jerzy Posted July 31, 2013 Share #1 Posted July 31, 2013 Advertisement (gone after registration) 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 Quote Link to post Share on other sites More sharing options...
Advertisement Posted July 31, 2013 Posted July 31, 2013 Hi jerzy, Take a look here Elmar 3.5 5cm and focal length. I'm sure you'll find what you were looking for!
giordano Posted July 31, 2013 Share #2 Posted July 31, 2013 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?- No. Standardized camera bodies have the distance between film and lens flange of 28.8mm. Correct? Yes 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?- Not necessarily. Sent from another device.. Quote Link to post Share on other sites More sharing options...
jerzy Posted July 31, 2013 Author Share #3 Posted July 31, 2013 thx John. My first assumption was based on school knowledge, as I found in the meantime (with help of another Forum member) is that it is valid for single element optic and thin lense only. For multi element optics there is no simple relation between focal length and distance to the film plane. Therefore my other assumption was wrong as well. Still, it would be great if someone could verify the list of focal length and group numbers.thank youjerzy Quote Link to post Share on other sites More sharing options...
Orient XI Posted August 3, 2013 Share #4 Posted August 3, 2013 As noted in earlier posts 50mm f/3.5 Elmars had a code number stamped upon the mount to denote the actual focal length. Other lenses, particularly up to about 1975-80, had a small number stamped at the end of the focussing scale. This was given in tenths of a millimetre; thus my 1963 vintage 90mm f/4 Elmar carries the number 10 which means its actual focal length was 91.0mm. If it had been stamped "95" the true focal length would have been 89.5mm. Quote Link to post Share on other sites More sharing options...
jankap Posted October 17, 2013 Share #5 Posted October 17, 2013 (edited) 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. 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. thank you jerzy jerzy I have one with the number 1201xxx. The group is 7 and I measured 32.4 mm. This corresponds with your measurements. Mines has a red distance scale with triangle. The best point for the diaphragm is where the focal length could be measured. There the picture reverses. But we cannot reach this plane from the outside. There is another point. A different focal length means a different distance scale. I assume the helicoid has been of the same properties. What did you find in this respect? Leica manufactures in batches. The lenses of one batch will have more or less the same focal lengths. One batch was combined with a corresponding mount 0 to 7. Jan Edited October 17, 2013 by jan_kappetijn Quote Link to post Share on other sites More sharing options...
Michael Geschlecht Posted October 18, 2013 Share #6 Posted October 18, 2013 Hello Jerzy, The focal length of a lens is the measured distance from the "second nodal point" to the film plane. The position of that second nodal point is dependent on the type of lens & a number of other parameters. Sometimes the second nodal point is in the middle of the lens front to back. Sometimes it is in front of the lens & sometimes it is somewhere else dependent on lens design. Some of the Leica Manuals, Such as Gunther Osterloh's in the 1990's show the position of some of these second nodal points in some lenses. Best Regards, Michael Quote Link to post Share on other sites More sharing options...
jerzy Posted October 18, 2013 Author Share #7 Posted October 18, 2013 Advertisement (gone after registration) Thx Jan and Michael. Yes, my assumptions that there is a direct relation between focal length and length of the lens (barrel) were wrong. Another forum member, on German thread (str) explained it to me. best regards jerzy Quote Link to post Share on other sites More sharing options...
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