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I found this interesting also. In support of this suspicion I once had a VC 35/1.2 ASPH and M 35/1.4 ASPH ... and they both metered exactly the same on a step-less M7 set to A. My friend Irakly theorized that the Leica glass was simply clearer, or a bit better at light transmission.The approximate aperture is now provided (calculated) in LR3 Beta. I have found it very accurate and interesting that it may reflect the true working aperture rather than the stated aperture. The Leica M glass reads fast especially wide open (e.g f1.0 with a 35 summilux in a dark situation)...but normal around F5.6. I am not saying the 35L is really 1.0 but rather other 1.4 lens may actually be a little slower. In any event close enough for my uses and a nice addition.
You have to be using version 5.6 of ACR or you won't get that data. With LR, you have to be using 2.6 (which uses ACR 5.6) or LR3 Beta. Alternatively, if you don't have Photoshop CS4 or the latest version of Lightroom, you can use CornerFix to read the Leica value or estimate the f-stop on its own, and then write it out to the files. I used to use CF to do this on all files before ACR 5.6.I use CS4 and ACR and while the exif data reads the maximum aperature, I have yet to see an actual exposure aperature. It may be time to check for updates or to search through all the exif data categories.
And this occurs with coded lenses on "auto detect" too - no aperature.