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192 CINEMATOGRAPHIC ANNUAL
valve gamma is derived. The three factors whose product is formed may be written thus:
(1) (2)
visual negative density print visual density
X
log valve spacing negative visual density
print projected density print projected density
= = over-all gamma
print visual density log valve spacing
It is customary to control the processing by the use of suitable sensitometer exposures developed along with sound negatives and sound prints. If, for example, the daylight time scale sensitometer is used to expose the sensitometer strips, we have the following relations between sensitometer gammas and the factors listed above:
(1) Light-valve gamma — 1.05 X daylight time scale negative gamma
(2) Apparent printer gamma = 0.95 X daylight time scale positive gamma
X negative printing coefficient = true printer gamma X PC.
(3) Projection factor = 1.25
If a Cinex tungsten time scale sensitometer is used to make the control strips, it should be remembered that the Cinex gamma is 5 per cent greater than the daylight time scale gamma and 10 per cent greater than the true printer gamma. Concisely stated, the over-all gamma equals 1.25 X product of positive and negative daylight time scale gammas X negative printing coefficient, or 1.12 X product of Cinex negative and positive gammas and negative printing coefficient.
The sound negative bath which gave a daylight time scale gamma of 0.50, the curve of Fig. 1, would have given this same curve for Cinex exposures had the development of the Cinex strips been a little shorter. The equality of Cinex gammas and light-valve gammas permits us to use the curve of Fig. 1 as representing the light-valve gamma of 0.50. The bath from which the curve of Fig. 1 was obtained produced sound negatives with a printing coefficient of 1.2. The curve of projected print density vs. negative visual density, shown in the upper left-hand quadrant of Fig. 8, describes sound positive processing to a d'nex gamma of 2.08, of prints made from a negative of printing coefficient 1.2. From what has been said just above, the over-all projected gamma, instead of being unity, is 1.4.
It has been the practice to set the recording lamp to obtain an exposure on the sound negative equal to 10 times the exposure at which the negative H 8 D curve begins to be straight. For Eastman positive emulsion in the usual sound negative bath, the visual densitv at which the H^D curve becomes straight, is about 0.35 or 0.40 X gamma for values of gamma between 0.3 and 0.7.
It is seen from Fig. 1 that the density at the toe exposure is 0.20. The density for the unmodulated negative track which the standard recommendation calls for is then 0.70 visual diffuse (corresponding to a visual transmission of 20 per cent) . This setting of the negative exposure means that 90 per cent modulation of lightvalve spacing