Rangefinder accuracy & focusing ease

[Erwin Puts <imxputs@knoware.nl>]

Since I posted the figures on the rangefinder accuracy of the M-series a
number of correspondents asked about the ease of and possibility of
focusing by the M.
Let me say at first this: the Leica M is (and has been) the only camera
designed and built to true engineering standards of mechanical measurement
apparatus. In its way there is no better or even an alternative. The AF
mechanism, while adequate for most tastes and uses is far below the
standards of the Leica. It is ironical and an indication of our technical
culture (no contradiction here) that we are quite often not able to see the
differences between these different standards of engineering.
But every measurement instrument has its limits. The critical aperture
calculations show the limits. What is meant by these numbers? When you
focus on an object at distances and aperture/focal-length combinations
within the limits of cac, the engineering of the Leica ensures that every
single mesurement is spot-on. If you use combinations outside these
limites, the chance is that your measurement is not precise. It might be,
but the system can not guarantee. The ease of focusing is not at stake
here. For your eyes (only?) you see the double images in the viewfinder
align  precisely. But the system tolerances allow a certain latitude that
is not visible but inherent in the tolerance chain. So you  can focus on an
object at a real distance of 5 meters and see the images coincide when the
distance on the lens is anything between 5.20 and 4,80. These small
differences are beyond the system capabilities (and the eye) to detect.
You can easily simulate this effect. Put a Tele-Elmar 4/135 (one of the
best optically in the current line-up) on an M6 and focus on an object at =
=B1
5 meters.On a tripod of course!!  Now focus as accurately as you can.Then
very slightly move the focusing collar left and right. You will notice that
there is a certain movement of the focusing ring without a corresponding
movement of the viewfinder images. Look at the depth of field markings of
the lens and note the margin. If you repeat this excercise with a 50mm, you
will note that the margin now is much less.
Erwin