Archived posting to the Leica Users Group, 1998/05/07

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Subject: Re: [Leica] M6 HM focusing improvement
From: (David Morton)
Date: Fri, 8 May 1998 3:09 +0100 (PDT)

Shawn London wrote:

> Erwin Puts calculated the theoretical maximum f/stop at which accurate
> focusing could be possible on the M6 quite a few months ago.  I do not
> have the email saved, but I do remember that at 50mm the normal M6
> rangefinder is accurate enough for an f/.85 lens or something of that
> sort.  At least at 50mm, you are quite within the capabilities of the
> regular M6 with a Noctilux.

Here it is:

- -----------------------------------SNIP-----------------------------------

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 reliaby focused when
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
Hope this info has some value

- -----------------------------------SNIP-----------------------------------

            David Morton  |  "saepius ventis agitatur ingens  |  pinus est celsae graviore casu
   Islington, London, UK  |  decidunt turres feriuntque summos
      (+44) 171 272 8908  |  fulgura montis." (Horace)