Patent GB-224930-A (GB)
Title: Photographic exposure calculator
IPC: G01J1/42
Filed: 1923-07-16
Abstract: 224,930. Gray, J., (Hewett, J. R.). July 16, 1923. Exposure - In a photo- 'graphic exposure calculator in which table of light factors for different light conditions, latitudes, periods of year and times of day are used, a table of factors is mounted on a rotatable plate and only the portion for immediate use (e.g. that for a particular place or latitude and for a particular month) is exposed to view through an opening in the face of the instrument. The instrument mav be of annular ring form and fit on the lens of the camera. A plate speed scale may be included or excluded, and the construction may be such that only limited portions of the calculator scales and of the subject lighting scale of the factor table are exposed to view when the calculator is set. In one construction, a number of discs 9, Figs. 1 and 2, each marked as illustrated in Fig. 5 for a different latitude, are enclosed with a packing-ring 10 in an annular casing adapted to fit on the lens mount of a camera. The front 2 of the casing is removably held on the back support' 1 by marginal indentations 4, and carries the exposure time scale 20 which is also the light factor scale. It also has an opening 15 through which a sector of the front factor disc, e.g. for a particular month (June), is seen, and a flange extension 17 adjacent the opening 15, with markings as shown in Fig. 1 which include the subject' lighting indications. A flanged ring 8, rotating between the front 2 and central flange of the back 1, carries a subject scale 21 on the extension 23, and a ring 24 on this flange 8 carries a stop scale 22 and a plate speed scale 26 the reading of which is visible through an opening in the flange 23. The ring 23 is notched and the flange 8 with the extension 23 can be set relatively to the ring 24 for any particular plate-speed (e.g. 512) by a resilient tongue 28 and thumb projection 29 on the extension 23, so that normally the subject scale 21 and stop-scale 22 move together over the factor and exposure scale 20. The factor disc 9 can be adjusted by the perforations 16 in the disc and a corresponding recess 16<1> in the upper edge of the opening 15 in the cover 2. The factor for the particular hour of the day and the state of the light is noted. The subject-scale 21 is set to the corresponding factor on the scale 20. and the exposure for any particular stop on the scale 22 is given by the same scale 20. The scale disc 24 carries scales on opposite faces for different svstems of plate speed and stop markings. To facilitate reversing or interchanging the discs 24 when the cover 2 is removed, the ring 8 may be in the form of a split spring ring. The back support 1 may also be fitted with graduated plates 30, 31 for the diaphragm-stop and shutter setting pointers; or alternatively these setting pointers may be adapted to move over the scale 20 with corresponding adaptation in the shutter and diaphragm mechanism. In another construction, Figs. 11 and 11<a>, for fixed-plate speed (P.S. 111) and in which only portions of the exposure and factor scale 20 and of the factor disc are visible which are available for use for a particular setting of the calculator, an outer plate 37 is rotatably mounted on the front 2 of the casing by means of a central turned-over edge 38 and an outer milled edge 39. This plate 37 carries the stop scale 22 and subject scale 21 and the indications 18<1> of the day division limits. The plate 37 has apertures 41, 42 through which readings of the scale 20 on the cover 2 are seen, and an aperture 15<1> exposing a more limited portion of the sector of the front factor disc 9 than is exposed by the opening 15 in the cover 2. The plate 37 carries a pointer 40 adjacent the opening 15 and moving over the flange 17.