DIAPHRAGMS (Fr., Diaphragmes ; Ger., Blen den) A diaphragm is the aperture, fixed or remov able, used in front of a single lens and between the combinations of a double lens, generally referred to as the " stop." The various forms of diaphragms, systems of marking, values, etc., are as follow : Various Forms.—The three patterns in general use are known as " rotating," " Waterhouse," and " iris." Very cheap lenses, and those of obsolete patterns, are usually fitted with fixed stops, or pierced circles of metal which are let into the lens tube. Rotating stops are mostly fitted to wide-angle and landscape (single) lenses, and are employed in many hand cameras ; a series of circular holes of various sizes are pierced round the margin of a revolving disc fitted to the lens mount. Waterhouse dia phragms were invented by John Waterhouse, of Halifax, in 1858, a circular aperture being cut in a thin piece of sheet metal and inserted into a slot in the lens mount, a separate piece— called a stop—being required for each opening. The " iris," the most modem form of diaphragm, consists of a series of curved plates of metal, vulcanite, or other material, fitted inside the lens tube and attached to a ring on the outside of the lens mount ; by revolving the ring the plates are made to open and close, thus form ing apertures of various sizes. Some Continental lenses have other forms of stops, but they are rapidly falling into disuse. The form in which the diaphragm is made does not affect its action on the lens in the least, and optically one stop is as good as another ; it is in the matter of convenience where one pattern has any advan tage over another. The iris pattern—so named, by the way, because it opens and closes like the iris of the eye—is undoubtedly the most con venient and popular pattern.
Systems of Marking.—There are at least half a dozen systems of marking. Stops are marked with numbers, such numbers, as a rule, appear ing upon the iris ring or lens mount when the iris system is used, upon the top or handle of a Waterhouse diaphragm, and upon the circular revolving disc in the case of rotating diaphragms ; in the last-mentioned case the number appears opposite to the aperture which it indicates, so that when the stop is in use in the lens itself its number is visible outside the lens tubes. In cluded in the stop-marking systems in vogue are the " f," " U.S.," " Dallmeyer," " Goerz," " Zeiss," and " Voigtlander," besides many others. The " f " and " U.S." systems are adopted for all but i per cent. of the lenses in general use, and of these the former is the more popular, especially in England, it being in a sense self-explanatory. The " f " number simply means the proportion of the diameter of the stop to the focal length of the lens. For example, f/8 is of a diameter one-eighth the " focus " of the lens. The " f " value of an unnumbered stop can be ascertained by dividing the focal length of the lens by the diameter of the stop. For example, a t-in. stop with a lens of 8-in. focal length is known as f/8 ; a +-in. stop with the same lens, f/16 ; a i-in. stop with the same lens, f/32, and so on. Thus the " f " number is not a fixed dimension, but always a relative one, varying with the lens employed ; obviously f /8 with a i6-in. lens would mean a
diameter of 2 in., whereas with an 8-in. lens it would mean a diameter of s in. Strictly, the " f " numbers should be +, etc., but the fractional form is ignored, and the numbers spoken of as 8, 11, 16, etc. Stops of definite " f " values may easily be cut to the required size. Suppose, for example, that with a lens of 6-in. focal length, an f/16 stop is required, then, 6 divided by 16 = ; therefore, the stop must be in. in diameter.
While, in the above rough-and-ready system of measuring, the " f " value of a stop may be accurate enough in the cases of ordinary rapid rectilinear and single lenses, extreme accuracy is essential in the case of modern and improved anastigmat lenses ; for while a slight error may be of little moment with a small stop, the same amount of error—which error is, of course, proportional to the aperture—becomes serious with large stops, which are a feature of anas tigmat lenses. It is necessary in such cases to consider the ellective aperture. The stops on a modern lens by a good maker are always correctly numbered, and it is only when a worker attempts to check the optician's calculations by dividing the focal length by the aperture that he finds an imaginary error. The division system described above leads to false conclu sions when some modern anastigmat lenses are measured by it, because of the great condensing power of the lens in front of the stop, from which lens measurements are taken. To quote an example; one of the most expensive of anastigmat lenses has a stop the value of, and marked, f/8; but the aperture of the stop is one-ninth the focal length, and, according to the rough and ready rule given above, would be f 19; while the stop marked f 15, although accurate, is f/7 according to the division method. The following method of finding the effective aperture of a stop is more reliable with all kinds of lenses. The camera is set at " infinity," or a distant object is focused upon the ground glass. A piece of card is then put in the place of the ground glass, or pasted thereon, so as to entirely cover it. In the centre of the card, and on the spot exactly opposite the lens, is made a hole the size of a pin's head. The camera is then taken into the dark-room, and by the assistance of a ruby or orange light a disc of bromide paper is cut to fit the inside of the lens cap. The cap, with the sensitive paper inside it, is then placed on the lens in the usual way, the sensitive side towards the stop. A lighted candle is then held against the hole in the cardboard for about half a minute, so that the light may travel through the camera, lens and stop to the bromide paper. The latter, after exposure, is taken out and developed, when a circular black spot will be found thereon, and the diameter of this spot will be the effective diameter of the stop used. If the exposure is made with the largest stop, the developed spot gives the effective aperture at which the lens will work, and the focal length of the lens divided by this, the true aperture, gives the " f " number.