The history of aspect ratios in film and video is a topic that deserves more than a simple blog post. Others have already provided us with a nice outline of the various formats and even an online museum of it’s various technical manifestations. In short, just as technology has changed so too has the aspect ratio. It is a combination of the size of the recording medium and the size of the crop. In film it is determined by the size of the film itself, e.g. 8mm, 16m, or 35mm, and the size of the film gate that exposes light on this film. In digital, it is the same but the film is replaced with the digital sensor.
The general trend in cinema has been to expand the field of view by having a wider aspect ratio. Formats like Super 8 and Super 16 represent decisions to expand the size of the exposed area to make better use of the same size film gauge. The era of “scope” films represented a move to extremely wide apsect ratios. Shawscope, Cinemascope, Tohoscope, and many others were all different brand names for this trend. Much as 3d projection has recently returned as a selling point, so too wide screen was used to attract audiences to the cinema with the promise of a more realistic field of view. These ~scope films were largely made possible by anamorphic lenses which optically compress a wider field of view into the same aspect ratio as 35mm film. During projection, these films were restretched to their intended anamorphic aspect ratio by an lens attached to the projector. Recently, the rise of large digital sensors, such as those in dslrs, has sparked a renewed interest in these anamorphic lenses and an equal increase in prices within the second hand market.
The expense of these specialty anamorphic lenses, during the first wave of their popularity, lead some to adopt the practice of simply cropping to a wider aspect ratio. This was branded as Techniscope. Below you can see an example of two films from Godard, one shot in anamorphic Cinemascope and the cropped Techniscope. Both have the same 2.35:1 aspect ratio. Also below you can see examples of the other most common aspect ratios.
In terms of television, 4:3 was the aspect ratio of “standard definition” for years. This is a relatively square format and presented the problem of how to adapt the wider aspect ratio of films. The most popular choice was “Pan and Scan” which means cropping to a section of the film and panning around to include relevant imagery. The other option is to include black bars on the top and bottom to allow for the display of the full original aspect ratio. The advent of “high definition” or “HD” is an increase in resolution, officially 1920×1080 with a smaller 1280×720 also available. It is commonly know as 16:9 or what is effectively a 1.78:1 ratio. this is a wider aspect ratio than 4:3. This reflects the same move to wider aspect ratios that took place in film and makes it easier to show the full frames of wider cinemascope films.
HD video can be cropped to other aspect ratios, just as was done in with Techniscope. Here are a couple of mattes for turning 1080 HD footage into either 1.85:1 or 2.35:1.