Archived posting to the Leica Users Group, 1997/12/17
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The infinity focus is defined as any distance beyond =B1900 meters. That is mostly used as the 'test' distance to check the infinity marking and the focal plane. For computational purposes it it is defined in relation to focal length (f) and now it is more than 1000f. The '1000' in the equation is used because the same number is part of the equation to compute the depth of field figures. Therefore a 21mm lens could have its infinity beyond 1000 x 21mm =3D 21 meters and a 50mm lens could have its infinity beyond 50 meters. The essential criterium is always: all lightrays from the distant (at "infinity") object must be parallel to each other. (all incident rays have an angle of zero). Look at the distance markings: the 28 and 35mm M lenses have 10meters and then infinity. The 135 has 50 meters and then infinity. Erwin