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How does the camera know...


David Monkhouse

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I have noticed for some time that the aperture is recorded in the information stored in a shot as well as the shutter speed. The camera is an M9P so the shutter speed I can understand. So how does it know the aperture...

It uses an external sensor to provide data to estimate the aperture used.

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I wonder why there is no estimated aperture for my Monochrom in Lightroom?
Only Adobe can provide an answer to that question.

As a matter of fact, the M Monochrom does record the estimated aperture value in each image file's metadata, just like the M9, M9-P, and M-E do. It just doesn't get displayed by Lightroom.

 

As to why that is ... no idea. Maybe it's because people at Adobe assume that a black-and-white-only camera will often be used with all kinds of filters. And filters that have a filter factor greater than one (i. e. those which are not just UV-, UV/IR-, Skylight-, or clear protection filters) will throw off the aperture estimation. So people who don't understand what's going on will complain. So I guess no aperture displayed at all is better than the display of massively wrong apertures. Only speculating though ... :cool:

 

Of course, the filter-and-estimated-aperture problem is the same with any digital Leica M ... but I suppose in the M Monochrom, it's considered worse because people will use this one with filters more often.

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The sensor gets things close though. Mine seems to usually be about a half stop above what I actually shoot at, for the most part. My guess would be that knowing about what the aperture is at capture helps determine what the camera does in terms of lens corrections, since things like vignetting are very different at f2 vs. f11.

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The sensor gets things close though. Mine seems to usually be about a half stop above what I actually shoot at, for the most part. My guess would be that knowing about what the aperture is at capture helps determine what the camera does in terms of lens corrections, since things like vignetting are very different at f2 vs. f11.

 

You're totally right on this. It's got to be for that.

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Not really. The guesstimated aperture is a bonus. The primary function is to set the camera to a primary value for flash M-TTL.

 

I don't think I'm following. I thought the TTL metering was inside the camera by the lens mount.

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My M9 isn't very good at guessing the aperture...it's off by a stop more often than not! I always wondered (still do) how it guesses and why the resulting exposure isn't off by a similar amount (that is when I use aperture priority).

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Mine isn't either.

 

I took this shot today:

 

Sarah - Group f/2

 

Relatively straightforward, +1/2 - +1 EV which is under exposed, but I was pushing it given the light. The camera thought it was f/1.2 as opposed to f/0.95 which is what it was. Seems like a standard middle-gray error. There's really no way that thing can be right, but I buy the argument that it's for lens correction at different apertures.

 

-jbl

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The camera thought it was f/1.2 as opposed to f/0.95 which is what it was. Seems like a standard middle-gray error.

No, this simply is vignetting. The geometric aperture is f/0.95 but the overall brightness behind the lens looks more like f/1.2 due to (minor) transmission losses and (substantial) vignetting.

 

Another thing that tends to throw off the aperture estimation by 0.5 - 1 f-stops is the character of the light. When the light is diffuse then the estimation is more accurate; in back-light situations the "blue eye" sees more light than the metering cell in the camera's bottom because a lens usually handles stray light better than the "blue eye" does—especially when the lens has a hood on. So in these cases the aperture will be estimated smaller than it actually is.

 

By the way, when the light is too dim for reliable light metering then the estimated aperture f/4 will be assumed by the system.

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I don't think I'm following. I thought the TTL metering was inside the camera by the lens mount.
Nope, it is a cascade of actions. The camera presets an estimated value on the flash unit by the blue dot, gives a preflash and measures off the shutter curtain, checks it - sometimes, if it is too far off from the estimated value it gives a second measuring flash based on the first- sets the definitive value on the flash and then takes the picture and flash.
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