After the image has been focussed as sharply as possible by examination of the ground part of the screen, a magnifying lens, adjusted to give a sharp image of the cross, is applied to the transparent area. It is now possible to see at the same time both the image of the subject and the cross in the plane of the screen. If the image is formed accurately in this plane, any displacement of the eye, either upwards or downwards, or to right or left behind the lens, will not alter the position of the cross relatively to the image. If, however, this is not the case, the direction of the displacement will indicate whether it is necessary to lengthen or shorten the camera extension. The extension is too long if the cross is displaced relative to the image in the same direction as the eye is moved. If the cross moves below the image as the eye is raised, the extension is too short.
309. Soft Focus. In photography there is occasion to consider two entirely distinct cases. First, it may be desired to obtain a result con forming to certain scientific, legal, or commercial requirements, in which case perfectly sharp definition is an essential quality. On the other hand, a photograph of artistic effect may be required, in which it is desired to suggest the impression conveyed by a subject without faithfully reproducing its finest details.
A photograph is not necessarily artistic because the focus has been softened. On the other hand, a rigorously sharp picture, in which details ordinarily imperceptible to the eye without close examination are easily discerned, can never create an artistic effect, whatever other merits it may possess.
The focussing in the second case is always much more difficult to judge, for the degree and distribution of the softening demands not only a certain aesthetic taste and knowledge of psychology but also a full acquaintance with the various methods used for the production of pleasing soft effects.
The degree of softness will obviously depend on a large number of factors, more especially on the size of the picture, on the subject, and on the artistic education of the public to whom the work is to be presented. It is evident that the outlines may be made much more diffused if the picture, owing to its size, is to be viewed from a distance. A portrait of a child or of a young woman can be treated with a certain amount of freedom, while that of an old man is often better rendered with a less degree of diffusion. And then, again, the uneducated will only approve with difficulty of a soft-focus portrait, which they will consider to have been bungled, whereas they will accept a photograph with softened outlines without any tendency to hardness. On the other hand, a photograph
with very softened outlines is much more pleasing to the cultivated taste, unless the effect has been exaggerated to a degree which is considered suitable only in studies executed for decorative purposes.
The method which most naturally suggests itself for the avoidance of the extreme sharpness afforded by the modern technically perfect lenses, such as the anastigmats, is to deviate more or less from the normal position of sharp focus. This method, however, invariably results in distinctly unpleasant portraits, in which the sharp definition, though absent from the main subject, appears in some minor part where it is least wanted.
An attractive portrait which is neither too sharp nor too soft can usually only be obtained by a combination of two images, one of which is perfectly sharp but of subdued intensity, while the other of softened outlines is used to obtain the desired effect.
Many different methods may be used to obtain this combination, either from a single negative, by double printing from a single sharp negative, or by taking a print from a combina tion of two negatives taken at the same time Fig. 165 shows in diagram form the appearance of a line in the principal cases considered.
310. Making Soft-focus Negatives. A recent variation (de Dalmas, 1923) of a very old method (A. Claudet, 1866) gives some very interesting results, which, however, its author admits (§ 312) to be inferior to those obtained by the Artigue method on account of variation in the scale of the image. During about two fifths of the exposure the extension is increased by one-eighth or one-twelfth of an inch for a wide-aperture portrait lens, or one-fifth of an inch for a lens of smaller aperture.
In a somewhat similar method the camera is first focussed on a point considerably in front of the subject, and the lens is then stopped down until sharp focus is obtained. Half the requisite exposure is then given, after which the aperture is opened to an intermediate position and finally to full extent. At each of these positions an exposure is given equivalent to a quarter of those necessary at the apertures used (E. Genet, 1923). This method is only applicable to still landscape photography, but is much more suitable to this class of work than the previous method.