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inconsistent focus depending on focus direction


hmzimelka
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So it took the Nokton 75mm f/1.5 to raise this issue, which kept me busy for a few days figuring out a focus inconsistency I was having with it.

Initially, I chalked it up to the narrow DOF and the lens' ridiculously short focus throw. 

Basically, what turns out to be consistently the case, is that pulling focus from infinity to my subject will result in perfect focus.
However, pulling focus from close focus to my subject will consistently result in a back focus. 

Now this has never been noticeable until the the Nokton 75mm came along. But now that I'm looking for the behaviour, I'm finding it with my Summicron and Ultron 28mm ASPH too, but with these two the difference is small. 

What bothers me is that this is 100% reproducible with my camera with any of these three lenses. If I focus on my subject from infinity, and repeat this multiple times, the focus always falls on the same spot. Taking multiple images but with each shot pulling focusing from close focus, the images are also all focused on the same spot but slightly behind the subject.

Now I cannot figure how this can be a rangefinder issue since I cannot discern any lag or slop on the rangefinder patch. It is responsive to even the slightest of lens inputs. But I don't see how its a lens issue either as there is also no slop there. I initially thought it could be the 75mm Nokton since a tiny amount of slop in the helicoid could have the focus ring move but not engage the optical block since the focus throw is so very short.

 

Now I'm sitting with a Nokton 75/1.5 thats arguably unusable. I'm not sure if my M11 is faulty, and I'm 11000km from the nearest Leica service centre so I can't just send it off to be checked. 

 

Edited by hmzimelka
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52 minutes ago, jdlaing said:

I discovered this over 40 years ago. Since that time my lenses are parked on infinity nd when I focus it is in one single motion from infinity to the focus point. It’s not a rangefinder issue. It’s a lens helicoid cause. 

Ok. I figured it would be something like that but I just wasn't confident with the assertion... but thank you so much. It does make me feel better about the problem.

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7 minutes ago, jankap said:

The LCD of your M11 is your hero. Trust him, he looks through the lens and that will be the real picture. Of course the helicoid should run smooth.😏

An EVF or LCD Live View Feed is not why I purchased a Leica M. For that I can use my infinitely more reliable Nikon D850 which has a better live view feed to boot.

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34 minutes ago, hmzimelka said:

An EVF or LCD Live View Feed is not why I purchased a Leica M. For that I can use my infinitely more reliable Nikon D850 which has a better live view feed to boot.

I wish I kept my D850 instead of trading it for a Z7...

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6 hours ago, hmzimelka said:

An EVF or LCD Live View Feed is not why I purchased a Leica M. 

There are limits to focus accuracy in a rangefinder. Lenses like 90/2 and 75/1.4 are close to those limits so better use focusing aids to get good hit rates with them. Used to be optical magnifiers i the past. Now LCDs and EVFs are recommended to get the best of those lenses at full aperture but optical magnifiers can still be used obviously.

Edited by lct
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I had an issue where the focus accuracy on my 90 APO Summicron changed based on the direction focus ring was turned. Turned to be a lens issue. Diagnosed and fixed by DAG.

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Edited by ravinj
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9 hours ago, ravinj said:

I had an issue where the focus accuracy on my 90 APO Summicron changed based on the direction focus ring was turned. Turned to be a lens issue. Diagnosed and fixed by DAG.

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Interesting. Thanks.

I suppose there may be other contributing issues in the system that could also create a similar issue. I wouldn't be surprised if a too light grease, or too little grease on the inner helicoid, would contribute more to this slop.

I did open up my Nokton using instructions found on https://yukosteel.wordpress.com/2021/06/18/disassembly-voigtlander-75mm-f1-5-nokton-vm/

It's a relatively easy lens to take apart and put together but there some catches with regard to fine tuning guides, infinity stop, etc.
Reason being my lens' infinity position on the ring didn't line up perfectly with the focus line and DOF scale. On top of that, the aperture lines didn't line up with the same line.
I've corrected this now.

I've also re-greased the helicoids with a lighter grease. The lens focuses better now but the issue still persists. I suppose at some point I'll try a slightly thicker grease on the inner most helicoid which is where the slop is likely the biggest. 

 

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2 hours ago, hmzimelka said:

Interesting. Thanks.

I suppose there may be other contributing issues in the system that could also create a similar issue. I wouldn't be surprised if a too light grease, or too little grease on the inner helicoid, would contribute more to this slop.

I did open up my Nokton using instructions found on https://yukosteel.wordpress.com/2021/06/18/disassembly-voigtlander-75mm-f1-5-nokton-vm/

It's a relatively easy lens to take apart and put together but there some catches with regard to fine tuning guides, infinity stop, etc.
Reason being my lens' infinity position on the ring didn't line up perfectly with the focus line and DOF scale. On top of that, the aperture lines didn't line up with the same line.
I've corrected this now.

I've also re-greased the helicoids with a lighter grease. The lens focuses better now but the issue still persists. I suppose at some point I'll try a slightly thicker grease on the inner most helicoid which is where the slop is likely the biggest. 

 


You are right, the amount of resistance has to be just right. I feel the focus ring on my 90 APO M is now a bit tighter than the factory settings, but it is very precise with absolutely no play which gives a high degree of confidence that it will not move away from the set focus point. And of course, there can be other factors as well like you noted.

When using lenses with extremely narrow DOF like the Hasselblad XCD 80 wide open at F1.9, even a few mm of body movement can result in oof images. And using focus and recompose with such lenses is a disaster! The reason I mention this is that it makes troubleshooting harder, unless on tripod with no focus and recompose. I know you ruled this out in your original post, but it is one issue that often trips me up :)

Edited by ravinj
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19 hours ago, ravinj said:

I had an issue where the focus accuracy on my 90 APO Summicron changed based on the direction focus ring was turned. Turned to be a lens issue. Diagnosed and fixed by DAG.

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Which lube is DAG using this days?

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1 hour ago, Photoworks said:

Which lube is DAG using this days?

Back in 2013, he fixed the sticky focus action on my 50 Summilux ASPH using his remaining supply of the old Leitz grease, after Leica NJ failed using whatever grease was then current.  Don said at the time that he had a good supply, but I haven’t checked with him since.

Jeff

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Is there any chance the camera rangefinder system can contribute to this error? There are series of linkages but I don't know much about the rangefinder mechanism to understand where this kind of error can come from. After all, only the the arm is sprung or has tension, and not necessarily the rest of the system that relies on gears?, bushes?,  etc

 

Anyways. With the Nokton 75mm f/1.5, the focus error is around 40mm at a focus distance of 2 meters.

If I set my camera on a tripod and land the focus position on the exact same mark, the the best of my ability, I get consistently accurate focus no matter the directions with little to no error. So either the RF helical is causing the issue or the camera's rangefinder mechanism.

Here are two images followed by crops of the same images. First, focus direction from infinity to subject. Second, focus from close focus to subject.

M11, with Nokton 75mm at f/1.5.

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This just feels like an overly large error and I'm concerned my camera is part of, or largely the issue... unfortunately I don't have access to another M camera.

 

 

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Well, I did another test... and I think its my camera thats causing the issue.

The following were all done on a tripod btw.

I mounted the Hektor 135mm on the M11 and noticed the rangefinder patch lagged behind a little on direction change. The lens has a huge focus throw, so the amount of focus ring movement was noticeable before the rangefinder patch moved. It had a tiny dead spot where the patch would not move but the ring would.

I then did the same test with the lens on the M3, and the lag was almost imperceptible.

 

Then, I mounted the Nokton 75mm on the M3, and focused on a thin object that made it easy and very clear when it was in focus or not via the rangefinder. 
First, I focused from infinity, and looked at the focus ring marks to note its location. Then I focused from close focus, and noted the focus distance. 

I did this with the M11 too. The M11 had a significantly larger margin of focus error based on where the focus ring position. The M3 was almost perfectly consistent based on the direction.

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