Standard Subject Size These are based on the principle that the height of a person or the size of a persons head does not differ greatly. Popular examples work on a quadrant and plumb bob (Primus, N&G) or built into a view-finder where a level indicates the distance which is marked on the finder lens (Watson). Subject Angle (Declination) These work by aiming a sight at an object, a level or plumb bob will then show the angle the object makes to the horizontal which can be converted into a distance. Some common arrangements are listed below. Originally these were called distance meters or telemeters, the term rangefinder applied to the 'two position' optical devices was used a few years later. With the increasing use of hand cameras where focusing on a ground glass screen was impossible or inconvenient, separate distance meters came to be used. Two scales were engraved, one for eye-level use the other for use at waist-level.Īs a finder and a level this instrument works well, it is less successful as a telemeter as the strong negative lens makes the numbers on the glass difficult to read. Its use as a telemeter is slightly different: the finder is levelled by ensuring the horizontal frame aligns with the centre line of the lens, the base of the subject is then observed against the distance scale engraved on the lens. The drawing of the second version shows the pointer to be suspended from the top of the rear section rather than the bottom. There were two slightly different versions, the first (the example shown here) is depicted in the British Journal Almanac of 1908, in the 1912 issue it is described as 'improved' and that a patent has been applied for. The finder was supplied with three masks to fit behind the lens for different angles of view, a mirror for use at waist-level could be attached behind the pointer. To use as a telemeter the finder is tilted downwards so that the pointer aligns with the centre line on the lens and the base of the subject, the horizontal frame will then point to a value on the distance scale engraved on the lens above the centre line. The finder is levelled by aligning the edge of the horizontal frame with a centre line engraved on the lens. This is a conventional direct-vision finder with a single negative lens and alignment post, at the rear of the finder a frame is hinged so that it remains vertical when the finder is tilted, to this frame a second frame is attached that will remain horizontal.
0 kommentar(er)
