When discussing aspect ratio, Display Aspect Ratio (DAR) is most commonly implied. DAR is the ratio of the width to the height of the display; examples being 4:3 (used in original SD productions) and 16:9 (now adopted for HD and UHD programming). When film or video material is recorded as a file, it may be stored at a different ratio, known as the Storage Aspect Ratio or SAR. For example, for video shot at 1280×720 pixels, the SAR is 16:9. Many HD storage formats such as DNxHD, AVC HD, and ProRes 422 store HD video as 16:9 such that SAR=DAR (“square pixels”).
However there are a number of frame sizes for which the SAR is not the same as the DAR. These are known as anamorphic frame sizes (with “non-square” pixels). Examples include 1440×1080 and 1280×1080. In these cases, a third type of aspect ratio, Pixel Aspect Ratio (PAR), is defined. The relationship between the three ratios is DAR = PAR x SAR.Movies intended for cinematic distribution may be shot at 1.85:1 or 2.35:1 thus requiring aspect ratio conversion for distribution to viewers typically using 16:9 display screens. Preservation of the entire width of the production may require “letterbox” presentation using black bands above and below the image. Display avoiding the black bands implies cropping the content horizontally, then scaling the picture to occupy the full display height and width.
Some audiences may find letterboxing distracting, especially on small screens as found on aircraft seat backs or mobile phones, so centre-cutting followed by zoom may seem quite appealing. However, the director’s artistic intent may be lost, especially if important action takes place away from the centre of the screen. Manual or automated pan and scan can be beneficial as the editor can choose the most important scene areas, but nevertheless, some of the content will inevitably be lost.