GB687256A - A photographic exposure meter - Google Patents

Abstract

687,256. Photographic exposure meters. RUSSELL, F. M. Nov. 16, 1951 [July 31, 1951], No. 18114/51. Classes 97 (i) and 98 (i). A photographic exposure meter comprises two plane-polarizing screens arranged one in front of the other, in a viewing device through which the scene, object, or image whose brightness is to be measured can be viewed, one of the screens being rotatable relatively to the other for weakening the light from the scene or object so that the angular rotation necessary to weaken the light until the scene or object or a significant part thereof is just visible serves as a measure of the brightness of the scene or object. The meter may be used to determine the exposure required either for photographing a scene or for printing from a negative. As shown, the meter comprises an eye-piece 2 rotatable in the base part 1, the parts 1 and 2 being held together by an oval steel ring 15 engaging in the grooves 16. The eye-piece 2 has a relatively small viewing aperture 3 and contains a plane-polarizing screen 4, while the base part 1 has a relatively large viewing aperture 5 and contains a plane-polarizing screen 6. An eye-shield 7 is attached to the eye-piece 2 and a hood 8 carrying a plain glass window 9 on which is marked a reference frame is attached to the base part 1. The part 1 has a scale marked thereon, and the eye-piece 2 has a pointer or index co-operating with scale on the part 1. The base part 1 is rotated relatively to the eye-piece 2 until the scene or a chosen part thereof is just visible, and the reading on the scale is used to determine the exposure required from appropriate tables. Alternatively, a calculating device consisting of an annular disc or ring rotatable on the base part 1 and marked with a number of concentric scales co-operating with complemental concentric scales on the part 1, additional to the original scale, may be provided for computing the exposure. The disc or ring may carry a scale of light values corresponding to the original scale and co-operating with a scale of film speeds on the part 1, and a scale of exposure times co-operating with a scale of F- numbers on the part 1. In using the meter for determining printing exposure, the darkest part of either an image of the negative projected on a sheet of white paper in an enlarger or of the negative itself placed on a sheet of white paper is viewed through the meter while the reading is taken. In this case, the meter is used in conjunction with a table computed to give the correct exposures for different printing papers. The hood 8 may be replaced by a hood carrying a colour filter excluding light of the colour emitted by the dark-room lamp, an allowance being made for the effect of the filter on the reading on the scale.