Backfocus and Focus Shift: The Plot Thickens

[Ted99]

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I have to focus back 1,2 mm to get focus. It's the same for all my lenses.

135/elmar is useles.

I can also focus with my M6 skift the lens to M8 it's the same effect as to turn the focusingring back 1,2 mm.

My M6 is adjusted for backfocus years ago.

 

 

/Ted E

[mjh]

I have to focus back 1,2 mm to get focus. It's the same for all my lenses.

135/elmar is useles.

I can also focus with my M6 skift the lens to M8 it's the same effect as to turn the focusingring back 1,2 mm.

Then evidently your M8’s rangefinder is at fault and needs to be adjusted.

[Chuck Hatcher]

As a brand new M8 owner, I've been following the backfocus saga with keen interest. Mainly I wanted to make sure "my" M8 did not have any unexpected problems while it is still possible to return it to the dealer. I am happy to say that I am unable to find anything wrong with it (other than the characteristic IR-induced color shifts). Along with my new M8 I bought an NOS LHSA 25th anniversary (1993) set with M6 body and 35, 50, and 90 Summicrons, all to actually use rather than collect. Anyway, my M8 with these lenses passes every focus test I try, including close up at all apertures and at infinity. The focus point of the pre-ASPH 35 Summicron is well contained within the depth of field at every aperture setting.

 

I am no expert in the field of optics, but it occurs to me that what should be a relatively simple to diagnose problem is being needlessly complicated. There are various things that could be wrong with a given lens or body that can cause a focus problem, and isolating which one or more of them is at play in a given situation should be the goal. In other words, if a lens is defective, adjusting the rangefinder isn't likely to provide relief. And if the rangefinder is out of adjustment or defective, blaming the lens design is not productive.

 

I was just thinking about the components of the lenses, and it struck me that part of each lens is part of the rangefinder system. Not only that, but the part of the lens that is part of the rangefinder system is replaced when lenses are coded (by Leica, anyway). The rangefinder roller follows the profile cut into the lens mount, and rangefinder accuracy can't be any better than the accuracy of the machining of that profile. In addition, the other function of the mount, locating the lens optics a precise distance from the sensor, would also depend on the accuracy of the machining and installation of the mount. So if there is a chance lenses of recent manufacture are any more prone to focus problems than older ones, there is also a chance having an older lens coded could introduce focusing problems that weren't there before. Note that I am not suggesting there IS a problem with new lenses, only that changing the mount on an older lens, which is what coding amounts to, COULD change its focusing characteristics. I would like to know how many of the lenses with the observed backfocus and focus shift problems are new, how many are retro-coded, and how many are uncoded.

 

As far as isolating the problem to the faulty component, I would recommend first eliminating the rangefinder from consideration altogether. In other words, focus by distance measurement. The rangefinder is only a tool, and it really has nothing to do with lens performance. If the distance markings on a lens barrel do not agree with the distance at which the lens focuses, there is a problem that can't be attributed to the rangefinder. (It may not be the lens, if the camera sensor is not adjusted to the correct distance from the lens mount, but at least you can eliminate the rangefinder.) If the lens when focused by distance is correct at one aperture but out of focus at another, in other words if the focus shift is severe enough to defocus the lens when focused by the distance marks on the barrel, I would have to conclude the lens is defective. While I believe there is some observable focus shift in the design of the 35 lux, there should not be enough to lose focus at the precise point of focus at any aperture. And I'm having trouble understanding what manufacturing defect would cause one lens to exhibit more focus shift than another of the same exact design.

 

Anyway, if the lens performs acceptably when focused independent of the rangefinder, but does not focus correctly when focused using the rangefinder, there still could be a problem with the lens, in that the mount may not be properly locating the rangefinder roller.

 

I can believe that a combination of marginal rangefinder accuracy and a lens design that requires precise focusing can result in unacceptable performance. But so far the failure to identify the real culprit has resulted in a lot of unnecessary fear, uncertainty, and doubt. I would never have felt the need to go looking for problems in a brand new camera had this controversy not been brewing. I wonder if the large number of Summilux 35's on eBay right now is the result of a minor panic.

 

I continue to believe there is a definite cause and solution for these problems that does not involve lens redesign. A redesigned lens may be less sensitive to tolerances in other components, but the existing designs work very well in most cases. We just need to know why they don't in the other cases.

[ho_co]

Good analysis, Chuck! And welcome to the forum.

 

The only problem I see with your suggestion of setting the lens by measurement is that the focus scale on a lens at this price point is just a guide.

 

If I'm not mistaken, the actual focusing mount (inside, between the optics and the focus ring) will vary with actual focal length; but the engraved outside portion may be the same for all versions of a particular focal length, particularly with wide angles.

 

In the same vein, the engravings themselves are a bit gross for actual measurement, since they have no indices.

 

That isn't to say that your recommendation isn't valid. It's worth trying. (I'm pretty sure of what I said above, but someone else may know better! )

 

Your idea of polling the forum members who feel they are experiencing backfocus is excellent! Even without clear results, we'll at least eliminate a cause for concern!

 

I would say the questions should be along the line of:

 

If and only if you are experiencing back- or front-focus or focus shift with a Leica lens on an M8, please answer the following:

 

1) Which lens is giving you the problem? (full identification with Leica catalog number if possible)

2) If that model is available in both chromed brass and also anodized aluminum, which do you have?

3) What problem do you have with this lens? (backfocus at full aperture; backfocus at most apertures; focus shift when stopping down; frontfocus at full aperture; frontfocus at most apertures; obvious inability of the lens to reach infinity)

4) If you have more than one M8 body, is the problem you described in question 3 the same on both/all bodies? If not, please submit responses to question 3 for all bodies on which you have tried this lens.

5) Have you personally adjusted the rangefinder of your M8? Please answer for each body in question.

6) Has a competent technician adjusted the rangefinder of your M8 at your request? Please answer for each body in question.

7) Have you had the lens adjusted by a competent technician?

8) What is the situation of the lens in question in regard to coding? (uncoded; purchased new already coded; purchased new without coding and subsequently coded; if the latter, specify the location of the coding facility: Allendale, Milton Keynes, Solms)

 

I've probably left out something important, so before you start the poll, doublecheck my ideas in depth.

 

In regard to question #4: I think we should confine the questionnaire to the M8. Adding the question of how the lens works on film bodies (Leica, Zeiss) would just muddy the water since the answer would depend on many factors--whether the respondent is commenting on b/w or color, whether slides or prints, what size prints, how careful the observer is, how powerful her/his loupe, quality of projector/enlarging lens, conditions under which scanned, etc. I also see no need to include the R-D1 for various reasons, a primary one being that there was no great complaint about this problem when the R-D1 appeared,

 

In regard to question #8: I don't know whether the "purchased new" category is important; if it is, perhaps we should add for each case a parallel "purchased used."

 

Just submitted for your consideration, Chuck et omnia!

 

--HC

 

PS--This grew to such a great amount of data requested that I think my suggestions have become unusable. I think the questions are all necessary to get a decent picture of the problem if there is any relevance of place coded, time of coding, version of lens, consistency between bodies etc--which is, after all, the basis of the query. But I think no one could sort all the data if the questions were presented as I've suggested.

 

Maybe the better idea would be to pose the question to all forum members: "Do you like Leica?"

[Jamie Roberts]

{snipped}Your idea of polling the forum members who feel they are experiencing backfocus is excellent! Even without clear results, we'll at least eliminate a cause for concern! {snipped}

 

Except we'd have another self-selecting survey again.

 

Look--I don't doubt for a moment that people who are having focus problems have actual focus problems.

 

But they should then get the lenses and camera checked.

 

Period.

 

There is no amount of conjecture or theory that will explain all those perfectly working 35 luxes, noctis, 28 crons out there in M8-land (mine included) without--for the love of pete (whoever that is)--taking into account the lens / camera connection first. No matter how many samples you've tried out of the box.

 

Case in point: my first M8 focussed just fine; the second one needed an adjustment.

 

But that does not mean camera or lens is flawed by design.

 

 

YMMV.

[ho_co]

Good point, Jamie!

 

My reason for not doing it was practical, and you've added a theoretical one that carries more weight.

[tashley]

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Hmm,

 

I see Jamie's point - the natural self-selection would ruin the exercise - but it is increasingly clear to me that a reasonable proportion of current issue lenses are not performing, for reasons which are no doubt in some cases related to the camera and in others down to the lenses themselves.

 

I straw poll of how many people are affected WOULD be useful, not to mention interesting, but I think the key point is that the dealers and Leica themselves are seeing the problem in some detail and will no doubt have a good idea of the proportion of problem lenses.

 

Jamie, I know you think it's most often the RF that is misadjusted. You may be right. But there are certainly a good number of people with problems where this is clearly not the case!

 

Best

 

Tim

[Jamie Roberts]

{snipped}Jamie, I know you think it's most often the RF that is misadjusted. You may be right. But there are certainly a good number of people with problems where this is clearly not the case!

 

Best

 

Tim

 

Tim--absolutely--it certainly seems like some people have significant problems on the lens side here.

 

I only harp on the RF adjustment on the M8 from what I've heard from dealers about a fair number of backfocussing M8s; those back-focus consistently though, it's true.

 

But there's obviously something going on at the lens level for a lot of folks as well--or in any case in the marriage of the two.

 

FWIW, this has happened to other manufacturers too; there are countless Internet threads out there about how badly the Canon 24-70 2.8L zoom backfocusses; the solution has always been (from CPS) to send both body and lens in for adjustment.

 

So I certainly hope all this has a happy ending for everyone concerned (and particularly you, Tim--you've had some really bad luck here!), and that we all get a calibrated set of lenses that interact well with the camera! I still have my 75 lux to go in...

[AlanG]

It seems to me that some of the lenses leave the factory without being tested and adjusted for correct focus. I can't think of any other explanation. Perhaps Leica feels that its overall tolerances are good enough that most lenses will be within spec.

 

I used to use Linhof medium and large format gear that had rangefinders. Prior to the Linhof 4x5 Technika V, all models had the lenses, cams and cameras tested and matched. (The model V and later ones didn't require the lenses to be matched to the individual camera.)

 

So my 6x9 Super Technika IV had a removable cam that was matched to the 53, 100, and 180 Zeiss lenses. It was sold as a set. The cam had the camera serial number and the serial number of the three lenses engraved on it. The system came with negatives that were photographs of test targets made with each lens. This was certification that all of the lenses met Linhof's standards. The rangefinder focus was perfect with all three lenses, although I almost always focused using the groundglass.

 

I think that most Leica users didn't need this level of performance until now. (The same with Canon and that is why their new model has focus calibration.) The resolution of the digital sensors and the magnification that we typically use for viewing simply shows any inaccuracies quite clearly. Few film users would have been as critical.

 

So you either need to have Leica adjust the lens and camera (as it should have been prior to delivery) or have someone else do it. I know in Washington DC that there are camera repair shops that have adjusted Leica rangefinders and and lenses for many years. I can think of two technicians who each have more than 30 years experience doing this. (Dick and Sebouh Bagdasarian at Pro Photo.) It can't be too tough getting a 28mm lens to focus to infinity when set to infinity. And assuming the focussing mount is properly made, the rest should follow.

 

I think it was easier for Linhof to make a cam that perfectly matches a specific lens than it is for Leica to make separate focus mounts and optical components that when combined will be as accurate - unless the focus mount characteristics can somehow be fine tuned for each specimen of every lens. (I don't see how this can be done.)

 

Perhaps a central point electronic rangefinder confirmation system instead of the mechanical one will be a better way for Leica to go in the future. That's what Linhof did on the Master Technika 2000. But Linhof eventually discontinued the electronic rangefinder as most users of 4x5 cameras only wanted to use the groundglass for focusing so they didn't sell too many rangefinders. If Leica goes the electronic rangefinder route, it would simplify their design and make it much easier to get focus confirmation with supplemental viewfinders. (Or make a built in zoom viewfinder.)

[ho_co]

... unless the focus mount characteristics can somehow be fine tuned for each specimen of every lens. (I don't see how this can be done.)

Yet this is exactly what Leica did for many years, and may still be doing: It's the reason for engraving the actual focal length on the focus ring of the lens mount (the two small digits following the infinity mark on many lenses 50mm and longer).

 

They measured the actual focal length of the lens head and matched it to a mount with the same characteristics.

 

With newer production techniques there may be less likelihood of deviation from plan.

 

Still, as you said, some problem may have crept into the procedure.

 

--HC

[t024484]

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!

The size that you specify is 3 nano meter, which is in the order of magnitude of molecules. Would be absolutely impossible to produce.

Producing a shim of 3 micro meter, which is 0.003mm is already a major challenge.

For your reference, a humain hair measures between 20 and 100 micro meter, depending on age and colour. Gold leaf, used for gilding, is 0.1 micro meter thick.

[tashley]

The size that you specify is 3 nano meter, which is in the order of magnitude of molecules. Would be absolutely impossible to produce.

Producing a shim of 3 micro meter, which is 0.003mm is already a major challenge.

For your reference, a humain hair measures between 20 and 100 micro meter, depending on age and colour. Gold leaf, used for gilding, is 0.1 micro meter thick.

 

 

Lord I love this forum! No shortage of technical expertise!

 

:-)

 

 

Tim

[jaapv]

135/elmar is useles.

/Ted E

 

And not advised for use on the M8 by Leica anyway. That is the main reason they eliminated 135 frame lines: focussing difficuly.

[GMB]

 

As far as isolating the problem to the faulty component, I would recommend first eliminating the rangefinder from consideration altogether. In other words, focus by distance measurement.

 

 

Chuck, I really liked your analytical approach to the problem. I had thought about the same approach before, but then did not pursue it because I could not figure out how to do it. For example, if you want to test the focusing at 1 meter, the object should be placed at a distance of 1 meter from the camera. But from which part of the camera? Front of the lens (I don’t think so), back of the lens, sensor? May be this does not make a big difference, but for some lenses, the DOF at full aperture is quite small.

 

In this context a different question. I read about people having a focus shift, for example, with a 35 mm at 5.6. However, at this aperture the 35 has a quite large DOF, and I thought this should normally be enough to ensure that things are in focus, even with a slight focus shift.

 

 

Georg

[Chuck Hatcher]

...So you either need to have Leica adjust the lens and camera...

 

The camera needs to be in spec, and every lens should be in spec, and in my opinion you should be able to use any lens on any camera without worrying about whether or not they have been matched to each other. I'd get nervous if Leica told me I needed to have my camera adjusted to work with a particular lens, or vice versa. If the performance we seek ever reaches a point that the normal tolerances from one lens and body to the next can't produce, then the M system's capabilities have been exceeded. But we aren't there yet! These focus problems aren't "normal", and most cameras and lens combinations don't seem to have them.

 

...It can't be too tough getting a 28mm lens to focus to infinity when set to infinity. And assuming the focussing mount is properly made, the rest should follow...

 

If there is a significant number of lenses being manufactured and sold with focus problems, that might be a dangerous assumption. I don't think the lenses with focus problems have any trouble focusing at infinity, or at least I didn't get that from the threads I've read. It could be a defective cam profile machined on the lens bayonet mount that causes the rangefinder to be correct at some distances and off at others. That would require testing at different distances.

 

...For example, if you want to test the focusing at 1 meter, the object should be placed at a distance of 1 meter from the camera. But from which part of the camera? Front of the lens (I don’t think so), back of the lens, sensor?

 

I would think from the sensor itself. I can't find a "film plane" mark on my M8, but I'm pretty sure it would be the same distance from the mount face as the film on any other M camera. But I really don't know if it is practical to focus this way, since the focus markings on the lens lack resolution and may or may not be accurate.

 

...I read about people having a focus shift, for example, with a 35 mm at 5.6. However, at this aperture the 35 has a quite large DOF, and I thought this should normally be enough to ensure that things are in focus, even with a slight focus shift.

 

As I understand it, and someone please correct me if I get it wrong, the lenses with the focus problems DO have increasing DOF as the aperture gets smaller, BUT the focus shift causes the point of focus to fall outside of that range, i.e an object at the exact focus distance is not sharp at all aperture settings. Whether this is caused by a lens with greater than normal focus shift, or by initially focusing to the wrong distance (caused by camera/lens rangefinder error) remains to be seen. I believe it is normal for the lenses in question to shift focus some as the aperture is changed, but not to the extent of ever being out of focus at the precise focus distance. Possibly a marginal error in focusing distance might result in loss of sharp focus at only some apertures, or at only some parts of the frame.

[Peter Branch]

.....As I understand it, and someone please correct me if I get it wrong, the lenses with the focus problems DO have increasing DOF as the aperture gets smaller, BUT the focus shift causes the point of focus to fall outside of that range, i.e an object at the exact focus distance is not sharp at all aperture settings. Whether this is caused by a lens with greater than normal focus shift, or by initially focusing to the wrong distance (caused by camera/lens rangefinder error) remains to be seen. I believe it is normal for the lenses in question to shift focus some as the aperture is changed, but not to the extent of ever being out of focus at the precise focus distance. Possibly a marginal error in focusing distance might result in loss of sharp focus at only some apertures, or at only some parts of the frame.

 

Chuck, One of the problems that have caused confusion in this thread is the difference between "Depth of Field" and "Depth of Focus". Depth of Field gives the variation in distance from the precise object focal point, in front of the camera, that appears to be “sharp”. Depth of Focus on the other hand is the variation in the distance between the lens and the sensor that can be tolerated and still give an acceptably sharp image.

 

With short focus – wide angle – lenses the Depth of Field is greater than with longer focal length lenses for a given sensor / object distance. However the Depth of Focus gets smaller. In simplified terms a small change in the lens / sensor distance becomes a much greater proportion of this distance. Wide angle lenses are more tolerant of focusing errors but very intolerant of lens mounting errors.

 

The M8 is the first M camera with sufficiently good image resolution, combined with a near perfectly flat sensor, that allows almost anybody to explore some of these effects on their computer screen.

 

There seems also to be a misunderstanding about Focus Shift with Aperture. The Noctilux for example does not become un-sharp at f/2. It actually gets sharper than at f/1 – but nowhere near as good as the current Summilux. What happens is that the point of critical focus moves and the lens has to be moved slightly further from the sensor to gain the maximum resolution. With care this effect can be reproduced but if hand held photographs are taken of three dimensional objects at about 2m, (6 ft), then all that happens is that the focus shift effect is masked by the increased resolution, increased Depth of Field and camera shake.

 

Some of these optical effects, which have always been there, are now on the verge of being seen by users operating under typical conditions. Previously they were well known to technical people but of no consequence as it took laboratory facilities and techniques to reproduce them. This does not mean however that they can be reliably demonstrated using an M8 without great care and appropriate technique.

[Ted99]

Hello!

I have made a' test.

1. M8 focus 31 cm in front, focus at the bow, no problem to focus with the 143 Elmar.

2. Focus with M6. then skift the elmar 135 to M8.

3. M8 fucus at the strings, fucus in 31 cm back take a' look at the broken railwayhook.

 

Proper adjustment will solve this problem. But I dont want to send the M8 away for a' while..

 

 

 

/Ted

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[Chuck Hatcher]

Chuck, One of the problems that have caused confusion in this thread is the difference between "Depth of Field" and "Depth of Focus"...

 

As you can tell by my sloppy use of technical terms, I am no expert in this field.

 

... Some of these optical effects, which have always been there, are now on the verge of being seen by users operating under typical conditions.

 

To my mind, the issue is which optical effects are normal and which are not normal for a given camera and lens design. Some examples of a particular lens design seem to exhibit focus problems while others of the same type do not. I don't believe the reason some do not is that their users are unable to adequately test them. I am very much an amateur but am able to mount my camera on a tripod, aim at a test chart or ruler at an angle, and focus on a given point. It may not be an absolute measurement of anything, but it allows me to easily see the range of distances that will be rendered sharp by my camera and lens at various apertures. If by closing down the aperture I could cause the precise point I had focused upon to go from sharp to unsharp, while a point some other distance from the camera became or remained sharp, I would conclude that I had either (a) focused incorrectly, or ( my equipment had a problem. If the correct answer was (, I would want to know if everyone with that equipment had the same problem, and if they did not, why I did and more importantly how I could fix it.

[charlesphoto99]

The size that you specify is 3 nano meter, which is in the order of magnitude of molecules. Would be absolutely impossible to produce.

Producing a shim of 3 micro meter, which is 0.003mm is already a major challenge.

For your reference, a humain hair measures between 20 and 100 micro meter, depending on age and colour. Gold leaf, used for gilding, is 0.1 micro meter thick.

 

Hmmm, maybe he said inches. I'd need to find his email, but there was a lot of 0s and then a three. He said it was about the thickness of a piece of scotch tape.

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[nzav]

 

To clarify part of your statement; the focusing cam that moves the rangefinder roller inside the M8 body is not associated with the lens mount on most M lenses. The cam is a separate component of the moving lens element assembly that moves independently of the lens mount and does not move at the same rate as the lens elements themselves. Some very complicated and extremely precise machining goes into the focusing cam helix assembly.

 

Most M lenses have a removable bayonet lens mount that is passive (no interaction with focusing mechanism); besides keying the finder frames, it only fastens the lens to the body. There are exceptions such as the older 35mm Summilux that has the lens mount machined with a focus helix and thus the lens mount is part of the focusing mechanism. These lenses cannot be coded because the mount is not easily replaced. Perhaps one day, Leica will develop a jig to directly machine the code element into the existing mounts of these lenses, but it obviously becomes a much more expensive proposition than simply unscrewing a mount and replacing it.

 

So as I understand it, when Leica updates most lens mounts for 6-bit coding, it is the bayonet that is replaced; the cam does not require replacement. If the mount is incorrectly spaced with the wrong shims, then lens focus will be off by that error.

 

PS. I have one of those 35mm Summilux lenses that Leica can't code. Fortunately it has enough unique qualities to make it as valuable as the current 35mm Summilux (ASPH) to many shooters!