Next in number comes the professional pho tographer who as a rule judges the exposure by the appearance of the image on the ground-glass.
But the class of photographers which proves most directly the absolute weakness of present day methods are those who endeavor by all means to secure correct exposures and yet, finding no help which is based on simple "cause and effect" reasoning, are compelled to depend on purely arbitrary exposure methods which are designed to "get the answer" by means of a "sliding scale." Still more to be lamented is the fact that the present methods have kept the whole matter of light measure ment and exposure so effectively separated from simple mathematics that these earnest people have quite concluded that there is no simpler way.
Then there is annually the immense money loss to the public due to exposures that are so wrongly guessed as to result total failures. In great part this loss is sustained by people who have been convinced by commercial advertising that photography was so simple that they could not fail.
Then there is to be added the injury sustained by the pride, the taste and the affections through the failures which are met with when certain greatly desired pictures fail to materialize.
In working out the following problems then, the f system is employed for conditions in nature outside of the use of lenses, and solely on ac count of its usefulness in arriving at the form of the light cone which converges from flames, openings and expanses and from which form the solid angle of the cone may be easily com puted mentally. That I have used the f num bers also in certain of the computations is due to the fact that, unlike the general practice of photography, only the illuminating engineer, the scientific photographer, the physicist and educator will be interested and such will com prehend the basic principles which underlie the solutions by the rules which are given for the f numbers.
The author desires not to be misunderstood regarding his opinion of the f system of num bers. The importance of this system is shown in the chapter on that subject and it would be extremely difficult to get along without its use in handling light problems of all kinds. His objec tion is to its irrational employment for numbering stops which need no numbers except the simple statement of their working dimensions in terms of a chosen unit.