Re: Leica meter MR-4

[pgs@thillana.lcs.mit.edu (Patrick Sobalvarro)]

Ben, you said that Dan's suggested way of adjusting the MR-4 was wrong:

   From: bholmes@frii.com (Ben Holmes)
   Date: Mon, 16 Dec 1996 10:12:22 -0700 (MST)

   >
   >If you have access to a meter that gives you readings that you trust (say
   >your Pentax spot meter), why not adjust the screw on the MR-4 until it
   >agrees with the Pentax when pointing at a uniform light source?
   >

   I'm pretty sure that analog meters are not linear along their range. The
   screw in the back is a "zeroing" adjustment. If it is set while the meter is
   registering any value other than 0 it would most likely be wrong at values
   above or below the measured value. 

...and you were kind of alarmed about it:

   From: bholmes@frii.com (Ben Holmes)
   Date: Mon, 16 Dec 1996 10:22:45 -0700 (MST)

   It alarms me when people give wrong advice on making an adjustment
   to a piece of gear. I'm not always right, in fact I'm frequently
   wrong - but if the original poster follows the advice given he'll
   end up with his meter out-of-whack AND he's in Nepal. Makes it
   difficult to run down to XYZ Photo for some help once the job is
   botched.

...but the argument about linearity is incorrect.  Dan's original
message gave a procedure that will work just fine, if this adjustment
screw shifts the needle monotonically.  To see this, first consider
that if the meter is correct at one value within its range, then it
has to be correct at other values in its range unless it's broken.
Then consider that if turning the adjustment screw in one direction
always shifts the needle further in one direction, there can only be
one setting of the screw that makes the meter read correctly for
values in its range.  If there is only one setting of the screw that
makes the meter read correctly for values in its range, then it has to
be the same setting of the screw for both no light coming into the
meter and some light coming into the meter.

So Dan's procedure should work fine.  The only danger with Dan's
procedure would come about if the meter were less precise for lots of
light than it was for a little light, and so much so that at high
light levels it was possible to move the screw around without changing
the reading, but at low light levels any movement of the screw changed
the reading.  Then it would make sense to set the screw with the meter
at its most precise reading (0), because otherwise its low light
readings would probably be incorrect.  But the meter would still read
correctly for more light than the light it had coming in when he set
it.

I think the manual's procedure should be followed -- it doesn't
require an external reference, and very possibly the meter is less
precise for high light levels than for low light levels -- but Dan's
suggested procedure was sensible enough that I don't think you had to
be alarmed.