M9 Detecting Aperture Automatically?

[rulnacco]

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Hi. I must confess, I'm quite mystified by something, and I haven't been able to find an answer in the wilds of the Internet, so I'm hoping someone here may know what's going on.

 

My local dealer has a second-hand M9. I won't be able to afford it, but I am of course quite interested in the camera. And he has graciously let me try some of my lenses that I employ on my film Ms on the body.

 

Here's what I find weird. I've tested my uncoded V4 50 Summicron and likewise non 6-bitted V2 35 Summicron on the body. When I open the DNG files in Adobe Camera RAW, it of course can't tell me what lens I was using, but it does tell me which aperture I shot the frame at. It does this whether I use the 35 or the 50, and it doesn't matter whether I'm shooting inside the shop, where there's relatively little light, or out on the pavement in front in bright daylight. The indicated aperture in ACR is occasionally off by a fraction of a stop, but it's generally very close indeed to what I had the lens set at, and usually right on the dot.

 

With a Nikon, this would of course be no mystery--there is a mechanical linkage between the aperture ring on the lens and the body, and so the camera knows which aperture I shot at. As there is no physical link whatsoever between the aperture ring on an M lens and the camera body, and furthermore I have not told the camera what lens I am shooting with, how does the M9 do such a good job of determining which aperture I used?

 

Is the camera psychic, or how does the M9 calculate which aperture you've used despite not even knowing which lens you've got mounted on the front of it, nor what the overall light level of the scene that you're pointing it is like? I'm flummoxed, would appreciate someone enlightening me!

[pico]

The M9 knows the chosen ISO, shutter speed, and light value. That is all it needs to approximate the F-Stop.

[rulnacco]

That occurred to me, but if I'm at 1/125 at 400 ISO--both of which the camera does know--how does it determine that I'm shooting at F2 inside the shop vs. F8 in mild overcast outside, when both settings would mean an equivalent amount of light was hitting the sensor? So I may be wrong--correct me if I am--but I don't think you've perhaps fully answered the question.

 

Although, does the little sensor just above the red dot measure the absolute ambient light level, in which case the camera does have a way to compare the amount of light hitting the sensor with the overall light level outside the camera? In that case, knowing the ISO and shutter speed probably would allow the M9 to approximate the aperture.

 

However it does work--and I may have just puzzled out my own complete answer to the question--it does a remarkably (and usefully) accurate job.

[pico]

Although, does the little sensor just above the red dot measure the absolute ambient light level, in which case the camera does have a way to compare the amount of light hitting the sensor with the overall light level outside the camera?.

 

Yes.

[rulnacco]

Cheers!

[250swb]

However it does work--and I may have just puzzled out my own complete answer to the question--it does a remarkably (and usefully) accurate job.

 

Ha, ha, wait till you put something like an ND filter on the lens and see what it guesses.

 

Steve

[amoebahydra]

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There is a round window above the Red-dot logo of M9, this window is actually an external light meter. When the reading of the external meter is compared with that of the internal meter, the camera can have enought information to approximate the aperture used.

[rulnacco]

Lol, Steve! Well, bless its little heart, it's at least gonna *try* its best, despite such an evil trick being played on it.

[Chuck Albertson]

There is a round window above the Red-dot logo of M9, this window is actually an external light meter. When the reading of the external meter is compared with that of the internal meter, the camera can have enought information to approximate the aperture used.

 

It *is* an approximation. I took a Noctilux to the ballpark Friday night and shot it wide open with the MM. When I imported the pictures into Lightroom, the EXIM data had the aperture for every picture well north of f/1.0, in a couple of cases up to 2.0.

[pico]

It *is* an approximation. I took a Noctilux to the ballpark Friday night and shot it wide open with the MM. When I imported the pictures into Lightroom, the EXIM data had the aperture for every picture well north of f/1.0, in a couple of cases up to 2.0.

 

There is simply no allowance in the Exif standard for an aperture wider than 1 (really 1.4142 [...irrational]). Leica can fudge to F/1 by reading lens coding or manual setting of lens, but not through the arithmetic..

.

[mjh]

Indeed the Exif standard specifies both the Aperture Value and the Maximum Aperture Value to be stored as logarithmic APEX values in unsigned rational format, i.e. as a fraction of two unsigned longs. So the smallest value that can be stored is 0 which corresponds to f/1.0. f/0.95 would require a negative value which, alas, Exif does not permit.