. The book of photography; practical, theoretical and applied. or orthostigmat. Astig-matism, although seldom entirely absentfrom any lens, is more usually found inlenses designed to obtain great flatnessof field. The introduction of Jena glasshas enabled the optician to devise com-binations in which this defect is almostentirely eliminated. The method of test-ing for astigmatism is described on p. 35. Curvature ob Field. One difficidty inseparable from thespherical form in which lenses are groundhas already been dealt with, namely,spherical aberration. There is, however,another. The image of
. The book of photography; practical, theoretical and applied. or orthostigmat. Astig-matism, although seldom entirely absentfrom any lens, is more usually found inlenses designed to obtain great flatnessof field. The introduction of Jena glasshas enabled the optician to devise com-binations in which this defect is almostentirely eliminated. The method of test-ing for astigmatism is described on p. 35. Curvature ob Field. One difficidty inseparable from thespherical form in which lenses are groundhas already been dealt with, namely,spherical aberration. There is, however,another. The image of a flat object isnot formed on a plane or flat surface, buton a curved one ; thus it follows that ifthe middle of the picture is focussed forAvith full aperture, the edges are out offocus, and vice versa. This fault is known curvature of field. In the exampleshown in Fig. 491, A represents a lensuncorrected for curvature of field, D thediaphragm, o the object, and f f the planeof the image. It is evident that if h hbe the position of the focussing screen,. Fig. 491.—CuRV.\TURE OF Field. the only portion of the image in focus willbe the central part b. Obviously, thebest effect, in the circumstances, will beobtained by placing the screen midwaybetween the extremes of focus, as shownby the dotted line, and using the smalleststop possible. Curvature of field is some-times due to faulty adjustment of thelens, in which case matters may oftenbe improved by slightly altering thedegree of separation between the com- 2 3 Fig. 492.—Method of Overcoming Curvature ofField. binations. The best modern high-gradelenses have an almost perfectly flat most satisfactory method of over-coming curvature, where it is known to bepresent in the lens, is shown by Fig. A B c D repi-esent the focussingscreen. Divide this with ruled pencillines, or mentally, into four equaldivisions, using the longest side of thescreen if it is rectangular. As will be 358 THE BOOK OF THOTOGRAPHY.
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