Both prisms and mirrors are best mounted behind the camera lens, the mounting being on a small board which is interchangeable with the lens board. The reflecting surface should be turned to make an angle of 45° with the optical axis of the lens, in such a plane that, after reflection, the optical axis is either horizontal or vertical, according to the work in This adjustment is only possible after repeated trials with the reflector mounted between the lens and the object to be photographed. Ordinary mirrors have occasionally been used, in the absence of a wide-angle lens, for the photography of interiors. In this way the effective optical distance between the object and the camera can be doubled, but in the case of brightly illuminated or reflecting objects the definition usually suffers from doubling of the lines, etc. 2 224. Lens Hoods. Any light reflected in the lens (§ 57) or which is scattered in the camera and distributed more or less uniformly over the image necessarily lessens the contrasts and tends to veil the shadow detail. Scattering of appreci able quantities of light can usually be traced to two causes.
In the first place, small defects in the polish of the lenses and moisture or dust on their surfaces uniformly diffuse a certain proportion of the light which should go towards the forma tion of the latent image. Moreover, if the sun, though not necessarily included in the angle or view, is shining on the front of the lens, any defects on the surface of this lens (or on any accessory such as a supplementary lens or colour filter which is placed in front of the lens) will cause a considerable quantity of light to diffuse into the camera. The same effect is produced, although to a smaller extent, when ever a photograph is taken in the open air, since the lens receives light from all parts of the sky, apart from that in the angle of view.
In the second place, the field illuminated by a lens (§ 55), already considerably greater than the field which is sharply covered, is naturally very much larger than the portion of field utilized. Any rays of light outside the useful field strike the interior walls of the camera, and these latter, even if matt black, always scatter an appreciable fraction towards the plate.' A lens hood is used to eliminate, or at least to diminish, the various causes of the incidence of stray light on the plate, by protecting the glasses of the lens from light coming from above and by intercepting as far as possible any light that the lens would transmit to the sides of the camera. The use of this accessory may be said
to be necessary for all out-door photography, especially for photographs taken against the light, and it is also of considerable advantage when working in a glazed studio.
It should be pointed out that of two lens hoods, each shading the lens to an equal extent, the one farther from the lens will be the more efficacious from the point of view of protection from the sun. Thus, in Fig. 91A, the two opaque screens AB and CD both shield the lens from the ray R, but only AB, that is, the one farther from the optical axis, entirely protects the front lens from the direct rays of the sun in the direction S.' Unless the lens hood is adjustable, it is not possible to confine the admission of light strictly to the beams received by the sensitive film, for if the lens be decentred, it will no longer cover the plate completely.
One of the best lens hoods would undoubtedly be a shade such as AB, hinged at A as high as possible, in such a way that the edge B can be raised or lowered to the limit of the field desired (B. T. J. Glover, 1920). Light from the sides is shielded by two small flexible curtains shown by dotted lines in Fig. 91A. In practice, one is usually confined to fitting on to the lens either a lens hood, represented in section by an obliquely truncated cone (Fig. 91B), or a cylin drical tube 2 with the end cut obliquely, which is slipped on to the lens, more or less, according as the lens is decentred or not. Before making a lens hood of any sort, it is best to make several trials with stout paper, so as to be certain that the lens hood does not cut the angle of view.3 On studio cameras the lens hood is usually formed by a piece of black cloth (extension of the focussing cloth), supported in front of the camera by a removable metal frame ; or the hood may take the form of a bellows connecting the lens front with an open-front frame, as used for the reproduction of transparencies.
125. Sky Shades. Landscape photography is simplified if the brilliancy of the sky and dis tance can be somewhat reduced without dimin ishing the intensity of the image of the ground or foreground. The desired effect can usually be obtained by the use of a yellow filter in conjunction with an orthochromatic emulsion, but unfortunately the use of orthochromatic plates is not yet general. For this reason various procedures have been tried which can be used by themselves to lessen the contrast between the sky and the foreground, or to supplement the use of orthochromatic plates or films.