34 



HANDBOOK OF PHOTOGRAPHY 



3-in. lens this drops to 15°48'. A common rule is that the diagonal of the field is 

 about equal to the focal length of the lens; this rule implies a semifield of about 26° 

 (see Table VIII for further illustrations). 



Effect of Inserting a Parallel Plate into a Light Beam. — There is optically no 

 effect whatever from the insertion of a piano-parallel plate of glass or other homo- 

 geneous transparent material into a parallel beam of light. Hence filters, prisms, etc., 

 required for use wdth a lens focused on a distant object should be inserted into the 

 beam before it enters the lens. 



Fig. 30. — Passage of a converging beam through a parallel glass plate. 



The situation is quite different if the parallel plate is inserted into a converging or 

 diverging beam. If the beam is such that its central raj^ falls perpendicularly on the 

 parallel glass slab, it is affected in the manner indicated in Fig. 30. Suppose the 

 beam without the slab comes to a perfect focus at B, then when the slab is inserted 

 the focus will be shifted away by about one-third of the slab thickness to B'. More- 

 over, the foci of rays at different slopes through the plate will be shifted by different 

 amounts, resulting in the introduction of considerable spherical aberration. 



But an oblique pencil is upset bj'- the slab in a much more complicated fashion. 



In Fig. 31, if 5 is a perfect image point before 

 inserting the slab, the image may go to a position 

 B' after inserting the slab; but ii A'B' is greater 

 or less than AB there will be distortion intro- 

 duced, and moreover the image B' will in general 

 suffer from spherical aberration, coma, astigma- 

 tism, and all the other aberrations due entirely 

 to the passage of the rays through the slab. 

 The amounts of these aberrations due to the 

 parallel-sided slab are independent of the position 

 of the slab but depend only on its thickness and 

 refractive index. These remarks apply directly 

 to filters inserted between the lens and the picture 

 and to reflecting prisms of all kinds, which are 

 optically equivalent to a parallel slab plus one or 

 more plane mirrors. 

 The firm of Taylor-Hobson has designed special camera lenses for use in Tech- 

 nicolor, for in that process a beam-splitting prism must be inserted between the lens 

 and the film ; ordinary lenses would be useless here on account of the great thickness of 

 the prism and the strong convergence of the raj^s passing through it. 



Simple Lens Tests. — Undoubtedly the simplest test for a photographic lens is to 

 photograph on a fine-grain film a distant vertical wall carrying on it sharp detail such 

 as pieces of paper covered mth printed matter of various sizes. The experiment 

 should be done outdoors, but not in direct sunlight, so as to secure uniformitj^ of 

 illumination. In this waj" any nonuniformity of illumination over the field, due to a 



Fig. 31. — -Passage of an oblique 

 pencil of rays through a parallel 

 plate. 



