CHOICE OF A LENS : TESTING : CARE OF LENSES 94. Preliminary Remarks. The qualities looked for in a lens, at least in certain types, are very different, according as it is required for artistic photography, record work, process work, en largement, or measurement by photography. The construction of an ideally perfect lens is not compatible with the fixed laws of optics. The optician in every case makes a compromise between numerous irreconcilable conditions which the ideal lens should satisfy. By cor recting the various aberrations as best he can, he can make a lens called universal, suitable for almost all work, which is therefore usually preferred by the amateur. But by neglecting all corrections not absolutely necessary for a particular kind of work, he can satisfy the necessary conditions more completely and thus produce a specialized lens which would give poor results if used under conditions very differ ent from those for which it was designed.
In pictorial photography, a simplified image is required, to give " firm " drawing without hardness, of breadth proportional to the scale of the image, and to suppress all unnecessary detail, translating only the general form. " When an artist paints from Nature he removes his eyeglasses if he is short-sighted ; if he has good sight he half-closes his eyes, otherwise he would not see the tree for the leaves nor the forest for the trees " (C. Puyo). This simplifica tion is best achieved by the use of lenses in completely corrected for spherical aberration, or, better still, uncorrected for achromatism (anachromatic lens). The fact that a very wide angle of view is never used in pictorial work prevents the oblique aberrations (astigmatism, coma, curvature of the field) from being very harmful, the more so as, to obtain a sufficiently uniform image in the different planes, small apertures must be used. Such lenses can, there fore, be made very simply, even from lenses made for other purposes, so long as combina tions practically free from distortion are em . ployed, in which spherical aberration is small, if not entirely removed.
Combinations such as these give excellent results in the practice of professional por traiture, but certain requirements, chiefly com mercial, such as the necessity of being able to make extremely short exposures under all weather conditions (for child portraiture), or the preference of some clients for a sharp picture which anyone with any artistic culture would condemn, compel the professional photographer to possess at least, as auxiliary instruments lenses of the greatest rapidity compatible with the great focal lengths which are necessary, if the photographer is not to approach too close to his sitter and so distort the image by exag gerated perspective. Some of these very rapid objectives, giving normally a sharp image for a scale of reduction from 1/2 to 1/10, include a lens the separation of which from the other components can be modified by the rotation of a ring, thus introducing into the image certain aberrations to soften the contours ( J. Traill Taylor, 1892).
The commercial photographer and the photo grapher for the illustrated newspapers, who are often unable to choose the most favourable viewpoint (i.e. to get sufficiently far away from the subject) are generally obliged, at least for a great amount of their work, to use lenses having a very large field (called wide angle lenses), and giving thus very sharp images without the necessity of stopping down greatly. Press work, and especially sports photography, requires the use of a very large-aperture lens, which is also necessary for colour photography on colour screen plates.
The process worker requires to obtain a plane image of a plane object, free from distortion within the limits of scale usually employed (same size or a little smaller). There is no need, in this case, for a large aperture, but the chrom atic corrections need to be very good if three colour work is to be done ; the different com ponent pictures obtained through selective filters should be capable of being exactly super posed and be of equal sharpness.