They don't know, chum. They don't have to know. But they know the proportions of your screen, which are the same for all North American TVs, namely 1.33:1 (or four by three). Nowadays most films are shot in wider formats, and adapting them for the profitable video market can entail a number of stratagems, some quite creative, some pretty kloodgy. Despite promised advances in TV technology, I'm betting the problem ain't going away, because of competing economic interests. But more on this in a mo'.
First some history. Prior to 1953 all movies were shot in 1.33:1, and the infant TV industry adopted the same format. But for just that reason, movie moguls decided they needed a different format to parry the perceived threat from the tube. They came up with two answers: 3-D, which was seldom more than a gimmick, and wide-screen projection, which became the industry standard. A variety of wide-screen formats were introduced, the most ambitious of which, Cinerama (2.62:1), required three projectors and a curved screen. That was too expensive for routine use, so most studios making "spectaculars" adopted processes variously known as CinemaScope, Techniscope, Panavision, et cetera. These required a 2.35:1 screen and special anamorphic lenses that squeezed the wide image onto a standard 35-millimeter film stock and then expanded it back to full width when projected.
Theater owners soon hit on an even easier way to create wide-screen movies. They used a special plate in their projectors to mask the top and bottom of the image, then projected what was left on a wider screen. Voila, instant spectacular! Two formats were widely used -- 1.66:1, now used for most western European films, and 1.85:1, which became the U.S. standard. (The standard for 70-millimeter movies is 2.21:1.)
There's only one problem: Creating wide-screen movies by chopping off the top and bottom of the picture sometimes cuts out important parts of the scene. So movie directors and cinematographers compensate by continuing to shoot most movies in 1.33:1 but composing scenes so that the important stuff will fit in a 1.85:1 or 1.66:1 frame when projected. (Lines on the camera viewfinder aid in this.) This process has gotten so casual that studios sometimes release finished films with boom microphones, cables, et cetera, visible at the top and bottom of some scenes, confident that theaters will cut the crap when they project.
So your movie has had its theatrical run, and now you're releasing it on video. What do you do? You could ship it out in the original 1.33:1 format, taking the risk that extraneous items would sometimes be visible. You could "letterbox" it, reducing the width so it fits the TV screen, with black bars above and below the frame.
Often the best choice (though one that offends cinephiles) is what's known as "span and scan," a lab process in which the copy camera moves around within the frame, allowing you to adjust what shows up on the TV screen on a scene-by-scene or even frame-by-frame basis. Let's take a dialogue closeup in which the actors are at the far right and left of a 1.85:1 frame. If you ran the shot at the original 1.33:1 (assuming this were even an option), you'd lose the close crop and thus some of the scene's intimacy and intensity. So instead, lab magicians cut or pan back and forth between the two actors. Only one appears in the frame at a time, but if you do it right, you retain the impact of the theatrical release. That's the mark of the unsung geniuses at the lower echelons of the movie business -- doing the job so skillfully that nobody knows.
Is there something you need to get straight? Cecil Adams can deliver "The Straight Dope" on any topic. Write Cecil Adams at the Chicago Reader, 11 E. Illinois, Chicago, IL 60611; e-mail him at firstname.lastname@example.org; or visit "The Straight Dope" area at America Online, keyword: Straight Dope.