Letterboxing

Every so often, while kicking back in front of our 52" flatscreens, we're reminded that television used to be different.  Not just more wholesome, but different.  For instance, take a look at these clips from ESPN's flagship show SportsCenter.

The TV on the left shows the intro to the very first episode, while the one on the right is more recent.  Why doesn't the first clip take up the whole screen?  What's up with those bars running along the sides?  Why is that necktie so awesome?  It all has to do with ratios (well, except for that last one).

Actually, it has to do with Thomas Edison.  In the Long-Ago Times (prior to ca. 1975), if you wanted to record video, you needed strips of light-sensitive material known as "film."  You loaded this film into the camera, which had some sprockets that would latch onto perforations in the film and pull it through the camera, exposing frame after frame as it wound its way through.

4x3

The physical dimensions of the camera determined its film size, but different cameras required different sizes.  Edison and his buddy William Dickson decided that this was pretty dumb, and that film sizes should be standardized.  In 1892 they were using 35mm film stock; any image recorded on that film had a width:height ratio of approximately 4:3.  This measurement is known as the film's aspect ratio.  Because everybody generally agreed that Edison was a genius, 4:3 quickly became the industry standard.  In fact, even today 4:3 is often referred to as "standard format" for video.

For a while, movies continued to be shot in 4:3; television followed suit, so watching a movie on a television screen was no big deal.  But then movies got wider: eventually, we got the 16:9 "widescreen" format and the 21:9 "cinematic" format.  Currently, 16:9 is the conventional format for HDTV, so newer shows are often filmed in this ratio.

Which brings us back to SportsCenter.  That first episode was shot when the 4:3 ruled the screen, but nowadays 16:9 reigns supreme.  And here's where we run into a problem: you can't just scale old 4:3 content to fit onto a 16:9 screen, because the ratios are different.  No amount of scaling can change the ratio.1  Here's an interactive that will let you compare an image on both a 4:3 and a 16:9 screen:

Is there anything we can do to have the image fill the frame in both screens?  Well, yes, sort of.  You could scale the width and height by different amounts.  For instance, to get a 4:3 image to cover a 16:9 screen, you could scale the width by a factor of 4 and the height by a factor of 3.  In fact, if your TV has a "Zoom Stretch" option, that's exactly what it does.  Are you sitting by your TV right now?  How does "Zoom Stretch" look?  If you answered "terrible," you are correct.  The image gets distorted, which isn't such a huge deal for rectangles, but it's pretty rough on things like human faces.

Another option would be to scale the image uniformly until it's as big as possible, without losing any of the original content, and then fill in the leftover space with black bars.  This is what's happening in our SportsCenter example, and it's called "Letterboxing."2  The bars are a bit annoying, and you waste a lot of screen area, but at least you can see everything.

When you're trying to go the other way, say watching a 16:9 movie on an old 4:3 TV screen, you can also zoom so that the image takes up the whole screen, which lops off a bunch of the content.  To compensate for the lost image when zooming, you can pan along the cropped image to show more of what's going on.  This technique is called "pan and scan," and was pretty common in the era of video tapes made for home viewing.  Directors hated pan and scan, because it effectively meant somebody edited the film to create a fundamentally different cinematic experience.  Here are some bigwigs on the subject:

Next time you're watching Lucy reruns on your widescreen, or Lawrence of Arabia on your old 13" set, think about how what you're seeing reflects some interesting decisions in entertainment history.  Think about the ways your technology tries to compensate for those incompatibilities.  And think about Thomas Edison.

Teachers, want to have this conversation in class?  Check out our lesson, Letterboxing!


1. No amount of isotropic scaling (i.e. scaling uniformly in all directions) will affect the ratio.

2. Presumably because the horizontal bars resemble mail slots. Technically, when the bars are vertical it's called "pillarboxing," but "letterboxing" is more common.


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