The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

Home | About | Photography | Contact






Astigmatism
r,r,r,r,r rays proceeding from a point at some distance from the axis, and from this point draw a line r p, cutting the axis at c, and draw through this point, and the principle axis of the lens, a plane, the lens will be cut by the line m. At right angles to this we draw the plane o n, through which the plane of rays, R",F,R"', shall pass. We shall now see that the rays passing through oc n will meet at the point p, but the rays, r',r', passing through
Fig. g.
represented as in fig. 7 c. Some of the later forms of lenses are free rom astigmatism over a large field, notably the Anastigmats of Zeiss and Goerz. In the old forms the optician has to make a compromise between curvature of the field and astigmatism ; and the best lenses are those which represent a point as a somewhat thickened star. In focussing a line at P, fig. 7, and racking the lens in to o, it goes through the shapes shown in fig. 9.
56