Re: [Leica] 13% or 23% ?

["Mark J. Rosen" <cmra.rosen@verizon.net>]

Emanuel,

He he he.  Yeah, I'm not worrying too much about it, but when I read a
manual I don't expect such a large inconsistency.

Chris,

Yeah, you're right.  I measured and clearly the white spot is in fact 12mm.
It's on the curtain right in front of the film plane, so I don't buy the
idea that somehow its distance from the film plane makes any difference at
all.  Instead, it seems to me that the manual must be correct when it states
that the spot is 13% of the negative size, and 12mm in diamter, and
incorrect when it states that the meter field is 23%, or 2/3 of the short
distance, of the bright line field corresponding to the lens in use.  Simple
arithmetic yields the straightforward result that the spot only occupies 13%
of the rectangular cross section of light reaching the film, i.e., a 24x36mm
rectangle.  Thus, the meter can only be reading 13% of the field.  And of
course this is completely independent of which lens is in use; changing the
lens just means changing what's in that 24x36mm field, but the same central
13% is what's being reflected to the metering cell.  Now obviously that
means that a short focal length will have more "stuff" being read by the
meter at any one time than with a long focal length, but the same percentage
of what's in the final composition is being metered.

I suppose the meter cell could actually be looking at 23% of the field, but
that would mean it's seeing the 12mm spot, plus an annular region extending
outward from the outside edge of the 12mm spot for 2mm; i.e., a 2mm wide
annular black region of plain black shutter curtain.  I find it somewhat
hard to believe that the meter would be constructed to read a 16mm spot, the
central 12mm of which is painted white to reflect light but the outer
portion being left black to reflect near nothing.

Thus, it seems likely that the illustration in the manual showing the
metering field is not accurate:  the spots shown are too large, 33% larger
than they should be (should be 1/2 the short side, or 3/6, but instead are
shown as 2/3, or 4/6, which is 33% larger).

I have to admit I'm quite surprised that there is so little in the LUG
archives about this.  Indeed, I'd also have thought a definitive answer
would have been issued by Leica itself by now.

Mark

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