We have already mentioned the use of diffusing screens (§ 126), which transmit a sharp image through the central part and a soft image through the outer part.' A very simple arrangement which can be used with large-aperture lenses yields some very pleasing results (G. Cromer, 1920). At a small distance behind the lens is placed, parallel to the plane of the sensitive plate, a very thin sheet of gelatine (such as is used by engravers), or glass (similar to extra-thin microscope cover slips) with imperfect surfaces. In this manner a certain fraction of each ray of light is slightly deviated from the direction which would give a sharp image, and the resultant effect can obviously be varied either by altering the thickness of the sheet or its distance behind the rear lens component. In a variation of this method (J. Sereni, 1924) a special type of grating, composed of glass strips separated by spaces about equal to their own width, is placed behind the lens. Upon the sharp image formed by the fraction of each bundle transmitted by the spaces is superimposed a soft image due to the interposition of the relatively thick transparent medium with non-parallel surfaces.
Finally, a negative with soft outlines can be obtained by the use of a lens which has been imperfectly corrected for spherical or chromatic aberrations, the resultant image in this case being composed of a large number of elementary images corresponding either with the various concentric zones of the lens or with the different coloured radiations. 1 There are on the market many types of lenses which are incompletely corrected for spherical aberration. They exhibit no peculiarity in use other than the fact that any alteration in the size of the aperture or in the length of exposure has an appreciable effect on the result, the image being much sharper when a small aperture, or, within normal practical limits, a short exposure, is used. Many lenses, more particularly those employed for portraiture, will give, as desired, a perfectly sharp image, or, after alteration of the distances between the various components (obtained by rotation of a ring in a spiral groove), a soft image clue to spherical aberration.
Anachromatic lenses (§§ 97, lox, ilo), which can easily be prepared by the photographer himself from very cheap components (spectacle lenses), have given some very remarkable results in the hands of numerous artistic workers. Their use is often criticized because an adjustment of the extension is necessary after the focussing has been done, and also because widely different results are obtained when light sources of different compositions are used successively (mercury arc, daylight, incan descent lamps). 2
311. Correction of Focus with Anachromatic Lenses. Time following rules apply only in those cases where the work is carried out with crown-glass lenses on " ordinary " emulsion, the subject being illuminated by daylight.3 For simple or symmetrical 1 an.achromats, the extension should be reduced after focussing, in order to bring the plate away from the plane of the sharp image formed by the yellow i adiations into the plane of the sharp image formed by the violet radiations.
For distant subjects this correction amounts to about one-fiftieth of the focal length ; for near subjects it should be calculated, once and for all, by means of the following formula, in which F represents the focal length and p the distance of the subject from the lens— p Y2 50 kp -F) For adjustable landscape lenses (telephoto anachromat), the extension should be reduced by a constant amount, obtained by adding one-fiftieth of the focal length of the combined lenses to one-twenty-fifth of the extension, reckoned from the ground glass screen to the rear lens.
For the telephoto studio lenses, the correction should be made by altering the position of the two systems (the lens objective and the divergent negative attachment). With telephoto ana chromats, the two systems should be brought closer together, the constant amount of this displacement being determined by trial once and for all for a given objective. The systems of a telephoto serni-an.achromatic objective should, after the focussing, be moved farther apart, the extent of the correction, which is practically constant for a given combination, being consider ably less than in the previous case. In this case again the correction should be determined by a series of trials.
312. Photography with Two Superimposed Plates. One of the most ingenious methods which have been proposed for the production of soft-focus photographs is that described in 1921 by the painter, E. Artigue. Two plates are exposed one behind the in the same plate holder, with the two sensitive surfaces facing the lens.
One of the sensitive surfaces is therefore in the plane of the sharp image, while the other is behind this plane, with the emulsion layer of the first plate acting as a diffusing medium in between. The normal time of exposure should be increased by about 50 per cent in order to reduce the contrasts in the first negative by over-exposure, and also to obtain an image on the second plate, in which the contrasts will be exaggerated by under-exposure.