Ok- Check this out (from the Leica Forum a few years ago):
Hello,inspecting a sample of 35 pcs of standartized, LTM Elmars 3.5 5cm I could observe differences in the length of the lens in extracted position. Within the sample there were different flavors like: without SN and with (SN range from 73xxx till 1320xxx), upgraded and in original condition, 11 and 7 o’clock, Nickel and Chrom, coated and non coated, few red scale (however all of them with triangle). So pretty much representative sample. So I measured the length of the barrel (without screw mount flange) but with the optical mount in the front (black ring where usually the SN is stamped) and compared it with the focal length group number (stamped usually on reverse of infinity knob). Results are as follows: focal length group 0 resulted in barrel length of 29.9 till 31.7mm, group 1 28.9-31.9mm, group 5 31-31.2mm group 6 30.5-32.9, group 7 31.2-32.6. While the trend may be observed (higher group number bigger length) there is an overlap and higher focal length group number does not mean bigger focal length. I have a possibility to do further measurements and I would appreciate your comments to my questions below:- Focal length is equal the distance between film surface and the rear glass, at least for the older optics like Elmar. In infinity position. Is this correct?- Standardized camera bodies have the distance between film and lens flange of 28.8mm. Correct?- During production time Leitz used various glass, modified as well the curvature slightly. While this may have impact on the length of optical elements (distance between front and rear glass) the distance between the film and rear glass shall stay the same for the lenses belonging to the same focal length group. Correct?- The list of focal length and focal length group number is a follows: 0-50.5mm, 1-49.6mm, 3-48.6mm, 4-50.7mm, 5-51mm, 6-51,3mm, 7-51.6mm, 8-51.9mm
. Is this list correct, respectively can someone correct it? ( I believe Puts published similar list as well, but I do not have it.)I appreciate your valuable feedback. After doing further investigations I may share the results upon requestthank you jerzy
Here's the thread it was from:
Looks like this practice was confirmed by a few other sources.
Here's a sample (not mine) Incorrectly labeled as a "10" engraving- it's really a "5" partially covered up