in front) the effective aperture will be greater than the actual aperture, and in the third case (concave lens in front) the effective aperture will be less than the actual aperture.
In the case of the old Chevalier doublet, with a negative front, a form with which we have worked, the negative front lens will so diffuse the light falling on the diaphragm that the effective aperture is less than the actual aperture, but in the case of an ordinary doublet the reverse holds good, the light falling on the diaphragm being concentrated by the front lens, so that the effective aperture of the diaphragm is greater than would be indicated by the absolute or measured diameter. Hence it is that in the case of all ordinary doublets the speed of the lens is understated if the measured aperture of the diaphragm is taken as the basis of calculation. That the effective aperture should be rightly the basis of calculation there can be no doubt, and we believe now that every maker of lenses gives the intensity on the basis of effective aperture."
In 1891 the Royal Photographic Society issued a new mani-festo on the subject of lens diaphragms ; the manifesto not only defining effective aperture, but also putting forward the focal aperture system in its simplest form. From this manifesto we quote the following.
(1) That intensity ratio be defined as dependent upon the effective aperture of a lens (and not upon the diameter of the diaphragm) in relation to the focal length of a lens.
(2) That effective aperture be determined in the following manner : - The lens shall be focussed for parallel rays. An opaque screen shall be placed in the principal focal plane, the plate being provided, in its centre (in the axis of the lens) with a pinhole. An illuminant shall be placed immediately behind the pinhole, and the diameter of the beam of light emerging from the front surface of the lens "shall be a measure of the effective aperture.
Note. - It will be found, except when the diaphragm be placed in front of the lens, that the diameter of the diaphragm itself is seldom that of the effective aperture.
(3) That every diaphragm be marked with its true intensity ratio, as above defined, in the following order of sequence : -