(§7o). Thus the effective actual aperture of each stop will be increased in the ratio of 13 to 15, that is, multiplied by 1•15, so that, taking into account the losses by reflection at two additional surfaces (about ro per cent), the effective speed of the lens will be increased by about 20 per cent. When using a less powerful supplementary lens, the gain in speed would be considerably less, and the gain may easily disappear or be converted into a loss by the reduction of light due to surface reflections. Moreover, the increase in speed is generally illusory, for if the image is required as sharp as that obtained when the objective is used alone, it is necessary to use a smaller aperture.
This reduction of the focal length also brings about, ipso facto, an increase in the depth of field, which at the same time is further aug mented, since an incompletely corrected optical system has always a slightly greater depth of field (86).
Positive supplementary lenses, while useful for record work, have unfortunately been used for portraiture; and as if the majority of photog raphers had not already a marked tendency to place their models too near the camera, the use of such supplementary lenses has been popularized under the name of "portrait attach ments." It cannot be too strongly emphasized that a portrait taken at too close a range is not far removed from a caricature of the sitter.
While formerly only bi-convex lenses were available for spectacle making, progress in ophthalmic practice has led to a preference for meniscus lenses, which also make the best supplementary lenses. The concave surface, which is turned towards the eye in spectacles and pince-nez, will also be that which must face the objective when it is used as a supplementary lens.
119. Tele-attachments. In the same way that the focal length can be reduced by using converging supplementary lenses, so it can be increased by the addition of a diverging lens (R. Viney, 1897). The use of such a lens is of great advantage in landscape or portrait pho tography with non-separable objectives. The increase in focal length causes a reduction of the angle of view. The softening of the definition resulting from the aberrations of the uncorrected lens, and the greater homogeneity of the image in the various planes, advantageously diminish what might be called, from an artistic point of view, the defects of a too well corrected lens.
It is possible to calculate the approximate value of the focal length (within 5 per cent) by assuming the power of the combined system to be equal to the difference between the powers of the objective and the supplementary lens. If an objective of 15 cm. (6 in.) focal length, or 6-7 diopters, is coupled with a diverging lens of so cm. (20 in.) focal length, or 2 diopters, the power of the combination will be in the neigh bourhood of 4-7 diopters, viz, a focal length of about 21-5 cm. (81- in.). For avoidance of ex
cessive aberrations, and the need of making an ad justment in focus as when using an anachromatic objective, the power of the diverging attachment must not be as much as half that of the objective. If, on the other hand, a supplementary lens of very low power is employed, its effect is very nearly negligible. In practice, the focal length of a diverging lens attachment should not be more than double nor less than a quarter of the objective with which it is being used.
Diverging meniscus lenses are to be preferred to cheap hi-concave spectacle lenses, and are used with the concave side towards the objective.' 120. Light-filters. Light-filters, the optical properties of which will only be considered here, are formed either by a coloured liquid contained in a plate glass cell with plane and parallel faces or by a film of coloured gelatine, which is either used as such or cemented with Canada balsam between thin, flawless plate glass, or thick optically worked pieces of glass.
With the exception of the plain gelatine filters, which are too thin to modify the rays of light to any appreciable extent, any filter which is placed in front of or behind an objective alters the position of the sharp image, and introduces various aberrations into it.
If a pencil of light is made to converge at a point P by some optical system (Fig. 89), and a thick sheet of glass L, or some other trans parent material with plane and parallel faces is interposed, it is easy to show that the image is displaced from P to F', its distance from the optical system being increased. If the inclina tion of the pencil is relatively small on the face of the sheet, the point P' is situated on the per pendicular drawn through P in a direction common to the two effective faces of the plate (L), and, if the latter is of glass, the displacement PP' of the image is approximately equal to a third of the thickness of the filter.' If the filter is placed in such a way that the optical axis of the lens is perpendicular to its face, this being the necessary condition for the whole to constitute a centred system, the effect of the filter will be the same for every pencil, and it will be sufficient to increase the distance between the plate and the objective by a third of the thickness of the filter, in order that everything may be in practically the same condi tion as before the introduction of the filter. If, on the contrary, the filter is not perpendicular to the lens axis, the differences of obliquity of pencils equally inclined to the optical axis cause deformation of the image. This may not be very considerable, it is true, but it would be particularly noticeable in negatives which had been taken through a badly placed filter, and which were required to give exactly super imposable images (three-colour work).