This is from The Evaluation of Negative Film Speeds in Terms of Print Quality. Jones speculates about luminance ranges outside of average. “It is quite possible that for lower and higher brightness contrasts the value of K (ratio of speed point gradient to average gradient) will be somewhat different from that derived for the luminance range used in this work. If this proves to be true, it simply means that the effective camera speed of a given photographic negative material will be dependent to some extent upon the contrast of the object being photographed. This will mean that the photographic material in question does not have a single, unique speed value and the complete information relative to this characteristic of the material could only be given in terms of a series of numbers, or perhaps a graphic representation of the speed number of object contrast. Such a course seems quite impossible to follow, for it is necessary, in many cases, to give a single number which represents the effective speed of the material.

In an footnote to this he states, “Since this manuscript was prepared, further statistical judgment of the minimum negative exposure required to give an excellent print when using object contrasts much lower and much higher than the luminance contrast of the test used in this work indicates quite conclusively that the minimum useful gradient is not to any appreciable extend dependent upon the object contrast. It appears, therefore, that the sensitometric criterion suggested will apply satisfactorily to a relatively wide range of object contrasts. The detailed results of this further investigation will be published shortly.