Rangefinder mechanics (was:Making pictures with the M Leica)

["Juan F. Sanz Cervera" <Juan.F.Sanz@uv.es>]

>From: KEVIN BURKE <KBURKE@iterated.com>
>
>I must be among the heretical few that rocks the focus of my lenses
>when trying to place the focus plane with my rangefinders.[...]
>
>I could be all wet, but I'm having trouble buying into the theory about
>excessive slop in the M rangefinder.  If one needs to "load" the system
>from one direction in order to provide a stable bearing surface, then
>approaching the same point from the opposite direction wouldn't
>necessarliy result in using the same relative bearing surface.  The two
>settings would be slightly different under such a condition.  It seems a
>good design should have taken the ambiguity of mechanical backlash
>into account and made sure it was below the practical limits of application,
>i.e. DOF.  Besides, the danged things are spring loaded.  If the spring
>tension is doing its job, the relative bearing surfaces should be the same
>regardless of the direction of travel.  The spring makes sure the system is
>loaded the same way each time in each direction.[...]

I totally agree with Kevin here. In this case, there is an easy test that
anybody can try: if you focus your Leica and get the same focus distance
with both methods, that means that the "no rocking" theory is just one more
of those photo myths. I tried it myself, and results are the same with both
methods with both my M3 and M6. Kevin also made a good point: if you have
defined vertical lines, it's not that difficult to get coincidence of images
in one motion. However, as Kevin said:

>If the focus point does not contain lines or edges with good contrast, the
>rocking lets me find the maximum contrast point of the superimposed
>images a little better.

That's exactly what I've found!

Anyway, I find it funny that my focusing method ("rocking") is supposed not
give me sharp focus when in fact it does in practice! :-) I am VERY critical
with focus in this respect. A good 10x lupe hides few focusing errors... :-)

Regards,

   - Juan 



                   ========================================
                             Juan F. Sanz Cervera
                      Associate Professor of Chemistry
                      Departamento de Quimica Organica
                       Universidad de Valencia, Spain
                 Juan.F.Sanz@uv.es    http://www.uv.es/~jsanz
                   ========================================