Characteristic curves of camera films and print films are frequently curved but in order to faithfully translate the exposures through a duplication system the intermediate film characteristic curves should be straight lines. All duplicating materials are designed to have long straight-line portions on which to expose.
To arrive at the correct exposure levels for any duplicating or intermediate film, one basic rule must be observed: The full tonal range of the camera original photographic image must be printed onto the straight line portion of the characteristic curves of all intermediate films.
The characteristics curve of one record of almost any camera negative original whether black and white or colour shows a short curved portion in the lower densities [the toe], a long straight line portion and a curved portion at high densities [the shoulder]. When manufacturer's exposure recommendations for a camera negative are followed, most of the important picture information falls on the straight-line portion of the curve as indicated by the circles, (see diagram). The brightness levels of the original scene are reproduced in the negative in an undistorted relationship except at the highest and lowest brightnesses. The ideal, i.e. the most visually acceptable, exposure is achieved by positioning the lowest exposures, the shadows of a scene, low on the toe of the curve, resulting in some compression of the shadow detail.
To produce an intermediate or duplicate negative of high quality, a straight line or linear relationship between the densities in the original negative and the densities in the intermediate negative must be maintained to ensure a minimum of distortion, or the same relative distortion in reproducing the curve shape of the original negative. All intermediate or duplication film stocks are designed to have a long straight-line portion in their characteristic curve adequate to accommodate the density range of the picture material as recorded in the original negative. By proper use of this straight-line portion a minimum of additional distortion may be introduced. However the straight line portion of any duplicating material's characteristic curve does not begin at an effective density of zero; there is always a curve in the low-density region, the toe. Therefore, the exposure given to any intermediate or duplicating film in both positive and negative stages must be sufficient to place the picture density range of the original camera negative on the straight line portion of the characteristic curve.
Original camera negatives often have important information in the lowest possible densities in a compressed form on the lower curved "toe" portion of the characteristic curve. Any intermediate or duplicate negative is therefore almost always denser than the original as it is always "placed" or exposed further up the curve in order to reproduce these relationships unaltered from the camera negative.
Using the curved lower portion of the duplicating film characteristic curve will produce a thin, duplicate negative. The degree of distortion of the tone scale which is introduced by this will be more or less noticeable in various scenes, depending on how much of the scene brightness range is reproduced on the non-linear portion of the characteristic curve. A high key scene, for example, may show little or no distortion, since the restricted brightness range causes it to be reproduced entirely on the linear portion of the curve. On the other hand, a night, low key or dark scene will, for the most part, occupy the non-linear, lower part of the intermediate negative curve, and poor tone-reproduction characteristics will be evident in the final print, usually seen as "smoky", low contrast shadows.