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Variation in LTM adapter thickness not critical if focusing solely by rangefinder?


ragarecords

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I have a number of CV and ELW adapters, varying in effective thickness between 0.990 and 1.010mm +/- .002mm. I just received a number of Chinese adapters with the 6-bit pits milled: .098 to 1.00.

 

Focusing on the antenna tower of the World Trade Center 1 mile away, those slight variations don't appear to affect infinity focus in the rangefinder (Coincidence in viewfinder with scale at infinity). That my eyes can detect.

 

However I've been thinking about the actual practical effect  of these variations on focus on the film plane using the rangefinder.

 

It seems to me that even a gross error in thickness would not lead to focusing error IF ONE FOCUSED ONLY BY RANGEFINDER. (Obviously major error would result from scale focusing; correct infinity focus would not be at infinity on the barrel scale).

 

My reasoning is that the position of the optical unit is transmitted by the lens' cam to the camera's cam follower. The measuring system is independent of how the optical unit is held in place, it's only interested in how far away the optical unit is from the film plane. Any error in adapter thickness would be covered by the helicoid, albeit with incorrect scale readings, and inability to reach infinity with a too-thick adapter.

 

Is there a hole in my reasoning?

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The rangefinder presumes a certain backspace to the film. The adapter makes up the 1mm difference between an LTM  and M. Any variance is trouble.

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Edited by pico
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I recently measured several of my adapters and, like you, found thickness variances, although not as extreme as yours. While I haven't noticed difference in results, probably due to DOF, I am convinced that for fine work/critical detail I'd only go with the 1mm thickness adapters. Of greater concern to me, as I posted in ths forum, was that I had a Leica lens which didn't properly line up (with the focusing stop at 12:00 o'clock on my M2) with the adapters I had, and the issue was corrected once I got a genuine used Leica adapter.

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ragarecords - your reasoning is correct (mostly). A thicker or thinner adapter will put the lens out of position, but also put the RF cam out of position equally, thus compensating the rangefinder movement.

 

Otherwise, the (for example) 90 Macro would not focus correctly with its really fat macro adapter installed between the lens and camera. The position of the back of the lens is moot - the position of the back of the cam relative to the lens optics postion is everything.

 

So long as the cam tells the RF that the lens is 31mm from the film plane - regardless of whether that is due to a faulty adapter, or normal focusing movement - the RF should show the correct focus point for a lens position of 31mm from the film plane. Period

 

The "mostly" would apply to:

 

1) lenses with floating elements (not many exist in screw-mount ;) ) because the optics change with focusing, and thus the optical layout would be out of sync with the subject distance, if your lens is set to 15 feet but the focus is actually on a subject 10 feet away. You may well get fuzzy corners, or lots of CA, or just incorrect focus. I speak as a user of several 75 Summicrons where the FLE would get itself out of sync with the focusing movement - Ugly!

 

2) In many older lenses (e.g. screw-mount era - pre-1960) the cams and lenses were individually checked and hand-ground or sloped at specific distances in the factory, and thus focusing at one distance with the cam surface in position (rotationally) for a different distance may not conform with Leica's original factory calibration and tweaks. Maybe Leitz/Leica machined in a slight "bump" in the lens cam surface, for correct calibration at 10 feet - and if, with your out-of-spec adapter, the bump actually occurs at 15 feet or 7 feet, it will throw off the focus accuracy at both 10 feet (no bump where there should be) and at the other distance (a bump where there shouldn't be).

 

As an aside - the RF geometry is designed around the actual movement of the lens elements for a 50mm lens - focusing at, say, 2 meters, the lens has moved forward exactly x-many mm. For any other focal length, a cam is needed, since the glass in a 90 moves many more mm to focus down to 2 meters, and the glass in a 35/28/21 needs far less movement to focus to 2 meters. The cams in those lenses move like the back of a 50mm lens (fool the RF into thinking a 50 is mounted) to give correct rangefinding, even though the glass itself is moving substantially more (or less).

 

It follows that the newer 50s with a true focal length of 51-52mm, need cams, whereas with an old 50mm f/3.5 Elmar LTM, the back of the lens cell itself moves the RF correctly (although, again, it may have had some machining to the surface for precise calibration at various distances).

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Some ltm to M adapters may have been hand adjusted for specific lenses:
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I find with digital .001 inch makes a very unacceptable difference.  25mm=1 inch=.025 mm.   

Yes, but the difference will only affect the two end points of the range of the lens. It might not quite reach the infinity setting, or it might overshoot. Since the range finder mechanism measures the distance of the optical group from the focal plane and not the distance of the lens case, its function is not affected by thick or think adapter rings, as Andy has explained above.

 

Both sides of the adapter have to be plane and parallel to the focal plane, or the lens will sit on the camera at an angle, which is certainly not useful.

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The rangefinder presumes a certain backspace to the film. The adapter makes up the 1mm difference between an LTM  and M. Any variance is trouble.

 

 

After reading Adan's explanation, I see that my post was wrong. Wrong! Wrong! Wrong! Gosh is my face red.

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