Understanding focus drift

[skinnfell]

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There is much talk about front focus and back focus, etc.

What I dont understand is how a lens can drift over time. My favorite lens, summicron 50/IV, started out being perfectly calibrated and is now WAY off. Judgning from my files throughut the last couple of years it has gradually become more and more erroneous. I have tried it on several different cameras and it behaves the same, likewise, my other leica lenses do not have this.

(It is now on its way for a Spa in germany, and I suspect it to be an expensive one.)

 

Does anybody have a good explanation to why focus drift happens?

Are there any measures one can take to prevent or alleviate this?

 

Thanks!

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[lct]

It might be that your lens was off from the beginning but you did not notice it on film. Digital is less forgiving. Now i have a 50/2 v4 that i bought new 30+ years ago and it has no focussing problems on my digicams so far (R-D1, M8.2, M240).

[AncientCityPhoto]

This happens to all equipment with use over time. Bumps, mounting and dismounting of lens, bouncing around in camera bags, etc... Some times its more noticeable than other times. I get my rangefinder equipment checked every year. My dslrs get checked yearly too. With rangefinders, it seems its the actual rangefinder that falls a bit out of calibration more often than the lenses them selves.

[jdlaing]

I believe focus drift is more prevalent in zoom lenses and focus by wire lenses rather than fixed focal length prime mechanical lenses.

 

I too think the rangefinder mechanism is easier to drift off than the lenses.

[TomB_tx]

For lens focus to change there must develop a difference between the optical cell position and the RF cam. There are different designs for establishing the relationship, but most Leica lenses are fixed so that I wouldn't expect changes. I have had a lens group loosen and cause changes, but that affected overall performance as well as focus position.

I believe some other brands of M-mount lenses use set screws to secure the focus adjustment, and this would be more likely to loosen and shift, but I wan't aware of Leica using this method.

[Michael Geschlecht]

Hello skinfell,

 

There is another possibility, depending on what you mean by 50/IV.

 

If you mean the first version of the 6 element 50mm Summicron that replaced the 50mm Rigid: Then there is the possibility that some of the earlier production pieces might have been made with divisible lens mounts for use in making close-ups. As per the preceding 50mm Rigid model.

 

If that is the case then simply tightening the lens head to the lens mount might solve your problem.

 

Best Regards,

 

Michael

[TomB_tx]

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If you mean the first version of the 6 element 50mm Summicron that replaced the 50mm Rigid: Then there is the possibility that some of the earlier production pieces might have been made with divisible lens mounts for use in making close-ups. As per the preceding 50mm Rigid model.

 

Best Regards,

 

Michael

Michael: that is correct. The 6-element version I bought in 1969 was from the first production batch in 1968. The lens head unscrews just like the "rigid" version. That construction makes it easy to shim to correct a rear focus error, but not so simple for front focus.

I believe these were still made where the focusing cam was finish-ground to profile to match the exact assembly. Mine does front-focus a bit at f2.0, but shifts to exact focus between f2.8 and 4.0. (So does my 1962 Rigid - so I wonder if that was normal so that focus shift wasn't all in one direction from perfect.) The v4 (2012) I have is exact at f2.0.