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.

Table 13.2: Definitions of factors obtained by comparison of characteristic curves.