[Leica] 13% and 23%

["mjr@grok.org" <cmra.rosen@verizon.net>]

In response to Chris's question, on the back of my manual it says IV/99/GX/L
which may mean it was printed in 1999.  In any event, page 24 says the white
spot is 12mm in diameter, and approx. 13% of the negative size..  On page
18, it says the brightline frames correspond to an image size of 23x35mm at
the closest focusing distance.  And on pages 30-31 it says the metering
field covers approx. 23% of the viewfinder image, or about 2/3 of the short
side of the image.  So, at the closest focusing distance, and assuming the
white spot is at the film plane (more on this below), we can calculate a
spot area of 113.1 sq. mm.  The brightline viewfinder image has an area of
23x35mm = 805 sq. mm.  So the spot is 14% of that area (or 13% of a 24x36
negative).  In order for the Leica manual to be accurate, which I assume it
is, the only thing I can think of is that the distance from the white spot
on the curtain to the film plane actually makes the difference.  Now I don't
know much about optical theory, but it occurred to me that due to the way a
lens works, it may be that if you looked straight down from above the camera
and took perpendicular sections of the light path as it progressed through
the lens, then past the lens on the way through the camera body towards the
film, the rectangular portion of that section that ultimately will achieve
23x35mm in size at the film plane may be smaller at the plane of the white
spot.  In fact, in order for the 113.1 sq. mm spot to be 23% of the image
size, the image size at the curtain plane must be 491.74 sq. mm, or, in
proper 2:3 proportion, about 18x27mm.  If this is a correct picture of
what's happening, then that would account for how the 12mm spot is able to
meter about 23% of the image seen in the finder brightlines.

Any optics experts out there?  Does any of this make sense?  Or am I
completely wrong?

Mark



- --
To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html