The term Aspect Ratio means the ratio between the width and the height of a picture. All standard TV sets for instance have a 4x3 aspect ration. This means that if your TV set is 28" wide, it will be 21" high (28/21 = 4/3) and will have a diagonal of 35" (sqrt(28^2+21^2)). The diagonal is the unit of measure that the TV Manufacturers have adopted as a standard to describe the size of a TV set.

  Up to 1954, all movies, except for some oddities, were filmed in an aspect ratio of 4x3 or 1.33 to 1. That ratio is also knows as the Standard ratio or the Academy ratio. The movies Gone With The Wind, Singing In The Rain, Fantasia and Citizen Kane for instance were all filmed that way and should be projected with that ratio. When TV started, it was decided that the ratio of a TV set should reflect that of the movie screens of the time so that movies would be shown perfectly on the TV set. As of 1999, the Internet Movie Database lists 5127 movies which have been filmed in the standard Academy ratio

  However, in the late forties and early fifties, the movie studios started to feel the pressure from TV and decided to invent new movie formats in order to add more value to the movie theater experience. It was decided that wide was the way to go, and various ratios have ensued. In 1954, Two high-end processes emerged which allowed wider ratios. Cinemascope and Panavision combine a special film and lenses to allow a ratio of 2.35 to 1 ratio or 7x3. It is almost twice as wide as a TV set. Many movies are filmed in that ratio and it is the favored ratio for big movies such as Star Wars or Titanic. Classic movies such as Casino, Manhattan or West Side Story were also filmed with this ratio.

  As of 1999, the Internet Movie Database lists 4057 movies which have been filmed in the 2.35 ratio. Here are some of the earliest movies in Cinemascope, Panavision or other cinematographic process. Notice how Disney and Hanna/Barbera are present.
   - 1954, Grand Canyonscope, Disney (Animation, Short)
   - 1954, Pet Peeve, Joseph Barbera and William Hanna (Animation, Short)
   - 1955, Lady and the Tramp, Disney (Animation)
   - 1955, Bad Day at Black Rock, John Sturges
   - 1956, Bus Stop, Joshua Logan
   - 1956, Et Dieu... créa la femme, Roger Vadim

  Over time, other ratios and processes have also become successfull. The most popular ratio has got to be 1.85 or 5.5/3. Many movies since 1954 such as Annie Hall, Vertigo, E.T. or Elizabeth, were filmed in that ratio. As of 1999, the Internet Movie Database lists 2018 movies which have been filmed in the 1.85 ratio. Here are three of the earliest movies to use that format:
   - 1954, Athena, Richard Thorpe (I)
   - 1955, All That Heaven Allows, Douglas Sirk
   - 1957, 12 Angry Men, Sidney Lumet

  Some films such as Ben Hur in 1959 have followed more esoteric ratios such as the Anamorphic SuperPanavision at 2.76 to 1! Given the success of wider formats, it was thought that for a great spectacle such as Ben Hur even wider would be a perfect fit. However, the cost of the camera, the lenses, the film and the projector were even higher than for the regular Panavision or Cinemascope processes, and only a few other movies have ever been filmed in that process with that ratio.

  Other movies such as Do The Right Thing, 37°2 le matin, Les Amants du Pont-Neuf or The Shining were filmed with an intended 1.66 ratio. As of 1999, the Internet Movie Database lists 752 movies with that ratio.   When you want to make a film with a wide aspect ratio, there are three main techniques: use a wide film, use an anamorphic lense, or use mattes.     This is the venerable 70mm film that used to be advertized everywhere for bigger movies and better theaters. By having a wider lense and a wider film, it is possible to capture an image that is also wider. This process also benefits from more resolution from the film used, so it generally delivers a better quality picture.     The concept here is simple. When filming, use a lense which expands the image vertically, and when projecting the film, use a reverse lense which re-contracts the image. The advantage of this process is that you can use a regular film and just use special lenses to expand and then contract the image. Anamorphic lenses have this property where the vertical resolution density (dots per inch or dpi) is not the same as the horizontal resolution density.
    If you have a printer which is 600x1200 DPI, you can see that effect if you print a square or circle using the native printer resolution: the image printed will be narrower or shorter depending on the ratio.
    The advantage of this technique is that it saves costs on the film and development process by using standard film and development equipment. On the downside, you generally lose quality compared to a true wide system simply because the film's area is just smaller.     Finally, one can use a regular film, but simply block the top and bottom part of the image during filming and/or projection. This is the most low-tech technique and results in the lowest quality overall because not only is the film smaller than a wide film, you also crop the top and bottom of the film compared to an anamorphic process, so you lose quality because the area of the film where the image is captured is even smaller.   One of the issues that started surfacing was the ability to show movies which were filmed at a wider ratio on the narrower TV set. What can one do to fit an image that is 7x3 in a rectangle that is 4x3? There are several ways one can achieve that: Pan&Scan, LetterBoxing, Open Mattes and Vertical Streching. Let's look at examples using the 2.35 ratio.
    The most popular solution up to recently is a process called Pan&Scan. The idea is to create a small window into the actual image and slide that window left or right depending on where most of the interesting content of the image is. If for instance, you have a character on the left looking at the distance, the window would be all the way to the left and one would not see the right side of the picture. If the character were on the right, the window would be to the right and one would not see the left side of the picture.
2.35 P&S
   On this sample picture, the window is centered and therefore, one does not see the "Di" and half of the 's' on the left nor the "om" on the right. The image has been cropped on each sides.
    This process has its advantages. Most notably, it makes use of the full TV screen to display the image. This means that the image looks as big as it can be, and it also uses all the vertical resolution of the set, resulting in an image with lots of details.

    However, at any point in time, almost half the picture is missing. Most movies with stunning photography or stunning exteriors or sets suffer a lot because the window will always focus on the character looking at the sunset for instance, rather than the sunset itself. Another downside of this technique is more serious. Let's take our example of the character looking at the sunset... What if you want to show the whole picture because that sunset is really stunning. The window must be made to slide from left to right in order to show the character and then the sunset. If you think about it, it looks as if the camera panned from left to right, but the camera never really did that, the Pan&Scan process added that in. This results in "fake" camera movements where originally, you had a static shot. This happens a lot in action movies with wide shots and for those who notice, it can become quite annoying because it breaks the composition of the entire scene. A great example of this is on the 5 Star DVD of Die Hard. If you have a copy, there is a featurette on the disk that explaines how the Pan&Scan process was done on a scene of the movie. Once you see this documentary, you'll understand how destructive the Pan&Scan process can be.

    Sometimes, it is even not possible at all to use that sliding technique simply because the action is equally distributed across the whole image. What can one do if two people are sitting at opposite sides of a table for instance and are having a conversation? The technique used is simply to show alternatively each side of the picture creating an effect where static shots alternate. This inserts heavy editing into a scene that originally was a static shot. Once again, this can have dramatic effect on the original intent of the filmmaker who chose a static shot where both individuals can always be seen at the same time. When one talk, often, the interesting part is the reaction of the other person listening. This is a cinematic technique that is quite powerful, but loses completely its effectiveness when Panned&Scanned. To see how dramatic that effect can be, you can watch the Widescreen and then the P&S version of The Godfather.
    The next technique is simply to open the mattes. You take the entire film and project it on the TV screen, "restoring" the full screen. This is probaly the most absurd thing for so many movies because during production, no one intended the movie to be opened in that way. Not only does it change the picture, but it can also open areas of the pictures not intended to be shown. Directors and Cinematographers aremuch better at this thiese days, but in the old days, you'd suddenly see a microphone on the top or bottom of the screen, or worse, unfinished sets. The most horrible example of that is in the climatic scene of "North By Northwest" where in the full-screen version, you can see the unfinished top of the set for Mount Rushmore. Nowadays, directors are aware of the TV screen and film their stuff "better" so that the open-mating process does not reveal horrible stuff. Nevertheless, it still changes the original vision of the Director and Cinematographer.     This is another one of those absurd process. Have you ever noticed sometimes how people in movies on TV seemed stretched vertically? For instance, watch the opening scene of Indiana Jones And The Temple Of Doom for a vivid example. Here, the idea is to compress the image on the side so you can fit more of it on a TV, but this results in the image looking stretched. Suddenly, circles look like vertical elipses, and people lose 10-20 pounds! The image is simply diformed.     Letterboxing the the favored technique among film enthusiasts, but it is often misunderstood by the masses, and as a result, rejected. Although the DVD format helped popularize this process and bring it to the masses, there is still a lot of confusion, which forces companies like Blockbuster, and some studios to still rent, sell, or release films that are full screen. In 90% of the cases, the process used is Pan&Scan.

2.35 LB Sample

    As the picture above shows, the idea for Letterboxing is simply to shrink the image until it fits horizontally on the TV screen. This causes black bars to appear on the top and bottom of the screen. A lot of people think that those bars hide something, but it is not true. It's like framing a regular picture in a square frame: the background of the frame shows more on top and on the bottom because of the different shapes.
    The advantages of this process are clear compared to the other processes: the integrity of the original picture as shot by the Direcvtor and Cinematographer is preserved. Now, you see the character looking at the distance and you see the sunset too, at the same time, in a static shot, without camera pannings. A scene where 2 characters at each end of the picture talk to each other is properly rendered too through this process, and you do not have added-on editing anymore. This process means that you see the movie exactely as you might have seen it in the theater. It preserves the theatrical presentation of the movie.

    Besides the criticism that the image is chopped off, which is absolutely not true and shows only the ignorance of the person who states it, there is another criticism which does have some merit: because you now use a third of the entire screen, the actual resolution of the picure is much lower than with full screen. Watching a Letterboxed movie on a 19" TV is quite a torture. The minimum is probably a 27" TV set, with 32" being recommended. But still, although a bigger screen makes the picture appear bigger overall, you still only use a third of your screen, and therefore, a third of the vertical resolution of your TV set. Several techniques have been used to compensate for this.
    The first idea tha comes to mind is probably the following: why not have TVsets that more closely match the shape of a movie picture? Well, taking advantage of the digital revolution, this exactely what the industry has done. The new HDTV sets have an aspect ratio of 1.78 to 1, more familiarely known as 16x9. It's wider than the standard TV sets which are 1.33 to 1. It is not quite the asbect ratio of a wide movie at 2.35, but it's much better than regular sets. Widescreen TVs try to create a compromise between regular TV programs and wide movies. So, for wide movies, you'll still have black bars at the top and bottom, but much less. And for regular TV programs, a funny thing will happen: there will be black bars at the right and left of the picture because now, the picture is narrower, instead ofbeing wider, than the screen it is displayed on.
    The second technique borrows from the Anamorphic process. What if your TV screen had a much higher resolution density and could compress the image when displayed? When you purchase DVDs, you probably have seen logos that say "16x9", or "optimized for widescreen TVs". Well, that's it. Those DVDs are what's called Anamorphic. The picture on the disk is encoded with much better vertical resolution and the image is therefore vertically stretched. When displayed on a TV set that can accept an anamorphic signal, the image is vertically compressed to regain it's normal shape. This results in a denser, more detailed picture since you gain close to 33% more vertical resolution. So, your image still takes about a third of your screen, but because the TV set has a higher vertical resolution, you lose much less detail than on regular sets, and you get a much improved image quality
    Finally, one can use a mix of those techniques all together. For instance, you could take a 2.35 movie, do a little bit of Pan&Scan and reframe it not to a TV format of 1.33, but to a "less wide" widescreen format like 1.85 for instance. Or you could do some vertical stretching and Pan&can at the same time. You probably also noticed how sometimes, for the credits, the image is letterboxed so that the tittles can all show properly. But as soon as the movie effectively starts, the image goes into a Pan&Scan mode. By combining different processes and techniques, people try to get the best of each technique at the appropriate time for a TV broadcast, but at the same time, tend to confuse viewers who then look at a more heavily processed movie.     Hopefully, the following table should give you a better sense, visually, of the most common aspect ratios. The first 2 columns illustrates how much of the picture you lose with Pan&Scan. The Gray area represents the shape of a TV screen, and the full rectangle represents the total image. With the 2.35 ratio at the bottom, you can see that close to half of the picture simply disappears. The first column shows the Pan&Scan window pushed to the left, and the second column shows it centered.

Left Justified Pan&Scan LetterBoxed WideScreen TV
1.33 or 4x3 1.33 Left 1.33 PS 1.33 LB 1.33 WS
1.66 1.66 Left 1.66 PS 1.66 LB 1.66 WS
1.78 or 16x9 1.78 Left 1.78 PS 1.78 LB 1.78 WS
1.85 1.85 Left 1.85 PS 1.85 LB 1.85 WS
2.35 2.35 Left 2.35 PS 2.35 LB 2.35 WS

    The third column shows how a letterboxed image shows on a regular TV screeen, and the fourth column shows the same concept, but on a widescreen TV. Notice that for smaller ratios, including the regular TV format, the black bars appear on the sides instead of the top and bottom. Open Wide, by Eriq Gardner on Slate
The Ultimate Guide to Anamorphic Widescreen DVD (for Dummies!) by Bill Hunt from The Digital Bits