Archived posting to the Leica Users Group, 1998/05/08

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Subject: Re: [Leica] M6 HM focusing improvement
From: Nathan Wajsman <nathan.wajsman@euronet.be>
Date: Fri, 08 May 1998 10:17:42 +0200

David,

Being new to Leica and rangefinders, this leads me to ask the question: does
this mean that I can assume that a high percentage (most?) of my shots with
the Summilux 75mm/1.4 will be erroneously focused when wide open?

Nathan


David Morton wrote:

> 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
> 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
> Erwin
>
> -----------------------------------SNIP-----------------------------------
>
>             David Morton  |  "saepius ventis agitatur ingens
> dmorton@journalist.co.uk  |  pinus est celsae graviore casu
>    Islington, London, UK  |  decidunt turres feriuntque summos
>       (+44) 171 272 8908  |  fulgura montis." (Horace)