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).