Society of Motion Picture Engineers : incorporation and by-laws (1922)

field or away from it. Its amount can be shown in a diagram somewhat similar to that one used for the representation of spherical aberration. Figure 8 represents such a case. If the coma were corrected all parallel rays striking the aperture of the lens under an angle with the optical axis will intersect each other at the same point. If the coma is not corrected the rays going to the upper part of the lens may come to focus at a greater distance from the lens than the rays going through the lower part of the lens. The consequence will be a distribution of light in the image plane similar to the little pear-shaped figure drawn just outside of the greatest concentration UNCQEEECTED COMA Hot8 ^■^A^ 1 V \ ^. ^——~~~ — ^-^^-^^^^^J<Xv^/ - X^^ ^5H[ .i i of rays. The curve shows the distances of the different points of intersection measured along the axis from a plane laid through the ideal image point. A perfect correction for coma will show the curve as a straight line. The curve for spherical aberration needs only be shown above the optical axis because by reason of symmetry it is the same on both side of the axis. This is different in the case of coma, because, as Figure 8 shows, the rays above and below the axis of the pencil strike the lens surface in a very different way, which fact is the reason for the one-sided light distribution in the image plane, called coma. Coma is a very difficult aberration to remove entirely, and although great improvements have been made in the recent high speed 91