[Leica] Metering question

["Mike Rivera" <mike_rivera@email.msn.com>]

Hi all:

I have a questions regarding metering on an M6 (non-TTL).

I was reading the "Leica Compendium" by Jonathan Eastland and I came across
this:  "...the photographer can accurately measure very small areas of the
object to be photographed using the preselector lever."  He goes on to
state, "By flicking the preselector lever to bring up the 90mm bright-line
frame, particular areas of the scene can be measured and interpolated.  Even
smaller areas an be metered using the 135mm projected frame, in effect
giving the photographer a selective metering facility."

Am I to assume then if I have my 35mm lens on the camera and I want more of
a "spot-meter" reading, I can simply flick the bright-line frame selector to
the 90mm frame and the metered areas gets correspondingly smaller?

How can this be?   I thought the metered area was determined by the lens
attached, mechanically.  If the above is true, how would the camera know how
much area to meter since the bright-line frames are shared by two focal
lengths (the 90mm and the 28mm share the same bright-lines)?

Confused in Sacramento and it's only 7am.

Mike Rivera