[Leica] IMGS: Tests with MM and M246 - Howard

[leica at jayburleson.com (Jay Burleson)]

Top center left on the front of an M body is a small window which 
contains a metering cell that meters the environmental light, in order 
for the camera to be able to guesstimate the lens aperture. This is 
relevant for in-camera vignetting correction as well as EXIF data.
It compares the light coming to it with the value of the max aperture as 
reported by the 6-bit coding and comes up with a (sometimes spurious) 
number. For example, use of the exposure compensation will really throw 
it off. I run -2/3 almost all the time on my MM and it reports to me 
that my Summilux wide open is f/1...

Jay

On 2/11/2016 5:35 PM, Howard Ritter wrote:
> Can anyone explain how Lightroom reports an aperture for images made with 
> M cameras, which don?t have a way to report their aperture setting to the 
> camera? All the camera knows is what the lens?s maximum aperture is, as 
> reported by the 6-bit code or manually.
>
> I just ran off a series of images with a coded 24/2.8, one at each usual 
> stop from 2.8 to 16. LR reports the apertures as 2.8, 3.4, 4.8, 6.8, 9.5, 
> and 13. The aperture could be inferred from the integrated light 
> intensity, but only if the camera knows the intensity of the illumination 
> of the subject, which of course it doesn?t. I?m puzzled not only by the 
> fact that LR reports an aperture setting, which the camera has no means of 
> knowing, but even more by the fact that the values are different for each 
> exposure, increasing continuously in the right direction, and most of all 
> by the fact that the values are, as Tina says, in the ball park. And the 
> Mac?s Preview app reports an Aperture Value, which for the same sequence 
> of images also increases correctly, but ranges from 2.97 to 7.4.
>
> With a non-coded 35/1.4 and lens data entered manually, LR reports the 
> apertures as 1.4, 2, 2.8, 4, 4.8, 8, 9.5, and then drops to 4 for f/22. 
> Preview does much the same thing but less accurately, reporting a 
> progression from 0.97 to 6.5, but then dropping to 6 for f/22.
>
> With the same non-coded lens, but the lens data manually entered 
> incorrectly as 90/2, LR gave the apertures as 2, 2, 2.8, 4.8, 5.6, 8, 9.5, 
> and 4. Preview gave them as 2, 2, 2.97, 4.5, 4.96, 6, 6.49, 4.
>
> I am completely baffled. Anyone have an answer? How do the programs derive 
> a value for f/stop? Since LR and Preview report different apertures for 
> the same exposure, it can?t be just information supplied by the camera. 
> The camera knows what the maximum aperture of each lens is, but what, the 
> camera or the program, or both, decides that a lens is set to that, or to 
> anything smaller?
>
> ?howard