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

[Peter Klein]

Jay:  I mentioned this in the other thread that in which Howard's question
appeared, but I'll ask you here as well:

Do you know why Capture One displays my M8's estimated f-numbers, but not
my (MM) Monchrom's?

--Peter

On Thu, Feb 11, 2016 at 7:51 PM, Jay Burleson <leica at jayburleson.com> wrote:

> 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
>>
>
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