Shooting tethered with the Leica M9          
    Freeware for PC/Windows 7          
  It started with an idea of a user. He suggested enabling M9Tether to overwrite the 'guessed f/stop' (as presented in Lightroom) with an f/stop you could set yourself.

I liked the idea so I dove into the specifics.

ExifTool shows the specific makernote field and calls it ApproximateFNumber, but the field didn't show an f/stop. Analysing photos, it seemed to contain only values between 0 and 10 in steps of 0.5. After some thinking and checking through Lightroom, I concluded the whole numbers were the F-stops engraved on the Leica lens, and the steps of 0.5 (e.g. 3.5 or 6.5) then should be the clicks in between the whole stops.

So far so good, but I was wrong.

I noticed on a value of 5 Lightroom claimed f/5.7, which didn't make sense to me, because I expected the Leica f/stop of f/5.6. 5 is a whole number. Why would Leica store values that didn't reflect their own f/stop? I had to feed the makernote a value of 4.97 to get Lightroom to report f/5.6. Then after some research I realised my mistake: The 0 to 10 values are a logaritmic table, based on the doubling or halving of light with every step of 0.5. To get to the right f/stop, you have to throw some math at it. Once I had the formula, indeed: value 5 translated to f/5.7. It turns out it's a table based on half f/stops calculated starting with f/1.0. And in that table, the 'whole' f/stop (based on the reference of f/1.0, not on the reference of the f/stop before) is f/5.7, not f/5.6.

To add to the confusion, I couldn't find a clear answer on what the intermediate f/stops actually are, especially since the table is based on the starting point of f/1.0. If in that table the whole f/stop is f/5.7, then what should be the half f/stop between f/5.6 and f/4?

In the end I decided to gather the information and just put all the values in claimed here and there and let the user decide (always the easy way out). I also added f/stops of some Leica lenses that do not fit the table, like f/2.5 on one of the 90mm lenses, and a f/3.8 I believe on the 18mm lens. Theoretically I also could have presented f/stops based on steps of 0.1 (f/1.0, f/1.1, f/1.2 etc) but decided the list would become too extensive. Although Lightroom does report those values correctly once put in.


I wasn't able to get Lightroom to show f/0.95. The makernote table starts at 0, which translates to f/1.0. Calculating the right value for f/0.95 leads to a negative number, and storing the negative number in the makernote field leads to Lightroom not reporting aperture at all. So unless a Noctilux user can actually confirm seeing f/0.95 in Lightroom, and send me the photo to analyse the makernote field, I'm assuming for now it's not possible to show (which might be simply a Lightroom limitation, because it's no problem in itself to store a negative value in the makernote field). The reason I have not included f/0.95 in the aperture list of M9Tether is because I don't know for sure if the negative number produced for an official f/0.95 is the right one (the negative number needs to be multiplied before it's written into the makernote field, and I can't test the multiplier. I'm assuming it's the same as the one for the positive values, but I might be wrong).

Another snag

Then I ran into another snag: when firing the camera through the 'Shoot' button the makernote field wasn't updated after an aperture change.

I would take a shot, change the aperture (a big change), take another photo, and find exactly the same value in the makernote field as the previous picture. The value would only change if I took a third picture. It was lagging behind one photo. All the time.

Until I tried shooting manually (still tethered, but using the camera's shutter button). Manually this problem didn't occur. It was infuriating, because I couldn't figure out why this didn't work on the PTP shoot command.

Then I suddenly realised: it's the metering system. The PTP shoot is like 'soft advance'. Metering and shooting occur at the same moment - there is no exposure lock - and most likely the light reading comes too late to be properly stored in the EXIF. I don't know if the reading actually comes too late technically or if this is a mistake on Leica's part, but some experimenting confirmed my suspicions: If I take a photo through the 'Shoot' button, change the aperture, engage the meter by pressing the shutter button to the second stop (without taking a photo), release it, take the second shot through the 'Shoot' button, then the value is updated.

Obviously I can't solve this problem. It means, when shooting tethered through the 'Shoot' button, the guessed aperture makernote field will always contain the value of the previous photo after an aperture change. This was always the case, I simply never noticed it before. Obviously you can now correct that mistake yourself by using the overwrite function.

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