STOPS.
A volume might be written on the subject of stops, but a few lines will suffice to give the amateur an idea of why they are necessary, and how they should be used.
The best part of a lens is its center, 1. c., those rays of light which pass through the lens at or near the center will be correctly refracted and will therefore give the image clear and sharp on the ground glass„while the rays which pass through the outer edges of the lens will not make such a dear and distinct image. It can thus be seen that the smaller the stop opening the sharper the because the outside rays will be cut off. But we find also that with a small diaphragm or stop opening we cut down the light to a great extent. If the beginner has a camera with focusing glass it will be well for him to focus on some objed on the ground glass, using the largest diaphragm and carefully noting the lines to see if they are sharp. Then let him put a smaller stop in position, noting the increase in sharpness and the decrease in light. The better the lens the larger the stop opening which we can use successfully, and consequently the " faster " the lens. Suppose we have one lens of S inch focus and we find that in a given light we can make a clear, sharp picture in 5 seconds with a stop one inch in diameter, while with another lens of same focal length we must use a stop only one-half inch in diameter in order to get a sharp phEture. How would they compare in speed? Nine people out of ten will jump at conclusions and say that we must give the lens with the half-inch opening io seconds. In this case the first impression is not correct. We must give four times the time or 20 seconds, because the area of the one inch stop is four times that of the half inch stop. \Ve find that we can get a simple little rule from this and if we know the correct exposure with one stop we can figure for our selves the correCa exposure for the others : The time rwriation between faro slops is as the square of their diameters.
With our single lens cameras there are stop openings of three sizes, the largest, for ordinary snap shots, the second, (which has about the diameter of the largest) for snap shots on the water and in tropical or semi tropical climates or for time exposures indoors, and the smallest, ordinarily used for time exposures out of doors—never for snap shots.
With the double lenses there is a greater number of stop openings and they are arranged upon what is known as the Uniform System, commonly abbreviated to " U. S." Of course when we say in a general way that the speed of a lens depends upon the diameter of the stop opening we do not mean, for instance, that a Pocket Kodak stop opening must be as large as the diaphragm in the lens of an S x io camera in order to have the same speed, but it must be as large inpro,6ortion, and that proportion we find is based upon the length of focus (the distance between lens and plate) of the lens. The proportionate size or the "value" of the stop opening is designated by f, and is the quotient obtained by dividing the focal length of the lens by the diameter of the stop. Taking, for instance, a lens of S inch focus with a stop one inch in diameter and we find that S I = S. Hence, S is the f value of the stop and would be designated : f S. Suppose our stop is inch in diameter, we would then have S =f 32.
For convenience the Uniform System of marking stop openings has been adopted by nearly all manufacturers of iris diaphragms and the follow ing table help the amateur to understand the meaning of these mark ings by giving thef value for each one : U.S. S U. S. S =f II. 3 U. S. 16 =f 16 U. S. 32=f>2.6 U. S. 64=f32 U. S. 12S = f 45.2 The convenience of the U. S. system is at once apparent when we understand that each higher number stands for an opening having half the area of the preceding opening. Between each number, therefore, we simply double the time. For instance, the table given on page 20 is for use with stop No. S with the No. 4 Folding Pocket Kodak. If we use stop No. 16 we must give twice, or if No. 32 four times the time of the table, while with stop No. 4 we would give only one-half the time of the table.