Patent GB-519583-A (GB)
Title: Improvements in or relating to view finders for use with photographic or cinematographic cameras
IPC: G02B23/14
Filed: 1938-09-21
Abstract: 519,583. View-finders for photographic cameras. ZEISS IKON AKT.-GES. Sept. 21, 1938, No. 27547. Convention date, Sept..21, 1937. [Classes 97 (i) and 98 (i)] In a view-finder for photographic purposes, having optical means by which image bounding marks are caused to appear in the plane of the view, means are provided for preventing light rays from the field of view mingling with those which form the image of the bounding marks, thus rendering the latter more distinct. A number of different forms are described. In the finder shown in Fig. 1, a tapering hood-like member 4 extends between a glass plate 5 and. a viewing lens 6, the view included in the area bounded by the member 4 being seen unmagnined through a central flattened portion of the lens. The space 7 between the member 4 and the housing 1 is viewed through the marginal portion of the lens as an image-bounding frame, the member 4 preventing rays passing through the picture area 5 from mingling with those forming this image. In a modification, the member 4 is replaced by a glass block of similar form, with blackened side faces. In another form, the image-bounding frame 7 and the image field are covered by red or green glass respectively, the outer and the central plane portions of the lens 6 being respectively of the same colours, so that rays passing through the image field are not seen in the outer portion of the lens which forms the image of the bounding frame.' A number of forms are described in which the light rays passing through the image field and the bounding frame are separated by differential polarization. Fig. 5, shows one form in which the bounding-frame and the image field are covered by filters 12, 13 having mutually perpendicular planes of polarization, and an annular polarizer 14 the plane of polarization of which is parallel to that of the filter 12 covers the unflattened portion of the lens 6. A ray 16 from the field of view striking the polarizer 14 is thus prevented from passing to the lens, whilst rays 17 from the bounding frame are passed to form the image of the frame. In Fig. 7, the filters 18, 14 have mutually perpendicular planes of polarization, so that rays through the image field are visible only through the central plane portion of the lens. The bounding frame is formed by a member 19 of doublyrefracting material which rotates the plane of polarization of a ray 17 passing through it, and is thus visible through the filter 14. In a modification the planes of polarization of the filters 18, 14 are parallel and the image field is covered by a plate of doubly - refracting material. In Fig. 9 the viewing lens comprises a plano-concave component 21 of doubly-refracting material and a plano-convex component 22 of glass, the refractive index of which is equal to the smaller of the two refractive indices of the lens 21. The combination acts as a plane parallel plate for rays 15 passing through the image field area of the polarizing filter 18, and as a lens for rays 17 passing the filter 18 and the image-bounding frame 19 of doubly-refracting material, which alters their plane of polarization. The lens therefore forms an image of the frame which is unaffected by the light passing through the image field. In a modification, a negative lens is arranged between the viewing lens and the frame 19. The finder shown in Fig. 10 comprises a semi-transparent mirror 23 formed between glass plates, a mirror frame 26 with inclined sides, a polarizing frame 27 in front of it, and polarizers 12, 13 forming the bounding-frame and image field respectively. A ray 17 passing the polarizers 17, 27 is reflected by the mirror 26 to the semi-transparent mirror 23, which thus forms the image of the bounding frame. A ray 28 passing the polarizer 13 in the image field is stopped by the polarizer 26. A modification of this form is described in which the polarizer 13 extends to the walls of the housing and carries members 29 . . . 32, Fig. 13, of doubly refracting material which rotate the plane of polarization of the light passing them and are therefore visible as an image formed by the mirrors 26, 23. Owing to the relative inclinations of the mirrors 26 two images of each of the members 29 ... 31 are formed at the margins of the image field, giving the effect shown in Fig. 14. The finders may be combined with a rangefinder having a common range and view-finding image. The image bounding members may be individually displaceable, or exchangeably arranged, to suit objectives of different focal lengths. The view finder may also include means for indicating the centre or other points of the field of view, and for forming images of indicators showing, for example, stop and - exposure time adjustments, depth of focus or the length of film used. Specifications 147,107 and 203,668, [both in Class 97 (i)], are referred to.