrees clockwise), when compared to the view from camera 4, is known as a reverse angle shot. The term "reverse angle" is somewhat misleading, as camera 5 is not directly opposite from camera 4 (in fact, camera 7, which crosses the axis of action, is directly across from camera 4). Since camera 5 respects the axis of action, the spatial orientation of the people matches shot 1: person A on the left, person B on the right. The term "reverse angle" stems from the fact that the roles of the two people are reversed: person A is now the speaker, and person B is the listener.
If an editor cut together shots 1, 4, and 5 in sequence, that would correspond to a common editing pattern, shot/reverse-shot.
+------------------------------------+
| |
| __ |
| / \ |
| o | |
| / @| |
| -__/ |
| / \ |
| / A | |
+------------------------------------+
Camera 6: The view from camera 6 shows the result of crossing the axis of action: The spatial orientations from shot 1 conflict with the orientation of camera 6, which is on the opposite side of the axis. A spectator, having seen shot 1 first, would expect that person A is facing right. However, person A is shown facing left here. This is an effect purely from the placement of the camera; nothing has changed in the actual scene.
+------------------------------------+
| |
| ___ ___ |
| / \ / \ |
| | | o o | |
| | @| |/ | |
| \___/ \o__/ |
| / \ / \ |
| | B \ / A | |
+------------------------------------+
Camera 7: This view from camera 7 (remember to rotate the camera 45 degrees counterclockwise in the diagram) also crosses the axis of action. Compare this shot to the shot from camera 5: the people have switched places! Person B is now on the left and person A is now on the right, which is at odds with the scene shown in shot 1. While this shot is a "true" reverse angle across from shot 4, it is considered more disorienting than a reverse angle shot from camera 5. If an editor cut together shots 1, 4, and 7 in sequence,
Respecting the axis of action--the first and second rules of the 180 degree system--ensures three things: the relative positions between objects in a scene are consistent on-screen, the screen directions are consistent, and the eyelines are consistent.
Almost all narrative films are shot with the axis of action in mind. A simple example can be found in Everything 2's favorite film, Fight Club. Warning: Very mild spoilers ahead.
Around 14 minutes into the film, the Narrator (Edward Norton) pulls Marla Singer (Helena Bonham-Carter) aside during one of their shared support group meetings. The dialogue between them is shot in a traditional shot/reverse-shot pattern based around an axis of action. The axis of action in this particular scene is drawn from the Narrator to Marla. The filmmakers have chosen to place the Narrator on the left side of the screen, and Marla on the right; all camera angles during this scene will maintain this spatial orientation between the two (excluding, of course, when they move past each other). The scene alternates in framing from medium shots to close-ups. Nevertheless, the Narrator is on the left, and Marla is on the right. The camera never crosses the axis of action, thus providing the clearest and least disorienting way to show the action.
Most mainstream films, Fight Club included, are concerned with spatial continuity, and so all scene are shot obeying the 180 degree system. However, there are times in narrative films where the axis of action is crossed, without disorienting spectators.
Again, we can take a scene from Fight Club as an example. Around an hour and eight minutes into the film, Lou, the owner of the tavern where Fight Club is headquartered, has a rather physical discussion with Tyler Durden (Brad Pitt). Establishing shots set the orientation of the two: Lou is on the left side of the screen, facing right, while Tyler is on the right side of the screen, facing left. An overhead diagram might help clarify the situation:
[4]
^
_ _
/ \ / \
--------------|L )------------( T|--------
\_/ \_/
[2]< >[3]
v
[1]
crowd crowd crowd crowd
Lou is the lozenge labeled with an L, and Tyler is labeled with a T. The other members of Fight Club are labeled as "crowd". During most of the scene, Lou gets in a few free punches. The scene is shot conventionally, using establishing shots (camera 1) and shot/reverse-shots (cameras 2 and 3); Lou stays on the left, Tyler on the right. After a particularly vicious punch, the camera is placed at position 4, in order to get a shot of the crowd reacting behind the action. (There is an excellent match on action during this particular cut.)
Since the camera crossed the axis of action, Tyler is now on the left and Lou is on the right. For the rest of the scene, the filmmakers abandon the axis of action. The camera is placed to best capture the one-sided fight and the crowd in the same frame. This violation of the 180 degree system is not too disorienting, because the scene began very conventionally. By the time the camera crosses the axis of action, spectators are familiar with the space.
Narrative filmmakers will oftentimes align the camera on the axis of action to smoothly transition the camera across the line. This sort of shot is known as either a head-on or tail-on shot, depending on whether the action moves toward or away from the camera.
Experimental filmmakers are usually the ones who will ignore the axis of action. They eschew spatial continuity in order to emphasize some graphical theme between shots or to shock spectators. Keep in mind that obeying the axis of action and the use of continuity editing is a conscious decision by filmmakers. Since most films are narrative, filmmakers want to keep spectator disorientation to a minimum, and the 180 degree system is very effective at this. Non-narrative filmmakers may want to shock spectators, and will cross the axis of action as is their wont. The axis of action is a valuable, but not mandatory, filmmaking tool.
Notes
David Bordwell and Kristin Thompson. Film Art: An Introduction, Sixth Edition. (New York: McGraw-Hill, 2001).
/msg me with any corrections or comments.
belgand says re axis of action: I'd mention the excellent Gollum/Smeagol scene from LOTR:The Two Towers. It shows not only one of the consequences of breaking the axis, but how this can be exploited.