Besides the Hurter & Driffield method of

obtaining the speed numbers of plates and films adopted by a large number of makers in this country, there are also two standard English systems known as the W.P. No. (Watkin's power number) and Wynne F. No., both of which are used to a fair extent.

The "Actinograph" number or speed number of a plate in the H. & D. system is found by dividing 34 by a number known as the Inertia, the Inertia, which is a measure of the insensitiveness of the plate, being determined according to the directions laid down by Hurter & Driffield—that is, by using pyro-soda developer and the straight portion only of the density curve. If, for instance, the Inertia was found to be one-fifth, then the speed number would be 34 ÷ 1/5 = 170, and the plate is H. & D. 170. The W.P. No. is found by dividing 50 by the Inertia. Thus 50 ÷ 1/5 = 250, and the plate is W.P. 250, but for all practical purposes the W.P. No. can be taken as one and a half times H. & D. The Wynne F. numbers may be found by multiplying the square root of the Watkins number by 6.4. Thus

√250 = 15.81, and 15.81 × 6.4 = W.F. 101.

For those photographers who are in the habit of using an actinometer giving the plate speeds in H. & D. numbers, the following table, taken from the Photographer's Daily Companion, is given,

which shows at a glance the relative speed numbers for the various systems. The Watkins and Wynne numbers only hold good, however, when the inertia has been found by the H. & D. method.

Table of Comparative Speed Numbers for Plates and Films

H. & D. W.P. No. W.F. No. H. & D. W.P. No. W.F. No.
10 15 24 220 323 114
20 30 28 240 352 120
40 60 49 260 382 124
80 120 69 280 412 129
100 147 77 300 441 134
120 176 84 320 470 138
140 206 91 340 500 142
160 235 103 380 558 150
200 294 109 400 588 154

Although theoretically the higher the speed of the film the less the duration of exposure required, there is a practical limit, as besides the intensity and actinic value of the light admitted to the film a definite time is necessary for it to overcome the chemical inertia of the sensitised coating and produce a useful effect. With every make of film it is possible to give so short an exposure that although light does fall upon the film it does no work at all—in other words, we can say that for every film there is a minimum amount of light action, and anything below this is of no use. The exposure that enables the smallest amount of light action to take place is termed the limit of the smallest useful exposure.