[Leica] Consistent underexposure - what am I (a newbie) doing wrong?

["Anthony Atkielski" <anthony@atkielski.com>]

I got a Leica M6 a few days ago, to find out more about Leica for myself.  The
experience is largely positive thus far, but I'm having trouble with exposure.
I haven't done manual exposure in several years (my F5 does it automatically and
I rarely switch it to manual), and a majority of my photos with the Leica seem
to be underexposed.

I could understand random exposure errors, but what could I be doing that would
cause _consistent underexposure_?  I'm seeing it in the daytime, in shadows and
under overcasts, and indoors.  I'm shooting Fuji Provia 100 transparency film,
so my latitude is not great.  The exposures are still usable (after boosting the
scans in Photoshop), with only dark shadows completely lost, but I can't figure
out why they are that way to begin with.

I've tried to do the logical things.  I aim the center spot of the viewfinder at
something that I'd like to see correctly exposed, and that isn't completely dark
or brilliant or specular.  If my actual subject is kind of close to what I think
is gray, I use that.  I then take that exposure and run with it.  Sometimes, if
there is weirdness in the shot, I adjust--for example, if slivers of sunlight
are in the metered area but the subject is otherwise in shadow, I boost exposure
by a half stop or a stop.  Still, I seem to be getting a lot of underexposure.

What are the likely reasons for my errors?  I assume that the meter is working
correctly and that I'm just not accustomed to using it correctly.

I have absolutely no prior experience with Leica prior to getting this camera
(although I've been taking photographs with SLRs for many years), so even stupid
mistakes are not excluded here.  Yes, I'm setting the ISO dial to 100.  The M6
isn't one of those cameras that likes to underexpose as soon as there is
anything even slightly bright in the frame, is it?  (My Nikon FG used to drive
me nuts with that.)  I take care to calculate the spot size based on the frame
for my focal length (35 mm); the manual says the spot is 2/3 of the frame height
in diameter, so I use that.

My second two rolls were better than my first two, but underexposures are still
way too frequent.  I'd like to figure out what I'm doing wrong, because in every
other way I'm getting beautiful results (the resolution of the Summicron in some
of the slides is nothing short of astonishing).

All comments and suggestions are welcome.

  -- Anthony