THE SPEED OF PLATES AND FILMS A speed list, if incorporated in this book, would soon be out of date, since new plates and films are continually appearing on the market.
In chapter 5 it has been made clear that each exposure in practice is really a test and proof of the speed of the plate used. In this method the speed of a plate is the exposure which it requires with unit stop when the surface photographed is of unit actinicity. For these reasons then it is practical for any worker to test the speed of the plate or film which he is using in his daily practice and it will suffice here to give a general idea of the speeds of different types of plates so that an approximation may be arrived at for the first trials. The extra slow contrast or process plates may be represented by the Cramer "Contrast" and the Lumiere Red Label which have speeds of 32 and 128 minutes respectively.
The popular fast plates of both Europe and America are very close to 4 minutes, or better, 256 seconds in speed. That is to say, given a subject of unit actinicity and using in the lens the unit stop (or stop f / 64 or U. S. 256 of the scales now in use), that time would be required to effect a normal exposure. None of these plates would fail to give a good negative on this basis, so far as the time of exposure is concerned. But after a single trial with unit measurement of subject as has been made clear, should the plate or film seem to be rather over-exposed then it would naturally be understood that 256 seconds had over-exposed the plate and half that time or 128 seconds would be considered the correct speed, this to be altered at will by the worker to suit his own taste in the matter. If on the other hand the plate seemed under-timed then it would be evident that it should have been exposed twice as long and that its speed is 512 instead of 256 seconds.
The ultra fast plates on the market may be tried at first on the basis of a 128 second speed. The reader should remember that the speed time of emulsions as explained in this method is based on a chemical effect that will develop in a normal de veloper and in a normal time (or at least, in the case of the ultra fast plates, without undue forcing) into a normal negative. For purely practical reasons these ultra fast plates may be taken as having double or four times their normal speed (one half or one quarter their normal speed time) since they are rarely used except under extreme conditions where a certain amount of under-exposure must be allowed in order to stop motion in moving sub jects which are sometimes in the shade, as under the most exacting conditions of press photography.
I have shown how to arrive at the speed-time of a plate or film on the basis of correct exposure to a subject having unit actinicity, with the unit stop (f/64 or U. S. 256) in the lens. It is ob vious that there must also be a speed-time for every other stop regardless of whether the unit scale has been applied to the lens or not. Thus in lenses having the U. S. numbers the smallest stop is usually 128 and this stop will show a speed-time half of that for the unit stop which is U. S. 256. This being true the speed-time for any emulsion can be found for the smallest stop or in fact for any other stop in a lens and this speed time divided by the unit actinicity of the subject will give the exposure with that stop and the actinic unit may thus be employed for exposure whether the unit scale has or has not been applied to the lens.
This method is not recommended however except as a gradual means of trying out the unit method.