I am asked questions from customers from time to time, that I wish I had these articles on hand when being asked as it explains things so well and clearly. These are pieces I have taken from Leica related literature over the years. I have not written these articles myself. They are from publications such as the LHS Leica Historical society, or the Leica Newsletter magazine and other publications alike. I hope they shed light on certain interesting subjects. Enjoy the read.
Stemars - Additional Thoughts
The footnote inserted by our editor to the Stemar
note in LHS Newsletter 40 was interesting. It was(quoting Lager's Leica Illustrated Guide Volume III P 11):
All (bayonet mount Stemars) researched by the author are equipped with a screw to bayonet adapter held in place by a tiny removable screw. Catalogued and marketed as a bayonet mount lens the OIASO (11510) becomes a screw mount OISBO (11500) when the adapter is removed.
This of course is true, but only up to a point. As a parallel case take the 35 mm f/2.0 Summicrons SAWOM and SAWOO of 1958–63. SAWOM is of course the M2 bayonet version of this lens, and SAWOO is the screw mount version, made in small numbers only. Certain early SAWOM lenses have an adapter ring that can be removed. This adapter is dissimilar to the standard type having a counterbore that encloses the body of the lens. The locking screw is located in the wall of the counterbore, and removal of the adapter reveals a small-diameter plain blind hole in the lens body. A SAWOM lens with the adapter removed certainly functions as a SAWOO but is not generally accepted as such. As an example 35 mm f/2.0 Summicron (Wetzlar) 1686549 is shown, with adapter removed.
Similarly it is obvious that an OIASO with the adapter removed will function as an OISBO. Whether it is a true OISBO depends on whether any Stemar sets were originally made as screw mount OISBO units. It is the contention of this note and the previous note that such is indeed the case.
With Stemar sets it is not possible to produce an adapter of the form evolved for the Summicron, since the locking pin for the focus lever would prevent such an adapter from being screwed into place. The adapter would have to be of form closely akin to the standard adapter and, as the editor remarks, Lager is reported to illustrate the locking screw in the side of the adapter. This must be positioned radially through the wall of the adapter into the screw thread region of the Stemar mount. Two engineering problems become apparent: the effective wall thickness of the adapter is only 0.7 mm and this is barely adequate to accept a threaded portion alone; the effective wall thickness of the Stemar screw mount section is 1.5 mm excluding screw threads. This portion is threaded internally for the focusing movement as well as externally for the mount. Thus great care would be necessary in drilling and tapping the hole for the locking screw to avoid fouling the focusing threads. Because of the limited wall thickness it is likely that Leitz would drill and tap the assembled Stemar and adapter, rather than have a plain hole in one and a tapped hole in the other. A most unlikely option is a threaded adapter with no drilling on the Stemar mount, i.e., a locking screw driven into the Stemar thread with con damage. It may just problems with screw bayonet ad: caused alternative arrangements to be made for the 35 mm Summicron SAWOI (non-standard ; and the 90 mm Summicron (standard with thread locking compound]
The thrust of he argument is that any which shows no locking screw holes must be a true OISBO any conversions fro to OISBO would leave The writer contends that on the evidence of Stemar set 1124280/1386 Stemars were con as true screw mount OISBO versions without modification from other forms.
Lens unit 112 illustrated to show appropriate areas.
Top: SAWOM with its adapter removed, showing the threaded hole in the adapter and the blind hole in the lens body.
Bottom: top view of special adapter of SAWOM.
Top: OISBO 1124280 from 12 o'clock position.
Middle: OISBO 1124280 from 4 o'clock position.
Bottom: OISBO 1124280 from 8 o'clock position.
Photographs by the author, Leicaflex SL + 100 mm f/4 Macro-Elmar
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