Opticians have adopted standard " f " numbers, namely, 3•16, 5.66, 8, 11.3, 16, 22.6, 32, 45, and 64, but stops may be " in between " any of those named, or larger or smaller than and 64 respectively. As a general rule, 11.3 is spoken of as s s and 22.6 as 22, but in the case of larger stops the decimal point, when it occurs, is always mentioned, as, for example, 4.5, 5.8, etc.
The " U.S." system (now practically obsolete in England) has long been popular in the United States, for which reason the initials are looked upon as indicating that country. Such, how ever, is not the case. " U.S." stands for " uniform system," as an attempt was made, first of all in 1881, by the Royal Photographic Society to induce all lens makers to adopt a uniform system of marking diaphragms.
The Royal Photographic. Society's standards and recommendations (dated 19o1) are as follow :—(1) That intensity ratio be defined as dependent upon the efiective aperture (and not upon the diameter of the diaphragm) in relation to the focal length of the lens. (2) That effec tive aperture be determined in the following manner : The lens shall be focused for parallel rays; an opaque screen shall be placed in the principal focal plane, the screen being provided in its centre (in the axis of the lens) with a pinhole ; an illuminant shall be placed imme diately behind the pinhole and the diameter of the beam of light emerging from the front sur face of the lens shall be the measure of the effective aperture. Note.—It will be found, except when the diaphragm is situated in front of the lens, that the diameter of the diaphragm itself is seldom identical with the effective aperture. (3) That every diaphragm be marked with its true intensity ratio, as above defined, in the following order of sequence : f/4, f/5.6, f/8, f/11.3, f/i6, f/22.6, f/32, f/45-2, f /64, etc., each diaphragm requiring double the exposure required by the preceding dia phragm. Should the greatest effective aperture of a lens not conform exactly to one of the intensities set forth above, this aperture should be marked in accordance with the definition of effective aperture, but all succeeding smaller apertures should be marked in uniformity with the above sequence.
Stops marked by the Uniform System are commonly known as Nos. I, 2, 4, 8, 128, and 256; the respective " f " values are 4, 5'6, 8, II, 16, 22, 32> 45, and Most of the lenses produced in Prance are marked according to the method advocated by the Paris Congress of 1889. //to is taken as the unit aperture, the series advancing as the " Uniform " system ; No. 1 is f/io, No. 2 f /14, No. 4 f/2o, and so on.
Influence of Diaphragms on " Rapidity."—The "rapidity" of a lens depends upon the stop used. The " U.S." numbers indicate relative
exposures, but the " f " numbers do not, although the relative exposures are easily calculated from them. To find out the relative values of the " f " stops, first square them, and the exposure is then as one product is to the other. Thus, f/16, for example, requires four times the exposure necessary with f/8, because 16 x = 256, which is four times 8 + 8 = 64. When makers mark their stops as follows, f/8, 16, 22, 32, 45, and 64, each stop requires double the exposure of the preceding one and half that of the succeeding one.
The Use of Diaphragms.—The main functions performed by stops are as follow :—(I) They govern the definition. A large stop, such as that generally used for focusing, may produce an indistinct image upon the focusing screen or sensitive plate, and it may be necessary to insert smaller stops into the lens in order to secure better definition. If, say, an object a few feet from the camera is sharply focused with a large stop, the background and surroundings may not appear sharply defined. The insertion of a smaller stop serves to cut down the area of the base of the cone of light formed by the lens, and the result is increased definition (see " Depth of Definition"). (2) To correct functional errors (as covering power) in a lens. Lenses not of the modern and improved anastigmat pattern have certain optical errors which show themselves when very large stops are used, and many of the older and even modern common lenses do not yield a sharp image all over the plate, a defect that is corrected by the use of a small stop and consequent increase in exposure. (3) To add to, or subtract from, the number of planes in a picture. All views are composed of various planes, or distances. When a large stop is used, only one plane is in focus, and the smaller the stop the greater the number of planes made dear and sharp.
In process work, 'various forms of diaphragm apertures are used, with the object of promoting the dot formation. The principle is that the apertures in the ruled screen act as pinhole lenses and form an image of the diaphragm, so that in this way the shape of the dot image is controlled. The square diaphragm is most commonly used, but squares with extended corners are also employed to promote the joining up of the dots in the high lights. The Penrose diaphragm system A standardises the use of such stops, the apertures being arranged on the basis that each smaller stop requires an exposure of half as much again compared with the next larger size. The adjustable diaphragm B enables any size of square opening to be formed.