Backfocus and Focus Shift: The Plot Thickens

[Bob Ross]

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Hi Tim,

Thanks for starting this contiuing saga post. I have learned a couple of things about RF adjustments. Now, what I am seeing in your samples is that the 28cron is focusing beyond infinity and I have seen the same with manual focus lenses on my E-1. Since your 50Lux & 90macro operate fine, I think it is the lens that need some shimming adjustments. I know my 35cron focuses at infinity and I know from the untidy screws in the mount that it was adjusted before I bought it ten years ago. I don't know how much focus shift it has, but I haven't noticed it over the years. Solms told you that focus shift adjustments on the 35Lux were a compromise, but the infinity adjustment on a lens shouldn't be. Someone of this forum pointed out that infinity occurs at 1000 X focal length, or 28 meters for your 28cron. That is a whole lot closer than those buildings. A good repair tech with an optical bench should be able to confirm if it is out at infinity (which you already know) and be able to make the adjustments. I'm sitting here with five lenses that all seem to work and listening to your tale with a great deal of sympathy.

Bob

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[thpeters]

These threads are beginning to scare me....

 

I have tried my M8 with 5 different lenses, doing focus accuracy tests with all 5 lenses because of the comments I have seen here. Ever single one of these lenses (28 f2, 35 f2, 35 1.4, 50 f2 and 75 f2) have been as accurate as can be. Completely dead on.

 

What bothers me is that guys here are doing things such as their own fixes with hex keys, and using flashes designed for other camera manufacturer's cameras on this one, etc. etc.

 

Guys, be careful. You are toying with a $5000 camera. Are you sure you are doing the right things? Are you sure that just because you have been told that focus can be adjusted with a hex key that you know HOW to do so?

 

I do not mean to be overly critcal. There are some very knowledgable people that frequent this forum. I am just asking that everyone be careful and make sure you know what you are doing before you jump to conclusions and state that there must be something wrong with the camera or the lens.

 

 

Hi Jay,

 

Right to the point, clear to me, and what I believe already for many years and my own experience with leica, the lenses should be all dead on.

 

Thank you for writing this down.

 

This comment is exactly what I try to tell all of you, it is ridiculous to adjust the focus with a hex wrench, when you have to change lenses.

 

Theo

[bradreiman]

Theo-I have said this to you before. Noone is talking about adjusting the hex when changing lenses. The hex adjust is only for overall adjustment. If you have to adjust for each lens you have a bad lens, simple as that. If all of your lenses are working properly and you adjust the hex key on the fastest lens you own then you should have a complete working system. I still don't see anyone talking about adjusting when you change lenses, can you point me to this?

[ken_tanaka]

Just to add another point to the graph, I have been experiencing the same issues that Tim is reporting. It's been driving me a bit batty, too. In my case my 90mm Cron, when pushed to infinite focus, does not register as in-focus in the rangefinder. I don't know at what point things go haywire, as I've yet to experiment in a dedicated manner. My 50 Lux also seems like it may be slightly off, although not nearly as much as the 90.

 

This does seem like it's a matter of mis-calibration of the lens mount rather than a matter of the camera's rangefinder, or at least not exclusively the rangefinder.

[marknorton]

It's important to realise that lens interchangeability can only be realised if cameras and lenses are both adjusted independently to a defined standard. Using a lens to adjust the rangefinder or the rangefinder to adjust a lens is a flawed procedure because the assumption you are making is that what you are adjusting is incorrect and what you are not adjusting is correct.

 

Let's say I test an electronic circuit with a voltmeter and get an unexpected reading. I could adjust the circuit or the voltmeter calibration to get the reading I'm expecting. In practice, the voltmeter is my independent reference and if I loose it, I have no idea what the actual voltage is.

 

That's why I think adjusting the rangefinder to work with particular lens is fraught with problems and for all the work I did on "Anatomy", I didn't touch the rangefinder. Read into that what you will. Here's a picture which explains how the rangefinder works. Not the sharpest I'm afraid which is why I didn't use it, but it will help explain.

 

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First, you can see the roller whose centre is offset to its mounting on the lever arm; that lever arm is pivoted off to the left and as it moves in and out, the silver arm highlighted presses against the lever. This adjusts the angle of the rangefinder and therefore what the rangefinder is looking at.

 

[Also to note from this picture are the chamfered edge of the silver cylinder with adjustment slots which presses against the brass frame carrier and causes it to move diagonally for parallax correction; the tiny wire spring is what you hear click when the frames change; the orange circuit is the viewfinder LEDs; the gold spring is what you are working against when you move the roller in and out.]

 

You can see that there is a defined relationship between the position of the roller and the position of the rangefinder lever and therefore the distance at which the main and rangefinder images will be coincident. Rangefinder adjustments change the geometry and therefore that relationship.

 

Do-It-Yourself adjustment has focussed on adjusting the roller eccentric - moving the roller centre in relation to pivot point of the lever and it's commonly thought this adjusts just the infinity position. It does, but it also does more. As the centre of the roller is adjusted in a circle around its mounting point, three things happen:

 

1. As the centre of the roller moves in and out, the apparent infinity position does indeed change because, with the lens set to infinity, what the rangefinder is then looking at also changes.

2. The point where the roller makes effective contact with the lens cam moves from side to side.

3. The effective lever arm length - the distance from the point of contact of the lens cam to the lever pivot - changes.

 

That last point is important because it effectively sets the "gain" of the rangefinder - how much the rangefinder angle changes in total as you move from infinity to closest focus. The second adjustment is at the lever pivot itself which allows the lever in the lens throat to be adjusted relative to the silver lever and I think this will be the dominant adjustment for infinity. However, both settings will interact and require repeated adjustment.

 

That's why I didn't touch my rangefinder and I recommend you don't either. Adjusting the roller eccentric is only part of the story and if you are not adjusting it to an independent reference, all you are doing is destroying the ability to mix and match bodies and lenses.

[LJL]

Mark,

Thank you for that detailed, yet clear description of what is involved in all of this. As I mentioned in my earlier post, I just cannot bring myself to touch the rangefinder for any adjustment. Your explanation confirmed what my gut was hinting at, but my non-engineering background was unable to explain.

 

That being said, is there any way you know of, aside from sending lenses back to Leica or to a lens specialist, to make any adjustments on a lens? This would be keeping the rangefinder set as is, and trying to get the infinity focus accurate on a lens (75 Cron) that is not doing well, meaning it back focuses and get progressively worse at greater distances? My gut again is telling me that the lens mount itself may need a shim or something that would bring it closer to where is should be. This is a lens that was shipped back to Leica for coding, and came back with the back focus issue, as best I can tell. The only thing that changed was the lens mount and Leica doing its check.

 

Any thoughts would be appreciated by me, and maybe several others that are seeing these sorts of strange new problems that did not exist before.

 

LJ

[ho_co]

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Excellent description, Mark; and I'm glad to see the photo of the rangefinder linkage in the M8.

 

Again I'll point out what Rob Stevens and others have said: The rangefinder adjustment at Leica has always previously depended on a solidly floor-mounted jig with a suspended chart about three meters away which simulates proper adjustments at two different distances--close focus and infinity, I think, though I'm not sure of that.

 

On another thread a poster said that in the M8 the rangefinders are calibrated by eye on the LCD. I can only say I hope not.

 

--HC

[thpeters]

can you point me to this?

 

Bradley,

 

I dont remember exactly in what tread but it was for sure it has to do with back focusing, somebody wrote, he has his hex wrench in his photobag and used it for fine tuning.

 

And I stay with my statement that is ridiculous.

 

This is Mark Norton comments,

 

That's why I didn't touch my rangefinder and I recommend you don't either. Adjusting the roller eccentric is only part of the story and if you are not adjusting it to an independent reference, all you are doing is destroying the ability to mix and match bodies and lenses.

 

 

Theo

[charlesphoto99]

Mark, thank you for that. I agree that it is more complicated than just the "hex key" maneuver and shouldn't be played with lightly. That said, I did, after several hours of careful back and forth with both the near/far and infinity adjustments, get 5 of my seven lenses to focus properly from close to distance. The 50 pre asph lux DAG was able to shim for backfocusing (with a shim the width of scotch tape!); the 28 Summicron he said he couldn't find a problem but it exhibits the same behavior as Tim's sans the out of alignment rangefinder patch at infinity. I think it will have to go to Leica - it may be a mount issue or something (?!!!) that Don Goldberg couldn't find.

 

I think this type of self adjustment of the rangefinder would of course have been imposible pre-digital. But with preview it can be done, but one must go all the way, and be aware that without the right balance between the two adjustments focus in the middle can be thrown off even though it appears to be fine at both closest and infinity. It's a very delicate balancing act and I wouldn't recommend. I used my 90 apo as a base lens and then checked against the 35 Lux asph and then the 24.

[ho_co]

LJ--

Pardon my intrusion; Mark will have his own opinion. My feeling is exactly the same as what we've already seen: These things are too complex to be handled by people who don't know exactly what they are doing.

 

My suggestion is to send the lens to Leica and only to Leica. Others would say to send it to a recognized competent lens repair technician, and I won't argue with that so long as the repair agency is familiar with Leica's designs.

 

But in the lenses you've got just as tight tolerances as you have in the rangefinder mechanism (they have to match or they won't work), plus the understanding of how the particular design is supposed to work.--See for example the fact that Leica told Tim that an occurrence of focus shift was to be expected on a particular lens; see for example that another forum member said he asked Zeiss to adjust their C-Sonnar to work more like Leica lenses, i.e. focus properly wide open and let the focus shift carry the lens out of focus instead of vice versa as the lens had been designed.

 

It's easy enough for a lens technician to adjust a lens to work the way you want it to, but that may not be the way it's supposed to and may introduce other problems. In my opinion, you need to understand why the lens behaves as it does before you try to 'fix' it.

 

Respectfully,

--HC

[charlesphoto99]

 

That being said, is there any way you know of, aside from sending lenses back to Leica or to a lens specialist, to make any adjustments on a lens? This would be keeping the rangefinder set as is, and trying to get the infinity focus accurate on a lens (75 Cron) that is not doing well, meaning it back focuses and get progressively worse at greater distances? My gut again is telling me that the lens mount itself may need a shim or something that would bring it closer to where is should be. This is a lens that was shipped back to Leica for coding, and came back with the back focus issue, as best I can tell. The only thing that changed was the lens mount and Leica doing its check.

 

 

LJ

 

I would self-adjust my rangefinder WAY before touching a lens! The shim that Don Goldberg inserted in the rear element of my 50 Lux was something like .000003mm thick and it changed the focus by about an inch! With lenses you need much more specialised tools than a hex key and jewelers screwdriver. Do you have any other lenses that focus properly throughout the range and at all f stops? If so, then it is the 75 that is the problem and should be sent back to Leica.

[ho_co]

I think this type of self adjustment of the rangefinder would of course have been imposible pre-digital.

Charles, unfortunately that is not the case. There have always been people who thought they new more than they did and acted on their beliefs. I won't go into specifics, but I have met people who were completely out of their league in trying to adjust one thing or another in cameras and never realized it. They've been around longer than either you or I.

 

Maybe the reason we're hearing about it more now is the internet and forums like this one; maybe it's the fact that with digital we get instant feedback, as you said. But the modern age has always been with us to one degree or another, and being who we are, we always push our luck.

 

--HC

 

PS--I think we're all impressed by Mark's dissembly of his M8, but in another way I'm equally impressed by the description Bob Parsons gave of disassembling a Leica filter http://www.leica-camera-user.com/digital-forum/19863-need-help-wate-4.html#post233836. I wouldn't have had any idea how to go about doing that without his guidance. I know I would not be successful at disassembling an M8, let alone getting it back together again. But I think I can handle a Leica filter with Bob's help. Then again, I may find out that even that is beyond me. But first I need to get my Leica filters.

[DaveSee]

It's important to realise that lens interchangeability can only be realised if cameras and lenses are both adjusted independently to a defined standard. Using a lens to adjust the rangefinder or the rangefinder to adjust a lens is a flawed procedure because the assumption you are making is that what you are adjusting is incorrect and what you are not adjusting is correct.

[snipped]

That's why I didn't touch my rangefinder and I recommend you don't either. Adjusting the roller eccentric is only part of the story and if you are not adjusting it to an independent reference, all you are doing is destroying the ability to mix and match bodies and lenses.

Thank you Mark! While the picture alone leaves me with fewer than a thousand words, your hundred or so to explain your point is very much appreciated! And to find in this post regarding the complexity of the M8 system that there is no mention of cyan, magenta nor 8 bit data with linearization table applied, one must pause to consider the team of engineers who contributed to this system... and of course to you all who pull as much possible from it; hence Tim's conundrum of imaging (what?,) confidence?

 

Ever in awe with light captured frail+rgds,

Dave

[LJL]

Howard and Charles,

Thanks for your comments. I had been thinking this to be the case, but had to ask as sometimes there are simple solutions. My suspicion is that the lens needs something like the shim you mentioned, Charles, but knowing how thick, etc., is the gamble. As it stands now, only that lens appears to have a back focus issue, so it will probalby be making a trip back to Leica soon. Just hate being without yet another piece of gear for any length of time at this point. So far I am managing with my own internal correction method, but it needs to work every time, as it should. Problem is that when it came back from Leica with the new coded lens mount, it was proclaimed fine. Obviously that is not the case, so I guess I need to systematically take some carefully constructed shots to "prove my case" and then ship it back.

 

Thanks again for your thoughts and support. Lest you think I am a crazed tinkerer, I am not, but I feel it is important to be able to do some field repairs as needed, as I do not always have the time luxury of sending stuff off for repair and such....especially Leica, where things just take so much longer.

 

LJ

[marknorton]

LJ, lenses are especially tough to work on and there are certainly no "user adjustments". The mounts are made complex because as you focus, the lens barrel has to move through the required range AND the lens cam has to move through its required range as well. Add floating elements such as the 50/1.4 and 75/2 into the mix and it gets especially complex. Looking at the 75/2, there's a separate helicoid at the back for the lens cam and it looks like this needs to be rotated slightly relative to the lens body, definitely not something to attempt yourself.

 

I've been thinking about my comment that as you adjust the rangefinder roller, the lateral position of the roller and point of contact with the lens cam changes. Normally, that wouldn't make a difference because the lens cams are a slightly profiled brass ring which moves backwards and forwards as the lens is focussed (for example, the 21/2.8, 28/2, 50/1).

 

Not so the 50/1.4 ASPH which has the most complex focussing mount of any M lens. It also has a very steeply profiled lens cam so that moving the contact point even a fraction of a mm from side to side will have a material impact on the rangefinder adjustment. In other words, if you adjust the roller to work with a 50/1.4 ASPH which is itself out of adjustment, it (the rangefinder) is much more likely to have problems with other lenses.

[LJL]

Mark,

Thanks for your comments. As I mentioned to Howard and Charles, it pays to ask about some things, as there may be user fixes. I fully appreciate the complexities of the lens mechanics you mention, and I suspect the solution is there, but not for me to find on my own.

 

All this being said, I still am quite impressed with some of the things that are coming out for DSLRs, such as the Canons that I shoot, where the user does have the ability to adjust focus planes of the lens. This is possible because of electronics and the AF mechanisms, which do not exist on the RF lenses. Canon sort of had to come up with something like this as there are QC issues with lens build, and a host of other things, and the sheer volume of lenses being sent in for what are very minor, yet important adjustments was swamping the repair techs. This is really the same sort of problem, but there is no simple user correction that can be made, and hence folks tinkering with the rangefinder cams, not realizing all the ramifications.

 

Thanks again for sharing your insights and interests in all of this. Great stuff.

 

LJ

[russell]

Send it to Leica. Stay away from DYI hex wrench solutions. Run focus tests when you get the body+lenses back. If still off then you follow up step by step with Leica and not DIY. Sorry, but there are too many varibles involved. And what really stood out at me was the hex wrench statement -- I bet that'll be at the root of the problem.

[ho_co]

Lest you think I am a crazed tinkerer, I am not, but I feel it is important to be able to do some field repairs as needed, as I do not always have the time luxury of sending stuff off for repair and such....especially Leica, where things just take so much longer.

LJ--Understood. If you were a 'crazed tinkerer,' you wouldn't have asked.

 

When I worked for Leica some thirty years ago, I was in a store when someone brought in a Leicaflex for repair. The manager called a local repairman, who quoted a price over the phone--sort of a generic 'oh, yeah, shutter adjustment? No problem!' response.

 

I asked the manager why he didn't send the camera to Leica, and he said the company took too long and its prices were too high.

 

The repairman picked up the camera and twenty minutes later called back. I overheard the conversatiion: "I'm going to have to raise the price I quoted you. I've never seen a camera this complicated, and it's going to take more time than I thought."

 

In other words, the repairman didn't know the camera and raised his price because of his lack of knowledge. By some quirk I was also there when the repairman returned the camera a couple weeks later. It was obvious that he didn't know what he was doing because he had not secured the baseplate properly. (The Leicaflex models had a trick to securing the baseplate not only at the ends but also at the tripod socket, and he had missed that fitting.)

 

So, in trying to save his customer time and money, the manager had sent the camera to an agency of good will but without training; had charged the customer for the not well-executed work; had virtually guaranteed that the camera would in the end have to go to Leica for repair; and had likely made that repair more expensive for its having been done incorrectly in the first place.

 

True story, I'm sorry to say.

 

And I mean no disrespect to you or to anyone else who needs to repair the camera in the field. But having seen jobs done half-way a number of times by people who were trying to help out, my advice is: If you do it yourself, send it to Leica for a checkup as soon as possible afterward.

 

--HC

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[tashley]

These threads are beginning to scare me....

 

I have tried my M8 with 5 different lenses, doing focus accuracy tests with all 5 lenses because of the comments I have seen here. Ever single one of these lenses (28 f2, 35 f2, 35 1.4, 50 f2 and 75 f2) have been as accurate as can be. Completely dead on.

 

What bothers me is that guys here are doing things such as their own fixes with hex keys, and using flashes designed for other camera manufacturer's cameras on this one, etc. etc.

 

Guys, be careful. You are toying with a $5000 camera. Are you sure you are doing the right things? Are you sure that just because you have been told that focus can be adjusted with a hex key that you know HOW to do so?

 

I do not mean to be overly critcal. There are some very knowledgable people that frequent this forum. I am just asking that everyone be careful and make sure you know what you are doing before you jump to conclusions and state that there must be something wrong with the camera or the lens.

 

You may be right, and contrary to popular practice I wouldn't advise people lightly to try it if only because a fair number of us have ended up wishing on balance that we hadn't, though a goodly number have found it easy and effective. But this begs the question as to WHY people are trying it. Having paid their $5,000, many are more inclined to risk the Hex Wrench than send their camera off to Solms for what may be several weeks.

 

I think it is beyond doubt that a certain number of M8s are leaving the factory with their rangefinders out of whack. There have been too many reports of it to be a fireless smoke. There is also a growing awareness that people are having focus issues. And if you've just bolted a $4,000 lens onto a $5,000 body and all you get is a foggy day, the wrench looks mighty tempting!

 

I think it runs deeper also: the M8 produces astonishing images and you won't find me using any other camera for now at least, but however many races it may win it is a skittish colt. To take a great wide angled shot with it, for example, I might have to use a specially adapted filter holder/lens hood purchased from a member of this forum, with a Leica only IR filter on a proprietorially coded lens, using an additional finder on top of the camera which does not display exposure or focus information and which prohibits the use of a flash gun simultaneously. In other words, the whole art of M8 photography is about adapting, tweaking, bolting on, homing in on best performance. It is the camera that makes us experimental in nature, and the long turn around times for adjustments that make us experimental in practice.

 

Other than the odd minor irritation and inconvenience, I don't really mind any of the above. It's part of the cost of ownership, and the benefit is amazing results. But it's clear that we have all been invited into the lab here...

 

Best

 

Tim

[lars_bergquist]

Tim, that second 100% crop (with the 28) is done at a subject magnification which is 50/28 = c. 1.8 x larger than the first one (with the 50). This is not a fair comparison. It would get you thrown out of any decent physics department. Do compare apples with apples. It would be a miracle if the first crop didn't look sharper than the second!

 

The old man from the Age of the Wet Print