OPTICAL METHODS OF BORE-HOLE INVESTIGATION 



343 



Fig. 8 sho^vs the paths of rays in the well camera, obtained by means 

 of graphical construction; the ray paths and image of AB are shown by 

 a broken line for the case when the medium is wholly air, and the actual 

 path is shov,ii by continuous lines, where account is taken of the fact that 

 part of the medium is water and glass, which have refractive indices different 

 from unity. It is evident that this circumstance leads to displacement of 

 the image by B'B" = 5 mm relative to that observed in air. The image is 

 produced with fourfold reduction. 



A very important quantity for the camera is the depth of field which is 

 the distance within the limits of which objects at different distances from 

 the objective lens will appear sufficiently sharply on the image. It is obvious 

 that, in order to obtain a sharp image of the usually uneven surface of a well 

 wall, it is necessary to have as large a depth of field as possible. Increase 

 in depth of field can be obtained by increasing the distance from the objective 

 lens to the subject of the photograph and reducing the effective aperture 

 of the lens. 



Fig. 9. Test plate. 



The distance from the subject of the photograph to the objective lens 

 is taken as equal to 267 mm; further increase in the distance is undesirable, 

 since this would lead to considerable lengthening of the camera. 



In choosing the effective aperture of the objective lens it should be 

 remembered that reducing it leads to a sharp increase in exposure time. 

 In the well camera the relative aperture of the objective lens is taken as 

 1:5.6, calculated for a depth of field of about 2 cm; in the majority of cases 

 the distance from the viewing window to the rock wall lies within the limits 

 of this distance. 



