Testing IIIg rangefinder for focus point

[ironringer]

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I am not sure in what thread this technical topic should be posted, so I will place it here for now, and moderators can move it as they decide.

Could LTM technical enthusiasts comment on my procedure (see pictures) to compare the indicated focus point, and possibly identify the need for a rangefinder calibration, using my IIIg and IIIf cameras?

I have used my re-coated and CLA’d screwmount, collapsible Summicron 50mm lens, with a very good condition IIIg camera I purchased this summer. For pictures taken with wide apertures (f2 or f2.8) I see (by reviewing the scans of negatives) that the actual focus plane on the film is slightly BEHIND the focus point shown by the IIIg’s rangefinder. I conclude that there is a problem with the IIIg rangefinder, or possibly the lens:

a) either the IIIg rangefinder needs adjustment to make the actual focus plane on film correspond with the focus alignment indicated in the rangefinder,

or (less likely) the re-coated & rebuilt Summicron may have a slightly different focus plane than a “standard” Leica lens, i.e. the IIIg rangefinder shows what it expects from a properly calibrated lens, and in reality the rebuilt lens is behaving differently (and needs adjustment).

I set up a test rig as shown, focused on the centre pin, and recorded the indicated focus position on the Summicron’s distance scale. I first used the IIIg and marked the focus point with a green line on the distance scale. Then I used two IIIf’s with the Summicron, and took pictures of their focus points on the lens distance scale.

What I hope to show, is that a difference in how the three cameras’ rangefinders “process” the same lens, is related to the rangefinders, because the other physical variables (lens, target distance) stay the same. I understand there is a human variable in how consistently I adjust the focus (seeing the aligned images), but assume that repeating through several examples makes these personal errors a minor factor.

I see from my simple test that the IIIg rangefinder creates a different focus point (green line) than the IIIf (shows a focus point on the distance scale that is slightly further away). Using the second IIIf there is a similar difference, in the same direction. As you see, the lens may actually be in focus (on the film plane) at a further distance than the IIIg rangefinder indicates.

Based on this limited test, I initially conclude that the IIIg rangefinder needs calibration using the standard Leica test jigs for infinity, 10 feet and 4 feet (as described to me by a Leica technician). The rangefinder is a complex mechanism and there are adjustments for relative lever distances and angles, which in theory can calibrate it for accurate indicated focus over the full range of distances. That is an iterative procedure but it can be done, with the right knowledge and test equipment.

Please comment on the validity of my simple test, my initial conclusion, and whether I can learn more by using other LTM lenses (Elmar and Summitar) to gather more data.

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[Maarten]

I recommand first to test for infinity: turn the lens to infinity and see if a subject at more than a few hundred meters/yards gives full coincidence of the rangefinder. When not the camera needs adjustment.

I have lost a lot of time checking lenses and M8 focus accuracy using a method similar to yours. Later I learned you need to check the camera/lens combination for infinity and then for focus at 1 meter. When these two are okay the intermediate distances are okay as well.

I see a lot of people testing focus on bookshelves at 1m. The first and most important (and easy) test however is for infinity. This solves 80% of the problems.

Of course your lens might need adjustment. In that case it needs to be tested for infinity (again!) on a collimator. A job that needs to be done by an expert.

Your camera and lens look gorgeous by the way.

[gbealnz]

I love a challenge like this.

 

Perhaps also try another lens? And if you have one, yet another body.

 

This may establish a trend, lens or body at fault.

Gary