Archived posting to the Leica Users Group, 1997/12/17

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Subject: Re: [Leica] WA infinityfocus definition
From: Erwin Puts <imxputs@knoware.nl>
Date: Wed, 17 Dec 1997 21:16:33 +0100

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