Managing a motion picture film collection can have passive components such as maintaining cool dry storage conditions and active components, such as duplication of the film for preservation or access purposes. To accurately copy a photographic image requires care and attention to detail, there are many factors to consider.
This chapter provides a brief overview of the duplication process designed to give some background information for people with little or no photographic or film laboratory experience.
A photograph has certain physical characteristics that define the 'quality’ of the image, irrespective of the content. These characteristics can be assessed objectively, with instrumentation, or subjectively by visual assessment. Generally the quality of an image is determined by the initial choice of film, exposure and processing of the original.
The goal of any duplication or copying of a photographic image is to accurately reproduce the tones of the original.
Image quality definitions
The perceived (subjective) or actual (objective) difference between two or more parts of an image in terms of tone or luminance. In assessing the image, contrast can also refer to the compression of tones between the lightest white and the darkest black, tonal compression.
Fig 13.1 tonal compression
A measure of the 'light stopping’ or 'light absorbing’ ability. Density (D) is defined as the logarithm of the ratio of the light falling onto (incident) and light transmitted (for a negative) or reflected (for a print) from a sample.
D = log Io/I
D = density
Io = incident light
I = transmitted or reflected light
An instrument that measures the density of an image. The reading aperture is most commonly 1-2 millimeters in diameter and reads a fixed point of the image. Microdensitometers measure very small areas of a moving sample and the reading is given in a plot of changes in density over a given area.
The act of light as a radiant energy falling upon a photosensitive material. Photographic exposure is the intensity of the light combined with the length of time the light is permitted to fall upon the photosensitive material.
Exposure is a critical step in photography. In combination with processing exposure plays an important role in tonal reproduction and image sharpness.
H = Et
H = quantity of light per unit area
E = illuminance or intensity
t = time
A subjective sensation of non-uniformity in the image. It is often attributed to the individual grains of a processed emulsion, due to the very small size of an individual grain. This is not true. Due to the random nature of scatter of grains within an emulsion, including depth, they may appear to be clumped together. This forms an irregular pattern on a much larger scale than individual grains.
The objective measure of the lack of homogeneity of the photographic image is determined from the spatial variation of density recorded by a micro densitometer. This is the measure of the small fluctuations in density. It is the granularity of a negative that gives the graininess of the print.
A measure of the intensity of a light source. Luminance is properly defined as the luminous intensity per square metre, where luminous intensity relates to the output of a standard light source.
The arithmetic ratio of the original object to the final object; e.g. 1:2, 1:4, where a proportional change in dimension is produced optically. It can also be expressed as a fraction: ½, ¼ etc.
The ability to show separation between two or more elements (fine detail) in an image. Resolution of a photographic system is measured by the use of test targets that give a reading in line pairs per millimeter. Resolution in itself is not a good indicator of the quality of the image. It is but one factor that leads to the impression of a high quality image.
A subjective response to the combination of the many factors that are used in producing a photographic image:
- resolving power of the lens and the film emulsion
- subject movement
- camera movement
- exposure and processing
- subsequent printing or duplication
A poor response from any component will reduce the sharpness of the final image.
A characteristic curve is the graphical representation of an emulsions response to light and processing. The characteristic curve is the basic tool in sensitometry.
A curve is created by plotting the density of points of known exposure against a logarithmic scale of exposure. A logarithmic scale is used to compress the range and enable the features of the curve to be easily seen.
The curve is measured from a test sample of the film that is given a known and repeatable exposure and processing under normal conditions.
To ensure the exposure is repeatable a device called a sensitometer is used. A sensitometer is a well controlled power supply running a calibrated lamp. Between the lamp and the film there is a shutter to control time and a density step wedge to control and provide a range of intensities.
Curves for colour materials are plotted for each colour dye layer.
|Base plus fog||The density of the unexposed film base plus any chemical fogging that may occur during processing.|
|Inertia point||The point at which the film has absorbed sufficient light energy to start forming a latent image.|
|Toe||A non-linear region where shadow detail is recorded in negative materials (Highlight detail in print material). Compression of the density differences occur and contrast is decreased.|
|Straight line portion||The linear section where most of the information is recorded. This section is used to determine the processed contrast of the film.|
|Shoulder||Non-linear section. Compression of density differences occur and contrast is decreased. It is unusual to record information on this region of the curve, however this region may be reached with overexposure or extended development such as push processing to increase effective film speed.|
Using characteristic curves
The characteristic curve shows the relationship between exposure and processing for any given emulsion. By analysing the curve, information regarding effective speed, contrast and specific developer characteristics can be obtained.
Changes in exposure and processing can be quickly assessed without a subjective analysis of an image which may be influenced by viewing conditions and personal interpretations. Deviation from a desired standard can be easily seen by overlaying the standard curve and the curve under examination.
|Effective Speed||On the x axis, exposure increases from left to right. Therefore, the closer the curve is to the left the more sensitive or faster the film. By overlaying the curves, an estimation of any increase or decrease in effective speed can be seen by shifts of the curve to the left or right. Note that the actual speed of an emulsion as a number (ISO/ASA) is determined using fixed density points.|
|Contrast||The resultant contrast is calculated using the slope of the straight line portion of the curve. In simple terms contrast is the tangent of the angle formed by the straight line and the x axis and described as gamma. Other techniques use specific densities above base plus fog and incorporate part of the toe in the calculation.|
|Developer||While the general characteristics of an emulsion are inherent in the manufacture of the film they can be affected by the formulation and condition of the developer. Developing time, temperature, agitation and chemical imbalances in developers, caused by under or over replenishment, can show as a loss or increase in effective speed, contrast changes, lengthening or shortening of the toe and increases or decreases in the density at which the film enters the shoulder. When exposure and development are optimum, the desirable characteristics of a particular film can be readily seen on the characteristic curve.|
The light sensitive component of a film emulsion is a microscopic grain composed of a silver halide (the halide usually being bromine, or sometimes iodine or chlorine). When the silver halide absorbs sufficient energy (in the form of light), it separates into an atom of silver metal and a free atom of halide. The silver metal migrates through the grain to imperfections on the surface of the grain. The final density that is formed in the emulsion is therefore proportional to the total energy of the light and the number of molecules of silver halide that separate. This is the formation of the latent image. To make this image visible requires intensification by the chemical action of the developer.
Apart from the formation of the image there are other considerations with exposure. One is the movement of light within the film’s emulsion. Not all the light energy is immediately absorbed by the silver halide grains; some of it is reflected or refracted from grain to grain. This is referred to as irradiation (Fig 13.7) and the effect it causes is known as image spread. This reduces the film’s ability to resolve fine detail. It can be said that optimum exposure therefore lies in the narrow range that allows full recording of shadow detail to the loss of detail (usually in the highlight or highly exposed areas) due to image spread. On small formats this usually occurs before the shoulder is entered.
Another closely related effect caused by unwelcome light moving in an emulsion is halation. This is the reflection of light from the internal surfaces of the emulsion and film base. Depending upon the generation of the material (positive or negative) where this has occurred, it can show as either a halo of lighter density around a light object or of darker density around a dark object.
Control of halation is achieved by dyes in the base or a layer incorporated in the manufacture of the film. This layer is removed during processing.
The resultant contrast as recorded on the film is described as the 'density difference’ between the lowest density (darkest shadow) and the highest density (brightest highlight) on a negative.
|Subject||The maximum and minimum reflectivity of the subject. The photographic image is a recording of the amount of light reflected from an object. If one part of an object has 90 per cent reflectance and another part 5 per cent, the density difference recorded on the film will be greater than if one part reflects 70 per cent and another part 20 per cent.|
|Lighting||Strong direct light will give harsher, deeper shadows and a greater density difference than soft, diffused light.|
|Optics||The materials and design of optics can reduce or increase the contrast as the image is refracted. Focus is also a consideration at this stage.|
|Flare||Non image forming light. Reflections on the surface or within the optics or camera/printer interfere with the image. This can be equated with static on the radio.|
|Inherent Contrast||The film manufacturers design the films to have certain characteristics that make them particularly suitable for certain tasks: e.g. duplicate negative. One of the most important design characteristics is the inherent or 'built in’ contrast. If a film with a high inherent contrast is drastically underdeveloped, the contrast would still be higher than a low inherent contrast film, even one that had been overdeveloped. The contrast can be varied by development, but only within a limited range: e.g. for dupe neg changes in development may be able to vary the gamma between 0.4 and 0.9.|
|Development||The time, temperature and formulation/condition of the developer.|
Measurement of developed contrast
To measure the developed contrast (gamma method) of a processed film using a characteristic curve, the tangent of the slope of the straight line portion is calculated.
The simplest method of doing this is to extrapolate the straight line portion to the base line or x axis and determine the tangent of the angle formed.
The straight gamma method does not take into account any of the toe characteristics of the emulsion, which may vary from emulsion to emulsion. As it is a 'best fit’ method through plotted data points personal interpretation may alter the result.
The method recommended by Eastman Kodak uses calculated density points to construct a straight line from the curve. A different equation is used for each stock type to allow for toe and other characteristics.
Subject luminance range
Subject luminance is the amount of light a subject either reflects or transmits, this is the intensity part of the exposure equation (H = Et).
The luminance range is the difference between the maximum and minimum amounts of light reflected or transmitted by a subject. This is measured as a ratio e.g. 100:1. The luminance range of a subject must be placed on the characteristic curve (Fig 13.10) so that optimum recording of the tonal values within the range can occur.
To move the luminance range on the curve either the camera or printer controls are used to adjust either the length of time or the amount of light that can pass through the lens.
If the lowest value subject luminance is given insufficient exposure (underexposure) it will be recorded at a density that will either fail to record at all or place it on the lower part of the toe or where compression of tones will occur.
Accordingly, if the highest value is given too great an exposure (overexposure) the values will be recorded in the shoulder region where again compression of tones will occur. A change in contrast is also apparent.
Exposure, development and gamma
When a silver halide grain has been exposed to produce a latent image, the developer acts upon the grain to cause other molecules of silver halide to separate and form metallic silver. The small quantities of metallic silver on the surface of the grain act as a catalyst that allows the reduction of the silver halide by the developer.
This causes the exposed grains to be acted upon by the developer before the other unexposed grains. With prolonged development time the developer will act on unexposed grains causing an overall slight density increase, known as chemical fogging.
In simple terms, the more exposure, the more silver specks on the grain, the sooner the development will start. The longer the development the higher the contrast, up to a point.
Chemical fogging and highlights developed to densities within the shoulder reduce the density difference as well as the inherent contrast of the film material. Figs 13.12i and 13.12ii show the results on the density difference of a scene with manipulation of the overall exposure and development.
Total system gamma
To ensure that the final duplicated material will exactly match the original material, the densities and gammas of the duplication stages must conform to tight tolerances regarding exposure and processing.
The general theory states that 'the product of the gammas of the duplicate stages should equal 1’. For example, creating a duplicate negative from an original negative the gamma of the intermediate positive and the duplicate negative, multiplied together should equal 1. This is achieved by a gamma of 1 for both the intermediate positive and the duplicate negative in colour materials, but for black and white it is more usually around 1.5 for the intermediate positive and 0.65 for the duplicate negative.
Exposure is critical throughout the duplication process. Insufficient exposure will cause the loss or distortion of information during duplication. On duplicate negatives struck from original release positives and intermediate positives the exposure must be sufficient to fully record the shadow detail. The intermediate positive stage of duplication must be fully proportional otherwise compression of the tonal range will occur (distortions caused by the toe of the emulsion). This is achieved by only using the straight line portion of the intermediate stock. In practice, on black and white intermediate positive stock, this portion of the curve starts at a density of 0.60. Therefore, if the lightest highlight density is recorded at a density of 0.60, all other densities will be recorded in correct proportion and thus remain faithful to the original.
Film stock designed for duplication has a very long straight line portion and is therefore capable of holding all the information from any 'normal’ original and even those with a large density difference (contrast), although in this instance to produce an 'acceptable’ final release print some manipulation of exposure and development would be required.
While underexposure causes problems, over exposure is not desirable with irradiation (image spread) within the emulsion causing a loss of fine detail and, if gross over exposure occurs, the possibility of recording on the shoulder of the curve with compression of the tonal range.
To determine if the gamma, exposure and development are providing the ideal reproduction, a Jones or Quadrant diagram can be used (Fig 13.13). By plotting the densities of the intermediate stages a final reproduction curve is derived. This should have an angle of very close to 45° (tan 45° = 1).
Examine all copies that exist. Do not necessarily dismiss components that are obviously shorter than the title’s original length. Research the following technical details for each copy that exists:
- exact lengths – a shot list may be required
- picture content – differing versions, reel composition, outtakes, offcuts etc.
- physical condition and condition degree – damage, scratches, shrinkage, brittleness, decomposition
- photographic quality – print-through, generation details, exposure density, frameline, problems, picture steadiness
- colour – colour balance, grading/timing information, noticeable fading
- sound track – quality of sound, completeness
The final selection should be the item that satisfies the above and is the most complete and most original.
There are two main printing transport systems used in motion picture film duplication:
- continuous – the film is constantly moving
- step – the film moves intermittently and is exposed one frame at a time
There are also two image transfer methods:
- optical – a lens is used to project an image onto the receiving film
- contact – the two pieces of film are in emulsion to emulsion contact, no lens is used
As the name implies, continuous printers move the film continuously, including during the moment of exposure. The original film passes over a narrow slit, through which the exposing light passes. Mostly continuous printers employ emulsion-to-emulsion contact, contact printing. This allows a very high speed process but does permit a degree of slippage between the films that will degrade the image. If the film being duplicated is at all shrunken, the slippage factor becomes very significant. Usually contact printing is used for high production runs, such as release prints.
One form of duplication that requires continuous printing is that of optical sound tracks.
Step printing is a generally slower process. The film is held stationary during the moment of exposure, which removes all slippage induced artefacts. The whole frame is exposed at once with a shutter controlling the exposure duration, making the process more like a cine camera. Step printers can use either an optical or a contact image transfer.
Step printers are used in commercial laboratories for the production of duplicating materials, dupe positives and dupe negatives where maximum image quality is required and since the production run is low, time is less important.
Due to the gate used to hold the image still during exposure step printers are entirely unsuitable for optical sound track printing.
Optical printers may allow compensation for shrinkage of the image by optically enlarging the image frame.
|Step printers||no image slippage during exposure||cannot print optical sound tracks|
|can generally deal with shrinkage better||slower printing times|
|can print sound tracks||slippage during printing|
|Continuous printers||faster printing times||loss of resolution|
|less able to cope with shrinkage|
|resizing shrunken frames||optical losses|
|Optical||correcting register problems (shifting frames in respect to perforations)||potential for dust on optics|
|Contact||no optical losses||cannot resize frames|
|cannot correct register|
As with rewashing, wet gate printing will reduce the effect of the scratch but will not restore lost image information, as can be seen in Fig 13.15ii).
Fig 13.15 Wet gate printing and the effect on scratching