Contrast-value as a function of logarithmic value of the ratio between two score values (solid curve). Ratio R = S2/S1, in which S1 and S2 are the first and the second score value. When there is no difference between S2 and S1, both the Contrast-value and the value of log(R) are zero (hatched lines). When S2 is zero and S1 has a positive value, log(R) is undefined (infinite negative) whereas the Contrast-value is −1.0. When S2 has a positive value while S1 is zero, log(R) becomes undefined (infinite positive) whereas the Contrast-value is +1.0. If all Contrast-values were numerically identical to log(R) values, the relationship between Contrast and log(R) values would be depicted by he dotted straight line. Note that although not identical in general, Contrast and log(R) values are numerically similar (deviation with respect to the mean of both types of values < 15%) for a wide range of log(R) values on both sides of the zero point (range −0.80 to +0.80).