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

[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