Archived posting to the Leica Users Group, 1998/06/09
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Charlie Trentelman wrote, in part: > That "steeeper cut" [on the 40 Summicron for the CL] is really a different sort of cam -- on most M lenses turning the lens mount to focus moves a solid cam back and forth at an even rate-- accomplished by several sleeves of metal machined on threads together so focusing the lens moves the cam against the little focusing wheel at the same rate no matter which lens you use.< I've seen this statement, about different kinds of cams, before (probably on Gandy's site), but I've been a bit mystified because on "most" of my M lenses the rangefinder wheel simply rides on the back of the rotating focusing helical in the lens - as on the 40 Summicron. The only exceptions that I've seen personally are the longer lenses with removable heads (my 90/2 Summicron and 135/2.8 Elmarit have the non-rotating "cam", and I suspect the 135/4 Tele-Elmar I used to have did too.) But my 35 and 50 Summiluxes have rotating cams like the 40 Summicron, as did the 35/2 Summicron, 50/2.8 Elmar, 50 DR Summicron, and 50/3.5 LTM Elmar I've had in the past. I don't remember for sure, but I think both the fat and thin 90/2.8 Tele Elmarits that I've owned also had rotating cams. So in my experience, most lenses have the rotating type of cam. Is my experience too narrow, or do most of the other short lenses use the non-rotating cam arrangement? (I have no experience with any of the lenses shorter than 35mm.) The 40 Summicron focuses from infinity to ~2.6 feet in about 90 degrees of focusing ring rotation, while the 50 Summilux takes about 180 degrees of rotation to cover the same range. So a degree of misalignment or centering of the rangefinder wheel will make twice the difference for the 40 Summicron as for the 50 (probably not an optically exact statement, but you get the point). On the other hand, if the rangefinder wheel is properly aligned, the "steepness" of the lens cam shouldn't make any difference. (As best I can tell, the wheels on the rangefinder arms are the same size on the M6 and CL, so they should roll along the lens cams identically.) And in my own experience, my 40 focuses just fine on my M6. These are remarks of a tax lawyer, not an optical engineer, although I have been looking at Leica rangefinder cams since the late 1950s. Cheers, Kip Babington