Re: [Leica] 13% and 23%

[Christer Almqvist <chris@almqvist.net>]

As you say below, at the shortest focusing distance, what you see 
within the brightline frames will cover 23x35mm on the negative. (If 
you use slide film and mount the slide, then the viewable area 
corresponds to the viewfinder at the shortest focusing distance.) 
That means that, at the shortest focusing distance, you will get a 
bit more on the negative than that is within the framelines. With the 
lens set at infinity you will get a negative that show yet a little 
bit more than what the framelines indicate. I have been told it is 
about two framelines width more at infinity. Anyway the framelines 
seem to move a little bit as the eye moves, so I would not try to be 
all that exact talking about framline coverage.... As an average for 
your calculations, I think it would be practical to assume that what 
is within the framelines is what you get on the 24x36mm of your 
negative.

I think you are correct in assuming that the distance between the 
film plane and the white spot makes some difference in calculating 
the area covered by the light meter. With the distance between film 
plane and white spot being very small, this can never account for a 
13% vs 23% difference.

Commenting on your earlier post, I think it is possible that the 
light meter  _looks_  at an area which is larger than the white spot. 
But as the area surrounding the white spot is very dark and very non 
reflective, I think that it does not influence the reading to any 
relevant extent. As I said before, put a sheet of white paper on a 
black background and make some measurements around the paper and into 
the paper and you will see when the meter starts to react.

There was an earlier reply to you saying: "do not worry about meter 
coverage, just compose picture,line up the arrows, set the distance 
and press the button". If I agreed with that I would not use a Leica. 
I like to make the most out of difficult light situations and for 
that, the selective measuring with the Leica M is a great help. Not 
quite the Zone System, but trying to achieve something like it.

Chris

>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
>
>
>
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Christer Almqvist
D 20255 Hamburg and / or
F 50590 Regnéville sur Mer

please look at my b+w pictures at:    http://www.almqvist.net/chris/new
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