Archived posting to the Leica Users Group, 2000/05/18
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At 9:47 AM -1100 5/18/00, Takeshi Hashimoto wrote:
>
>--
>
>On Thu, 18 May 2000 16:00:21 Peterson Arthur G NSSC wrote:
>>
>>I don't know, and so can only presume John is right, that the "M rangefinder
>>is more accurate than a SLR at 50mm." But my question is this: given the
>>fact that with an SLR one is actually looking through the lens at (almost
>>always) full aperture, does it not then stand to reason that an SLR would be
>>able to focus any lens, of whatever focal length and whatever maximum
>>aperture, with sufficient accuracy for that particular lens? I'm not being
>>argumentative, just asking what seems like a reasonable question.
>>
>
>
>But the accuracy of the M system is fixed by the width of the two
>viewfinder ports. As the lens gets faster/longer, it (the margin of error)
>becomes too great to focus within the DOF of the lens. In other words,
>with an f/1.0 lens, the SLR is probably a little better.
A while ago Erwin Puts posted the following on the maximum aperture that
can be focussed accurately at any given focal length with the various Leica
cameras:
- --------------
Focal length Max aperture Max aperture Max aperture
M3 M2, M4, M5, M6 M6HM, M6J
21mm 0.12 0.15 0,13
24mm 0.15 0,19 0,16
28 mm 0.21 0.26 0,22
35 mm 0.33 0.41 0,35
50 mm 0.67 0.84 0,71
75 mm 1.5 1.88 1,59
90 mm 2.17 2.71 2,29
135 mm 4.88 6.1 5,16
Computations based upon
physical baselength = 69,25
effective baselength:
M3=0,9
M2 etc = 0,72
M6J and M6HM = 0,85 (0,86 is also mentioned)
I noted in an earlier post that ALL M types share the same physical length
of 69,25. Many persons give different figures for the physical baselength
of several M models (68,5 etc). These figures are not correct.
Erwin
- -------------------------------------------
While the calculations are no doubt correct (Erwin wouldn't stand for
anything less;-)), there are a number of assumptions made in arriving at
these numbers. Whatever they are, if you apply the same assumptions to an
SLR with a good ground glass (split image and microprisms actually are
worse for accuracy) you come to the conclusion that you can usually focus
an SLR lens at its maximum aperture, just barely. If conditions are not
that good, maybe you should stop down. Focussing accuracy can be increased,
of course, by putting a magnifier behind the eyepiece.
To get to the point, with the poorest of the M's re focussing accuracy, the
Noctilux is well within the capability of the camera, and the .85 and M3
have a stop or more to spare. An SLR does not do as well.
Also, autofocus systems do not do quite as well (yet) as manual focus on
SLR's, so you're gambling if you use your lens at maximum aperture with AF
on your AF camera.
* Henning J. Wulff
/|\ Wulff Photography & Design
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