Tag : Film

10-bit log

This usually refers to a 10-bit sampling system that maps analog values logarithmically rather than linearly. It is widely used when scanning film images which are themselves a logarithmic representation of the film’s exposure. This form of sampling is available directly from some digital cinematography cameras.

The 10-bit data can describe 210 or 1024 discrete numbers, or levels: 0 – 1023 for each of the red, blue and green (RGB) planes of an image. However, as all electronic light sensors have a linear response and so produce an output directly proportional to the light they see, when scanning film they represent the transmittance of the film. Usually it is negative film that is scanned and this means a large portion of the numbers generated describe the scene’s black and dark areas (representing bright areas of the original scene), and too few are left for the light areas (dark scene) where ‘banding’ could be a problem – especially after digital processing such as grading and color correction. Transforming the numbers into log (by use of a LUT) gives a better distribution of the digital detail between dark and light areas and so offers good rendition over the whole brightness range without having to use more bits. A minimum of 13-bit linear sampling converted to 10-bit log sampling means sufficient detail in the pictures is stored to allow headroom for downstream grading that is common in film production.

10-bit log is the basis for sampling in the Cineon and SMPTE DPX formats that are still widely used in the post production and DI industries.

See also: 10-bit lin, LUT

2:2 F1/F2

A film frame being transported as 2:2 (pSF) is placed into two consecutive video fields. F1/F2 denotes that the film frame is carried in field one and the following field two. This is commonly referred to “normal dominance” or “perfect cadence”.

See also pSF

2:2 F2/F1

A film frame being transported as 2:2 (psf) is placed into two consecutive video fields. F2/F1 denotes that the film frame is carried in a field two and the following field one. This is commonly referred to “reverse dominance” or “reverse cadence”.

See also pSF

Answer print

The answer print, also called the first trial print, is the first print made from edited film and sound track. It includes fades, dissolves and other effects. It is used as the last check before running off the release prints from the internegatives.

Camera negative (film)

Camera negative film is designed to capture as much detail as possible from scenes. This not only refers to its spatial resolution but also its dynamic resolution. Modern camera negative stock has almost 10 stops’ (over 1,000:1) exposure range and so is able to record detail in both the low-lights and the highlights which are well beyond the range that can be shown on the final print film. This provides latitude to compensate for over or under exposure during the shoot or to change the look of a scene. The latitude is engineered into the film stock by giving it a very low gamma of around 0.6.

Exposed and developed camera color negative film has an orange tint and is low in contrast – differing greatly from the un-tinted and high contrast print film. As not only the blue, but also the red and green layers of the film are sensitive to blue light, the orange layer is added below the blue layer to stop blue light going further. All types of film stocks use orange dye but for print films it is bleached away during processing.

There are numerous stocks available. High-speed stocks work well in lower lights but tend to be more grainy. The opposite is true for low speed stocks.

Color timing (a.k.a. Grading)

The color of film exposed and processed in a laboratory is controlled by separately altering the amount of time that the red, blue and green lights are used to expose the film. This is referred to as color timing and its effect is to alter the contrast of R,G and B to create a required color balance.

In a lab, color timing is usually applied at the point where the edited negative is copied to the master interpositive, but can be done later at other points, if required. This contrasts with the digital intermediate process where color timing is applied at any required time. In addition there is far more flexibility for color control with gamma, hue, luminance, saturation as well as secondary color correction. Also the results can be seen immediately and projected onto a large cinema screen, and further adjusted if required. The images have precise color settings to show the results as if output via film, or digitally.

See also: Grading, Timing

DCI

Digital Cinema Initiatives, LLC was formed in 2002 with members including Disney, Fox, MGM, Paramount, Sony Pictures Entertainment, Universal and Warner Bros. Studios. Its purpose was to establish and document specifications for an open architecture for Digital Cinema components that ensures a uniform and high level of technical performance, reliability and quality control. It published the Digital Cinema System Specification in July 2005 (freely available at their website) and established a set of technical specifications that allowed the industry to roll-out Digital Cinema. It is a measure of the DCI’s success that now well over half of the world’s cinemas are digital.

There are three levels of images, all with a 1:1 pixel aspect ratio, 12-bit 4:4:4 sampling in X´Y´Z´ color space.

LevelPicture SizeAspect RatioFrame Rate
14096x21601.89624Hz
22048x10801.89648Hz
32048x10801.89624Hz

The specification includes requirements for JPEG 2000 image compression, X´Y´Z´ color space and a maximum playout bit rate of 250 Mb/s. To prevent piracy by copying the media files there is AES 128 encryption (Advanced Encryption Standard able to use keys of 128, 192, and 256 bits to encrypt and decrypt data in blocks of 128 bits). There is also forensic marking to deter and trace the bootlegger’s camcorder pointed at the screen. Such schemes include Philips’ forensic watermarking or Thomson’s NexGuard watermarking.

DSM → DCDM → DCP → DCDM* → Image and Sound

DCI describes a workflow from the output of the feature post production or DI, termed the Digital Source Master (DSM), to the screen. The Digital Cinema Distribution Master (DCDM) is derived from the DSM by a digital cinema post production process, and played directly into a digital cinema projector and audio system for evaluation and approval.

The approved DCDM is then compressed, encrypted and packaged for distribution as the Digital Cinema Package (DCP). At the theater, it is unpackaged, decrypted and decompressed to create a DCDM* with images visually indistinguishable from those of the original DCDM.

Website: www.dcimovies.com

 

Densitometer (Film)

An instrument used to measure the optical density of film, usually over small areas of images. The instrument operates by measuring the light passing through the film. When measuring movie film density, two sets of color filters are used to measure Status M density for camera negative and intermediate stocks (orange/yellow-based) and Status A for print film to correctly align with the sensiometric requirements of the stocks.

Density

The density (D) of a film is expressed as the log of opacity (O).

D = Log10 O

Using a logarithmic expression is convenient as film opacity has a very wide range and the human sense of brightness is also logarithmic.

See also: Film basics (Tutorial 2)

 

Digital negative

Digital image data that contains all the detail (spatial and dynamic/latitude) held in original camera negative (OCN) film. This allows all latitude headroom to be included on the material for use in a DI process, so adjustments of color and exposure can be made to the same degree as with film.

See also: Camera negative

Film formats

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 resolutionFull frameAcademyCinemascope
4K4096 x 31123656 x 26643656 x 3112
2K2048 x 15561828 x 13321828 x 1556
1K1024 x 778914 x 666914 x 778
Aspect Ratio1.3161.3721.175

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.

FormatWidth (mm)Height (mm)
Full Frame24.9218.67
Academy21.9516.00
Cinemascope21.9518.60
1.66:121.9513.22
1.75:121.9512.54
1.85:121.9511.86

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

Film recorder (digits to film)

Equipment which inputs digital images and outputs exposed negative film. For this, CRT, laser-based and D-ILA LCOS imaging device technology recorders expose high-resolution images onto film. Here there is some emphasis on speed, taking a few seconds per frame, as well as image quality. Laser-based models can scan a 35mm image in about 2s for 2K, (4s for 4K), CRT-based recorders 1s (3.5) and D-ILA imagers can expose 3 f/s for 2K (4K not known).

While the use of film for acquisition and presentation has greatly diminished, in one respect the workflow has reversed as film recorders are needed to create film copies of digital motion pictures for presentation in the many thousands of remaining film-based cinemas. For archiving, many believe that film has a far greater shelf life than digital media, is immune to the digital world’s technology creep and obsolescence, and proven to survive well for many decades.

Websites:
www.arri.com
www.celco.com
www.imagica.com

Film scanner (film to digits)

A general term for a device that creates a digital representation of film for direct use in digital television or for digital intermediate work. For television, film scanners replaced traditional telecines that had to work in realtime. For use in digital film production, they should capture the full detail of the film so that, when transferred back to film, the film-digital-film chain can appear as an essentially lossless process. For this, film scanners are able to operate at greater than HD resolutions (1920 x 1080); 4K is now predominant in the movie busininess. The output is data files rather than the digital video that would be expected from the old traditional telecines.

For movie production the output must retain as much of the negative’s latitude as possible, so the material is transferred with a best light pass and recorded by linear electronic light sensors, typically of CMOS technology to a high accuracy – at least 13 bits of accuracy (describing 8192 possible levels). Using a LUT, this can be converted into 10-bit log data which holds as much of the useful information but does not ‘waste’ data by assigning too many digital levels to dark areas of pictures.

Note that this is different from using a telecine to transfer film to video where, normally the film is graded as the transfer takes place. Additional latitude is not required in this digital state so 10 or 8 bits linear coding is sufficient.

Flash

Double flash is commonly used in film projectors so that each of the 24 f/s is shown twice; a total of 48 f/s. This means the movie presentation has less flicker. Triple flash is better still with a frame rate of 72 f/s.

When presenting 3D cinema, the left and right eyes want motion and the parallax to appear at the same time but the sequential frame presentation of 3D, often using a single projector, naturally offsets motion timing. Double, or better triple, flash improves the motion portrayal. Here total frame rates are double that of 2D, so:
single flash is 48 f/s
L1, R1, L2, R2, etc.

double flash is 96 f/s
L1, R1, L1, R1, L2, R2, L2, R2 etc

triple flash is 144 f/s
L1, R1, L1, R1, L1, R1, L2, R2, L2, R2, L2, R2 etc.

Note that the cine player offers only 24 left and right frames/s. It is the job of the projector to present each frame two or three times. Of course, the projector has to be capable of clearly showing frames at that rate.

Frame

A whole television picture. A frame has shape, its aspect ratio. Today all new TV frames have a 16:9 aspect ratio. Some motion pictures are presented on TV with a wider aspect ratio, typically with a black border above and below. A frame has a specific resolution and is either using interlaced (I) or progressive (P) scans. Most productions now originate in an HD format of either 1280 x 720P or 1920 x 1080(I or P) pixels. Some still use SD with 701 x 576I or 701 x 480I frames. These two SD standards do not have square pixels, all other DTV frames do. In UHD a frame could have 3840 x 2160 (4K) or 7680 X 4320 (8K) pixels. UHD only uses progressive scans. Interlace makes a relatively low frame rate of 25 or 30 f/s (shown as 50 or 60 fields/s) suitable for portraying motion quite well but, without further processing, stop motion freezes can look poor.

Another property of a frame is its color gamut, as defined in its standard. As TV video standards have progressed, so the associated color gamut has expanded. Some say this is the most striking change from HD to UHD. UHD frames may also have a higher dynamic range (HDR) – again enhancing the look of the pictures. A frame has a specific time. Usually 1/25 or 1/30 of a second. Larger frame formats, especially 4K and 8K, require faster frame rates to reasonably portray smooth movement on a big screen. See ‘Frame rate below’.

See also: Interlace

Grading (a.k.a. color timing)

Grading is the process of adjusting the color of a clip to get the best out of the material or to match shots perhaps taken at different times or in different lighting conditions. With film, grading was traditionally performed when going from internegative to print film by controlling the exposure of the film. In television it was traditionally done off the telecine for commercials or tape-to-tape for longform programs. Either way, both processes were, by their nature, linear.

The advent of non-linear grading systems (such as Quantel’s Pablo Rio) has changed the rules for color grading and correction. While there is still a requirement for an initial technical scan for film-originated material, from this point on grading can – and often does – happen at multiple stages in the post production process. For example, color correcting individual layers within multilayer composite shots (which may be shot under different lighting conditions) to ensure that the result is harmonious within itself. In addition, non-linear editing means that scene-to-scene comparisons and corrections can be made as the edit unfolds.

This eases the final grading process when the finished work is reviewed interactively with the director/client.

Secondary color correction is aimed at controlling a particular color or a narrow range of colors – such as those on a car or product. Here typically the hue, gain and saturation can be changed. There are also several methods available for defining the object, area or ‘window’ that requires color correction such as using wipe-pattern shapes, drawing an electronic mask by hand or a combination of automatic and by-hand methods. Some of the most sophisticated tools are provided by media workstations such as Quantel’s Pablo Rio.

See also: Film scanner, Telecine

Grain management

Controlling the amount of ‘film’ grain visible on a film or digital movie. Its appearance is considered by some to add a certain look to the production. Modern DI equipment can include grain management that can increase or decrease its visibility on film or digitally originated material. Aside from aesthetics, grain affects compression systems as they see it as extra movement and so can waste bandwidth by coding it – adding another reason for controlling the amount of grain according to the different coding requirements for, say, digital cinema and mobile reception.

HFR

High Frame Rate – a frame rate higher than normal. For instance, movies (films) are normally shot at 24 f/s but some have been shot at 48 f/s – HFR. Some audiences say they do not like it as it’s too real and does not look like film.

It has been observed that when viewing UHD, motion judder is often very apparent and so a higher frame rate (say 48 f/s) is recommended by some. When shooting fast-action sports, such as football, then the UHD result would look better using, say, 50 or 60 f/s. In fact the UHD standard Rec 2020 includes frame rates up to 120 f/s.

Internegative

As a part of the chemical lab film intermediate process internegatives are created by contact printing from interpositives. These very much resemble the cut negative. The stock is the same as for interpositives: slow, very fine grain with a gamma of 1, and the developed film is orange-based. To increase numbers, several internegatives are copied from each interpositive. These are then delivered to production labs for large scale manufacture of release prints.

See also: Film basics (Tutorial 2)

Interpositive

This is a first part of the chemical lab intermediate process where a positive print of film is produced from the cut (edited) camera negative. Interpositives are made by contact printing onto another orange-base stock. In order to preserve as much detail as possible from the negative, including its dynamic range, interpositive material is very fine grain, slow and has a gamma of 1. During the copying process, grading controls are used to position the image density in the center of the interpositive material’s linear range. As a part of the process of going from one camera negative to, possibly, thousands of prints, a number of interpositives are copied from the negative.

See also: Film basics (Tutorial 2)

Keycode

A machine-readable barcode printed along the edge of camera negative film stock outside the perforations. It gives key numbers, film type, film stock manufacturer code, and offset from zero-frame reference mark (in perforations). It has applications in telecine and film scanning for accurate film-to-tape or data transfer and in editing for conforming neg. cuts to EDLs.

MTF

The Modulation Transfer Function is a measure of spatial resolving power. It can refer to a medium, such as film, or a lens, or any part of the scene-to-screen chain. It is akin to frequency response in electronic images. To assess the MTF of film, it is exposed to special test images comprising sine-wave bars of successively higher frequencies. The results on the processed film are assessed by measuring its density over microscopically small areas to obtain peak-to-trough values for the different frequencies. These results should then be corrected to allow for the response of the lens, the test film itself and any D/Log E non-linearities.

In a practical film system, the film images pass through many components including the camera lens, intermediate stocks and contact printing to the projection lens. Each of these has its own MTF and the system MTF can be calculated as follows.

MTFsystem = MTF1 x MTF2 x MTF3 etc

See also: Resolving power

One light (pass)

A one-light pass refers to a film-processing lab giving the same exposure to a defined length of film, during printing. This is the simplest, quickest and cheapest way to print all the film and the results are typically used for making rushes, dailies, etc. These are often subsequently telecined and recorded to videotape as a reference for the offline decision-making process.

See also: Best light

Perf film

Short for perforations. It is a way to describe some information about the format of images on 35mm film by how many of the perforations, or sprocket holes, are used per image. For example, Full Frame is 4 perf.

Print film

Film stock designed specifically for distribution and exhibition at cinemas. Unlike negative film, it is high contrast and low on latitude. This is designed to give the best performance when viewed at cinemas. Obviously a release print has to be clear of the orange base so this is bleached out during processing.

See also: Film basics (Tutorial 2)

Printer lights

The illumination used to expose film in a processing laboratory. ‘White’ light is passed through red, blue and green filters so that the exposure to each can be individually controlled. Film is contact printed, placing the new film stock against the processed film that carries the images. The amount of light can be varied to provide the required exposure to show more detail in the highlights or the shadows or to keep to the mid-range of the scene brightness. To print an overexposed negative will require higher values and underexposed lower values of printer lights. A change of 1 in the value represents 1/12th of a stop adjustment in exposure. Differential adjustments of the values provides basic color correction (timing). The values for the lights are recorded as grading (timing) numbers onto disk or paper tape.

See also: Color timing, Film Basics (Tutorial 2), One-light pass, Timing

pSF

A Progressive Segmented Frame (pSF) format splits a progressive image into two sequential fields. It is identical to 2:2 in terms of motion profile.

Sequence Detection

This is the act of finding film frame boundaries. For “perfect” pSF or 2:3 sequences, this will produce a regular pattern of frames; for “non-perfect” sequences the pattern will not be regular and might have discontinuities at edit points for example.

Stop

A ratio of amount of light where one stop represents a x2 change – doubling or halving of the amount of light. The operating range of film and electronic light sensors, such as CCDs and CMOS, are quoted in stops. Typically, a camera’s shutter speed and the lens’s aperture setting restrict the light arriving at the sensors/film so the mid brightness of the required scene corresponds to the middle of the sensor’s or film’s sensitivity range.

Stops are simply the expression of a ratio, not absolute values. As they represent doubling or halving of light, they are actually powers of 2. So

1 stop = x 2
2 stops = x 4
3 stops = x 8
4 stops = x 16 etc.

F stops are the simple calculation taking focal length and aperture into account. This does not fully solve the problem of how much light gets to the sensor/film when lenses are not 100% clear. The T stop takes the transmissive quality of the lens into account. The transmissive quality is affected by the glass used to make the lens and any additional anti-reflective and conditioning coatings applied to the lens elements.

For example:

Use the formula:-  T-stop = F-stop/transmission_fraction. Where transmission ranges from 0 (opaque) to 1 (perfectly clear)

For a perfect lens the calculation is T = F/1 ->  T == F

For a lens with 80% transmission (0.8) then T = F/0.8

So for an F stop setting of 4 the real measure of light getting to the sensor is equivalent to 4/0.8 = 5.0

Note that the depth of field will still be calculated from the F stop setting – only the exposure is set using the T stop.

Telecine

Device for converting film images into video in realtime. The main operational activity here is color grading which is executed on a shot-by-shot basis and absorbs considerable telecine time. This includes the time needed for making grading decisions and involves significant handling of the film, spooling and cueing which risks film wear and damage, besides the actual transfer time. The output of a telecine is digital video (rather than data files).

Digital technology has moved the transfer process on. Now, adding a disk store or server can create a virtual telecine enabling the film-to-digital media transfer to run as one continuous operation. Whole film spools can be scanned in one pass, with useful footage selected by an EDL. In this case the telecine may be termed a Film Scanner – creating image files (rather than digital video) that contain sufficient latitude for downstream grading.

See also: Grading, Film Scanner