Re: [Leica] 13% and 23%

[Henning Wulff <henningw@archiphoto.com>]

At 9:38 AM -0400 8/29/02, Mark J. Rosen wrote:
>Chris,
>
>But I wonder if even a small distance between the film plane and the white
>spot plane could actually account for that difference because we're only
>talking about a change in image size of 23x35 to 18x27, which doesn't seem
>all that huge a difference.  Still big, but it's not the same ratio as 13 to
>23% (which would be about a 75% increase in size).  It's only about a 25%
>gain in image size over that distance that would be required for all the
>numbers to work out.
>
>In the end, I don't know.  But at least it seems conceivable that the Leica
>folks worked out these numbers from sensible application of optics theory.
>I'm just trying to follow the logic.
>
>Mark

It's quite certainly a typo. If the distance of the metering circle 
from the film plane (even though it is just a few mm) were to make a 
difference by diffusing the area measured, it would cut both ways, as 
in that some rays which normally hit the 12mm center now hit the 
black curtain, and vice versa. Then, the aperture and focal length 
would have to be taken into account. A non-telephoto 135mm lens set 
at f/22 would have essentially no diffusion or variance from 
measuring spot to film plane, and a Noctilux wide open would have 
both a greater angular discrepancy between spot and film plan and 
much greater diffusion, and a 21 Elmarit  would again have greater 
angular discrepancy, so that instead of 13% it might now measure 16%.

These are the only variables, and the location of the lenses' rear 
exit pupil with respect to the distance of the measuring spot to the 
film plane the only factor that can affect the percentual coverage 
area.

The way that the sensor measures the light off the spot is a far 
greater variable than the percentage of coverage. The sensor measures 
at a steep angle, so that parts of the spot are weighted differently 
than others, and with wide angle lenses, some nearly specular 
reflections of some steep rays will have a greater effect than others.

In any case, think of the spot as being half to two thirds the frame 
height, and go and shoot. In the end, if you want precise results, 
get a good spot meter. Better yet, a spot meter and a larger format 
camera. This is the wrong area to obsess about.

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    *            Henning J. Wulff
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