Aspect Ratio

The image resolution is something most of us quickly comprehend because everyone can understand that an image has height and width. So it would seem that everything is pretty simply expressed in width and height terms, or x pixels by y pixels. That is assuming everything is treated squarely, unfortunately in digitized data, pixels are often not considered square in their basic form. Most images pixels have a fixed size in pixel terms but a differing aspect ratio when viewed, are far from square.

Square & Non Square Pixels - What Are They?

Pixels in the graphics world are deemed square. However pixels in SD are non-square. For example a 100 pixel vertical line may be longer or shorter than a 100 pixel horizontal line on a video monitor, depending on the video system being viewed. Pixels in HD are fortunately, square.

The term which describes this squareness or non-squareness is pixel aspect ratio, expressed as a fraction of horizontal (x) pixel size divided by vertical (y) pixel size. The pixel aspect ratio for square pixels is 1/1.

If you look at an older regular TV the ratio of width to height is 4:3, termed as the aspect ratio. If you divide out the ratio, it comes to 1.33:1, another way of expressing it. However, when modern movies are made, they’re not even close to a square, but rather the shape of a wide rectangle (some wider than others).  Originally films were also shot in a 4:3 aspect and when transferred to TV pretty much the whole image could be viewed, it was cheaper so people stayed at home. However to tempt the public to return to the cinema the film industry introduced wider formats and imagery that simply that could not be reproduced on the television set at home. In order for us to see all of the movie’s image, it needed to be located inside of the square frame of our regular TV. This produces an image with blank bars at the top and bottom, which we refer to as widescreen or letterbox.

 

 

Although the widescreen image is physically smaller in real terms, there is one very important advantage that outweighs the bad:  We don’t miss out on any of the frame compared to the much older practice, called pan and scan where the left and right sides of the frame are cut off so as to fill the TV screen, have you seen the Magnificent Four And A Half - does not have quite the same ring does it, but I suspect you have? From an artistic point of view, the widescreen version is what the director intended for us to see, and it is what we see at the movie theatre.

It is interesting to note it can become very confusing because even though an image is stored on a four by three (4:3) piece of 35mm negative, the image can be projected and captured by using anamorphic lenses. Which will stretch and unstretch the image, changing the aspect ratio, the word anamorphic derives from the Greek words meaning formed again.

The most common aspect ratios used today in the presentation of films in movie theatres are Widescreen (Academy) 1.85:1 and Cinemascope 2.39:1.

Converting formats of unequal ratios is done by either cropping the original image to the receiving format's aspect ratio, by adding horizontal mattes (letterboxing) or vertical mattes (pillarboxing) to retain the original format's aspect ratio, or by distorting the image to fill the receiving format's ratio. See Deliverables for a greater insight.

 

Practical Limitations

In motion picture formats, the physical size of the film area between the sprocket perforations determines the image's size. The universal standard is a frame that is four perforations high. The film itself is 35 mm wide (1.38 in), but the area between the perforations is 24.89 mm×18.67 mm (0.980 in×0.735 in), leaving the de facto ratio of 4:3, or 1.33:1.

With a space designated for the standard optical soundtrack and digital versions, the frame size is reduced to maintain an image that is wider than taller (mimicking human eyesight), this resulted in the Academy aperture of 22 mm×16 mm (0.866 in×0.630 in) or 1.37:1 aspect ratio.

 

Common Film Formats

Before the advent of High Definition there were only two image formats 525/60 and 625/50, unlike 35mm film which has many. The most common are Full Frame, which occupies the largest possible area on the film, Academy and Cinemascope. The specifications for scanning different file formats are as follows:

The scan aperture generally represents the total frame size indicated by full frame. It is generally considered all scanning is done at full frame size as this avoids the complexity of adjusting the scanner optics or raster size with risks associated with repeatability and stability.

In addition different camera apertures can be used at different aspect ratios during principal photography. However these measurements are all based on standard 35mm 4 perf film - there are many more formats still in use.

3 Perf

Most motion pictures today are shot and projected using 4 perf, but cropping the top and bottom of the frame for an aspect ratio of 1.85. In television production, where compatibility is unnecessary, a 3 perf format is sometimes used, giving, if used with Super 35 the 16:9 ratio used by HDTV and reducing film usage by 25 percent, which can be a significant cost saving in production.

However because of 3 perf's incompatibility with standard 4 perf equipment, it can utilize the whole negative area between the perforations (Super 35 mm film) without worrying about compatibility with existing equipment; the Super 35 image area includes what would be the soundtrack area in a standard print. All 3 perf negatives require optical or digital conversion to standard 4 perf if a film print is desired. Though 3 perf can easily be transferred to video with little to no difficulty by modern telecine or film scanners. With digital intermediate increasingly becoming a standard process for post-production, 3 perf has become more popular with productions which would otherwise be averse to an optical conversion stage.

Super 35

Super 35 is a production format. Theatres do not receive or project Super 35 prints. Rather, movies are shot in a Super 35 format and are then either through optical blowdown/matting or Digital Intermediate, converted into one of the standard formats to make release prints. Because of this, often productions will also use Super 35's width in conjunction with a 3 perf in order to save costs on wasted frame area shot and allow too shoot for an extra 33% longer in time with the same length of film.

If using 4 perf, the Super 35 camera aperture is 24.89 mm × 18.66 mm, compared to the standard Academy 35 mm film size of 21.95 mm × 16.00 mm and thus provides 32% more image area than the standard 35-mm format. 4 perf Super 35 is simply the original frame size that was used in 35 mm silent films. That is, it is a return to the way the film stock was used before the frame size was cropped to allow room for a soundtrack. James Cameron was an early, consistent, and vocal supporter of the format, first using it for The Abyss.

2 Perf

2 perf camera systems use only 2 perforations per frame on 35mm film, which gives an aspect ratio close to the 2.39:1 aspect ratio used in anamorphic prints. It was first proposed conceptually around 1930, but was not put into practice until 1961, when Techniscope was developed at Technicolor's Italian branch. It has recently been brought up again with the advent of higher quality, lower grain film stocks as well as digital intermediate post-production methods which eliminate optical blowups and thus improve quality. A proportion of the spaghetti westerns shot in Italy at the time were shot using this methodology, such as Sergio Leone's The Good, the Bad and the Ugly.

Ultimately 2 perf is used because you double the amount of film that is being used in the camera, a huge cost saving. Again James Cameron championed the technique to capture the Titanic during the deep sea diving on to the wreck. Although the format still needs to be transferred to 35mm 4 perf for projection.

VistaVision

The VistaVision motion picture format was created in 1954 by Paramount Pictures in order to create a finer-grained negative and print for flat widescreen films. Similar to still photography, the format uses a camera running 35 mm film horizontally instead of vertically through the camera, with frames that are eight perforations long, resulting in a wider aspect ratio of 1.5:1 and greater detail, as more of the negative area is used per frame. This format is not projectable in standard theatres and requires an optical step to reduce the image into the standard 4 perf vertical 35 mm frame.

While the format was dormant by the early 1960s, the camera system was somewhat revived for visual effects by John Dykstra at Industrial Light and Magic, starting with Star Wars, as a means of reducing granularity in the optical printer by having increased original camera negative area at the point of image origination. Its usage has again declined since the dominance of computer-based visual effects, although it still sees very limited utilization.

A diagram of the VistaVision format, affectionately dubbed Lazy 8 because it is eight perforations long and runs horizontally (lying down).