Lightroom aperture readings on M10 w/ Summicron

[Chris O]

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Hi everyone,

I apologize if this has been asked before; I searched the forum but couldn't find any previous topics like this.

Recently, I bought an M10. I notice that in Lightroom my 50MM Summicron, which has a maximum aperture of 2.0, has a reported aperture of sometimes as low as 1.2. An example of this oddity is as follows:

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Is this normal behavior? Any thoughts? 

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[Michael Geschlecht]

Hello Chris,

Welcome to the Forum.

What you are reading as F 1.2 is 1:2 which is another way of writing: This is the relationship of the physical diameter of the lens to the physical length of the lens. The lens length is measured from the second nodal point of the lens to the film plane. One divided by two.

 Which can also be written as F 2

Best Regards,

Michael

 

Edited February 24, 2021 by Michael Geschlecht

[Chris O]

Hi @Michael Geschlecht

Thank you for the reply. That's what I thought at first. Take a look at the picture above, which is listed as F1.7. This was shot wide open at F2.0, not F7.0.

[lct]

There is no electrical nor mechanical communication between the body and the aperture rings of M lenses so what you see is just a guess. Normal Leica oddity so to speak. See: 

 

 

[Michael Geschlecht]

Hello Chris,

I am not sure since I do film photography. You will have to wait for someone who does digital to come along. Don't worry, it won't take long.

Best Regards,

Michael

[Chris O]

Thanks @Michael Geschlecht.

@lct, that's kind of what I thought but wanted to make sure there was nothing wrong with the camera. 

[hteasley]

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Camera doesn't know what the lens aperture is: there's no communication to the camera as you turn the aperture ring. So it guesses, based on the histogram of the image.

Gets it wrong almost every time.

[Jeff S]

14 minutes ago, hteasley said:

So it guesses, based on the histogram of the image.

 

Not exactly. As Leica states: “The camera compares the brightness measured through the lens with the measured ambient brightness to estimate the aperture used. This value is then written to the EXIF data. In some cases, the interpretation of the subject or scene may lead to incorrect estimation of the aperture value, for instance when shooting subjects or scenes with high contrast and black or white objects.”

The ambient light is measured by a sensor, which is incorporated in the little round window on the front of the camera.  I believe it’s also used for frame line brightness adjustment.

As a side note, Leica initially deleted this aperture ‘guess’ in EXIF for the M10, but reintroduced it via a FW update due to customer feedback.

Jeff

[jaapv]

The sensor compares ambient light with EV value to determine the aperture, as Jeff says. It is also used to determine the intensity of the pre-flash in  the M-TTL flash system.