Unlike pre-HD television, which had only two image formats, 525/60I and 625/50I, 35 mm film has many. Of these the most common are Full Frame, which occupies the largest possible area of the film, Academy and Cinemascope. The scanning for these is defined in the DPX file specification as follows:
|Scanning resolution||Full frame||Academy||Cinemascope|
|4K||4096 x 3112||3656 x 2664||3656 x 3112|
|2K||2048 x 1556||1828 x 1332||1828 x 1556|
|1K||1024 x 778||914 x 666||914 x 778|
These scan sizes generally represent the valid image size within the total frame size indicated by full frame. It is generally considered that 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. Although these digital image sizes came about as formats for scanning film, new digital cinematography cameras are also using them, exactly or nearly. In the file-based world of DI the exact size does not matter, as long as it’s managed correctly and, most importantly, able to produce high quality output for release prints and digital cinema – where the DCI specifies exact sizes.
2K has 3.19 Mpixels and a 1.316:1 aspect ratio. It is used for digitizing full frame 35mm motion picture film sampled in 4:4:4 RGB color space – making each image 12 MB. Sampling is usually at 10-bit resolution and may be linear or log, depending on the application, and is progressively scanned.
Note that the sampling includes 20 lines of black between frames because of the use of a full frame camera aperture. Thus the actual ‘active’ picture area is 2048 x 1536, has a 4:3 aspect ratio and is exactly QXGA computer resolution. Removing the aperture creates an ‘open gate’ format which may have no black bar between frames – then all 1556 lines carry picture information.
4K is a x4-area version of 2K, with 12.76 Mpixels. Once again the format includes ‘black’ lines – 40 this time, so the actual full frame image is 4096 x 3092. Historically many aspects of handling 4K have been problematic – not least due to the large data rate (over 1.1 GB/s) and the amount of data produced – about 4 TB/h. However modern technologies applied all the way from scene to screen have now made 4K far more readily accessible. For some time, 4K has been the format of choice for some complex effects shots where it was felt these needed extra quality (over 2K) to still look good after all the necessary processes are completed, specifically where the finished shots are inter-cut with the original negative. Now 4K is increasingly being used for whole movies.
DCI 4K and 2K sizes for exhibition in digital cinemas are not the same as the DPX values above. They are 2K (2048 x 1080), 4K (4096 x 2160), quite close to the 1.85:1 aspect ratio of Cinemascope.
In addition, different camera apertures can be used to shoot at different aspect ratios. All these (below) are ‘four perf’ (a measure of the length of film used) and so all consume the same amount of stock per frame. Note that scanners (and telecines) typically change scan size to maintain full 2K or 4K images regardless of aspect ratio. It is no longer normal for work to be scanned at a fixed full frame size.
|Format||Width (mm)||Height (mm)|
There are many more 35 mm formats in use but in general the use of film is rapidly diminishing as digital alternatives have become easier and more cost-effective. Still, some people want to produce the ‘film’ look.
For lower-budget movies Super 16 is sometimes used.
See also: MTF