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

[KEVIN BURKE <KBURKE@iterated.com>]

>>When focusing a rangefinder camera, you
>>should start either at infinity or the closest setting, and move the ring 
in
>>one motion until the two images coincide--do not rack it back and forth, 
as
>>you might with an SLR, to improve the focus.  The reason this works, I
>>understand, is due to the nature of the mechanical linkage between lens 
and
>>rangefinder--racking it back and fortyh produces inaccuracies.

>This is an old machinists trick. As the bed screw of a mill or lathe wears
>it develops slop. When moving the bed to make a cut you back up so that the
>screw is engaged fully when your tool touches the work. Same with the
>rangefinder.

I must be among the heretical few that rocks the focus of my lenses
when trying to place the focus plane with my rangefinders.  Having
tried the varieties of advice that exist on the subject, I find a higher
percentage of hits when rocking with longer lenses at wider apertures
and closer distances.  The lenses with smooth, free focus movement
have worked well with this method.  When possible, the best
technique I've found is to make sure I'm using vernier seeing and
aligning lines or edges at the top or bottom of the rangefinder rectangle.
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.

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 have heard about leaving one's controls in a default position (focus at
one extreme, etc.) so that the photog could react quickly and instinctively
to an image opportunity by always making adjustments from the same
rest point.  This technique is different from the issues described above.

Disclaimer: I've only had the pleasure of using later model rangefinders
(M4-2, M4-P, M6).  Maybe a more mature rangefinder would show
the slop in focusing due to its age.  Practically speaking, I haven't seen
it's effects on film.

Regards, Kevin
kburke@iterated.com