Archived posting to the Leica Users Group, 1998/01/31
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THis is my message from some time ago:
There is a mechanical/optical relationship between the accuracy of the
rangefinder and the baselength in relation to the maximum aperture/.
The equation will be published elsewhere, but here is the result:
Focal length Max aperture Max aperture
M3 M2, M4, M5, M6
21 mm 0.12 0.15
28 mm 0.21 0.26
35 mm 0.33 0.41
50 mm 0.67 0.84
75 mm 1.5 1.88
90 mm 2.17 2.71
135 mm 4.88 6.1
This equation then tells you that the 21 mm can be reliably focused when the
maximum aperture is f/0.12 (incredible!).
At 50mm the Noctilux approximates the theoretical maximum values when
attached to the M6.
The 135mm seems to be overstretched when used at its maximun aperture of 2,8.
Hope this info has some value
As you can infer frm these figures, fully reliable measurements are nor a
problem with the 2,8/90 on any M, but using the Summilux 75 is a bit
demanding on the rangefinder.
Erwin