THE OPTICS OF PHOTOGRAPHIC LENSES 



29 



In a spherically corrected lens, the aim is to make the extreme marginal ray cross 

 the axis at the point P, which can be accomplished by a suitable choice of lens shape 

 and construction. Then it generally happens that the intermediate rays do not cross 

 the axis at this focus but fall a little short of it, giving the situation indicated in Fig. 22. 



Fig. 21. — Diagram illustrating spherical aberration. 



This small residual aberration for the intermediate zones of the lens is known as "zonal 

 aberration" and is generally negligible, but it becomes of serious magnitude in micro- 

 scope objectives and photographic lenses of over //3 relative aperture. In large 

 telescope objectives, zonal aberration would be so serious if spherical surfaces were 



Fig. 22. — Diagram illustrating zonal aberration. 



used that one or more of the lens surfaces is invariably made aspherical by judicious 



hand retouching, and consequently the quality of such a lens ultimately depends more 



on the skill of the retoucher than on the care of the designer. 



The general effect of large spherical aberration in a lens is to produce an image of a 



point source consisting of a bright central point (the focus of the axial 



rays, represented by P in the ray diagrams) surrounded by a halo of 



light caused by the rays which miss the point F, as indicated in Fig. 



23. With an extended object, of course, every point will be imaged as 



a patch of this sort, all the central bright spots serving to outline the 



image, and all the halos merging together to produce a general fog or 



haze over the picture. On stopping down the lens by means of its iris 



diaphragm, the halo becomes smaller, the central brightness remaining 



virtually unchanged until the entire halo has gone. Further stopping when spher- 



beyond that stage diminishes the central brightness also. i*?^ .^ erra- 



tion IS Dr6s— 

 The author once examined an //1. 5 lens which had a large residual gj^^_ 



of spherical aberration of this type. The outer halo could be com- 

 pletely removed by stopping the lens down to//2.9, after which further stopping down 

 merely diminished the brightness of the central spot. As far as extended objects are 

 concerned, this lens would give its best image at //2.9. Any enlargement of the 

 aperture beyond //2.9 would produce a haze over the whole picture resulting in bad 



Fig. 23.— 

 Image of a 

 point source 



