Let's get on to some cinematography basics, starting with...
THE LINE (aka "The 180° Rule)
Within the first 20 or so years of the 20th century the "language" of cinematography was pretty well developed and has changed surprisingly little since then. You don't realize you know it, but thousands of hours of exposure to TV and film have trained you to subconsciously "read" film. We all understand a common visual lingua franca, and to fail to learn the basics makes you communicate badly.
One of the most common mistakes for beginning filmmakers relates to "
The Line" and eyelines. Heck even pros occasionally make that mistake, too, and have to fix it in post (if they can).
In short The Line is an imaginary line you draw between two players in a scene, and which extends through them. For instance, imagine if you drew a line between Kirk and Sevrin in this shot from "The Way to Eden":
The basic rule of The Line is that once you establish it, you
cannot cross it. This is because The Line establishes direction of looks, movement and relative positions in relationship to the camera. It makes it possible to maintain a sense of screen direction and where things are even when you change setups and angles.
Let's say you shot the above scene. You've established that Kirk is to screen left of Sevrin, and must look to screen right to face him. If Sevrin faces Kirk, he must face screen left. Now, if you wanted a closeup of Kirk without Sevrin in the frame, you'd have to stay on the same side of The Line, so that Kirk's eyeline still goes to the right side of the frame.
In all the camera positions to this side of the line, Kirk's screen facing is always to the right, even if we looked over his shoulder or over Sevrin's shoulder
Why is this important? Because if you cross the line you break up the sense of which character is where and what they're looking at. For instance, if we "Crossed The Line" in the Way to Eden scene to get a closeup of Kirk, he ends up looking to frame left, like this...
...with the result that he appears to be looking away from Sevrin, which makes the looks mismatch.
This is why you don't cross The Line.
HOW IMPORTANT IS THE LINE, REALLY?
Let's look at this same scene as it appears in the episode.
Sevrin starts here...
...then runs off to screen right ending up here...
...after which the editor chose to show Kirk looking at him, but (apparently) he didn't have a shot of Kirk facing the correct direction, so he did
this...
Yes, he
flopped the shot horizontally to make sure Kirk's eyeline remains consistent. In short, maintaining the eyeline trumps making sure the badge is on the correct side of the uniform. It's that important to the scene making sense.
Lots of beginning filmmakers have this idea that rules are made to be broken, and rules like The Line don't really matter. Well, they're wrong. If you want the action and relationships on the screen to be immediately comprehensible, you stick to the language everyone knows, and which makes intuitive sense.
The Line in scenes with more than two characters can get trickier, but we'll talk about that later.
REDRAWING THE LINE
The Line isn't a permanent thing. You can establish a new Line if the actors move around in the scene, or if the camera moves (say dollies) to a new position, but you need to see the move and establish this new Line.
Hope that all makes sense!