Leica 90mm f2.0 APo-- 6 bit coded. not registering correct apertures on M11

[mark1958a]

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I did a search and could not find others with the same issue but of course I could have missed it.

I noticed my coded Leica 90mm f2.0 App is not registering the correct aperture. It seems a bit random... The camera recognizes the lens as 90mm f2 but not the correct aperture.  Not sure if it is the lens or the camera or something I might be doing incorrectly 

[mark1958a]

If I turn the Auto lens off.. it seems to do better in recognizing.. It registers at F1.7 wide open and goes up 2.4, 2.8 3.4 ... so it is not random like with auto lens recognition

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[adan]

There is no mechanical or electronic connection between M lenses and cameras to tell the camera which actual aperture is in use. There is only the 6-bit coding (painted dots) to tell the camera the maximum aperture possible. This is due to the legacy nature of the M system - it has to work exactly the same for 1954 lenses as it does with the newest 2023 lens (or as close as possible).

The M digital cameras GUESS the aperture in use - and have done ever since the first M8 in 2006. You are only about the 2000th customer to ask the same question over 17 years.  

The CPU guesses the aperture you had set by comparing the actual scene brightness (as detected by the little round external sensor on the front of the camera, above<left of the red dot logo 🔴) to the dimmer light coming through the lens as recorded by the normal internal exposure meter.

If the external reference sensor detects a bright scene, but the lens is projecting a darker image, the camera "guesses" that the lens must be set to approximiately f/2.8 or 3.4 or 5.6 or 11, depending on how large the difference is between the light sensors.

Usually the guess is accurate to within 1.5 stops or so - but that just depends on the field of view of the lens you're using compared to the "roughly normal" field of view of the external sensor. It can be right on - or 3 stops off, just depending on the structure of the scene (black door in the middle of a white wall, or vice-versa).

Note the guess has nothing to do with the actual exposing of the picture, only the estimate of where you set the aperture.

Edited April 20 by adan

[Jeff S]

Since your profile says “always learning,” I trust that you will enjoy and benefit from reading the FAQ, which includes the answer to this commonly asked question, as well as to many other questions you’ve yet to ask.  And some useful tips, too.

Jeff

[rustyrus]

My 90mm APO is worse than other lenses. Maybe its the focal length that throws it off but I have shot wide open before and get an F/8 registered. 

 

I stopped looking

 

 

[mark1958a]

Yes.. all thanks for the responses. I understand now. Sorry for the repeat question. I have been learning a lot about the M system and reading and googling before posting but honestly I could not find the answer to this one but likely did not look enough.. thanks again. Mark

[Gregm61]

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Try using ND filters and see what happens. I often use a two stop ND filter with my 35mm f1.4 FLE so I can shoot in outdoors during the day at f2 or 1.4 with my M240 and/or M246 and the camera will often record a “used” aperture of f8 or f11 instead of the actually used f1.4 or  f2.

Edited April 20 by Gregm61

[lct]

11 hours ago, rustyrus said:

My 90mm APO is worse than other lenses. Maybe its the focal length that throws it off but I have shot wide open before and get an F/8 registered.

Never happened to me so far. In auto iso M mode i often get f/2.8 instead of f/2. JFI.

[Leica Filmmatic]

I just ordered lens and this is good to know.  Outside of the aperture how is the performance of the lens 

[mark1958a]

I just received mine and have not had a lot of time to shoot with it. Seems super sharp.. and nice bokeh but I need to spend more time with it.