Backfocus and Focus Shift: The Plot Thickens

[tashley]

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I went walkabout in London today with my new 28 Cron, one battery, one card and one M8 body (the older of my two) in order to give the lens its first try-out.

 

A bit of chimping and pretty soon it became clear there were focus issues, such that even during the course of the day I started to pull focus closer than indicated by the RF in order to get the degree of sharpness I expect from this lens and know it can deliver.

 

When I got home I pulled the files into Lightroom and looked at 100% and sure enough, I was right. So I started to scratch my head: am I personally damned, is every leica lens I buy shorter than 50mm going to have focus issues? And why did my CV35 give sharper performance than my Leitz lenses?

 

So I went up on the roof, slipped on a 50 Lux in order to test that my RF isn't out of whack (I allen keyed it a while back and it seemed fine before and ever so slightly better afterwards on my 'trusted lenses')

 

And I discovered the darndest thing: looking at an office block about five miles away, and a church steeple a similar distance, I found that the RF gave perfect alignment at infinity. But when I switch to the 28 Cron, it does not.

 

I'm sensing a possible clue to some odd lens performances here - but I don't know enough about the way the lenses interact with the RF to take this further without the help and advice of you, my very good friends!

 

All clues gratefully accepted... and I can't wait to try this on my other M8 body but I don't have it with me...

 

Best

 

Tim

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[charlesphoto99]

Hi Tim,

 

I have problems with my 28mm Summicron as well. It doesn't focus sharp when racked to infinity. It's better when close-in but not as sharp as I would expect with this lens. I sent it and my 50mm pre-asph lux to DAG for adjustment - he got the 50 spot on with something like a .000003 mm shim but said he couldn't find anything wrong with the focus of the 28 - maybe ever so slightly front focus but not enough to warrant an adjustment in his opinion.

 

Yes, it's frustrating, and hopefully someone (Leica?) can shed some light on this. It's to the point that I may switch to some Voigtlander lenses and sell the Leica ones and use the 10x $ elsewhere.

[tashley]

Nearly all my shots today were soft at the point I focussed on but you're right, it's better close in. When I bracketed a front focus at long distances, some of the results were stunningly crisp, as I would expect.

 

What is intriguing here is that the lens is hitting the RF arm in a different way to that in which the 50 lux does it, hence the strange difference in RF alignment at infinity. This might point to a problem with the mechanical linkage rather than the optics.

 

Intriguing, yes, irritating also. I don't want to trot out the old trope, but if a CV lens that costs multiples less can perform better, why pay more. Oh, coding. I knew there was something!

 

:-)

 

Tim

[gravastar]

............And I discovered the darndest thing: looking at an office block about five miles away, and a church steeple a similar distance, I found that the RF gave perfect alignment at infinity. But when I switch to the 28 Cron, it does not............

Tim, I'm not sure I understand that statement, could you elaborate. Are you saying the lens is hard up against it's infinity stop but the rangefinder still shows a split image of a distant object?

 

I have some CV lenses which do not produce rangefinder coincidence when the focus reference line on the barrel is set opposite the infinity symbol and the focus is against the end stop. They do however give a sharp image of a distant object when the focus is turned until you get rangefinder coincidence. The lenses will in fact focus slightly past infinity when set against the end stop.

 

With all the Leica lenses I have (except one, a Noctilux) the image of a distant object is in focus when the rangefinder is coincident and the lens is against the end stop and the infinity marker lines up with the reference line on the lens barrel.

 

Bob.

[newyorkone]

I'm noticing that the "hex" key fix may perhaps not be the fix that we all think it is. I noticed that adjusting the circular hex RF adjustment does make close to medium focus better but it throws off infinity focus on all of my lenses but most noticeably on the Noct.

 

I fear that adjusting this part may be detrimental in the long run and that there is much more work involved to getting things in alignment that we had hoped.

 

I tried to adjust things back to what the M8 was out of the box last night in hopes of fixing infinity focus and will just compensate by focusing closer for the time being. When I get some spare time and can part with the camera for a little while I'm going to personally drop off the camera and lenses in NJ and hope that Leica can sort things out for good.

[thpeters]

Hi,

 

I have wrote earlier about this in an other tread but no response from all the technical engineers here in this forum.

 

Clearly to me, and my guts feeling, there is a big problem with the focus and the problem have to be found in realizing that film is thicker then the sensor, the lenses are develop for film, not for digital.

Simple said, the distance from lens to film is different then from lens to sensor, for sure very small, but it gives big problems.

 

I bet, Leica knows this, needles to say or to write it down here.

 

See earlier treads about this and every day are coming in new problems regarding the focus.

 

I have in all these years never problems with focus with my 3 lenses and the M4, the focus was there where I want it,period.

 

Having said that, I am not an engineer to prove my statement here, but I believe 100%

I am right in this. The lens focus is a big problem for Leica and the costumers.

 

Beside that my APO 90 mm is doing great after the RF adjustment, wondering when the WATE is coming, I hope not, it start all over again

 

It is ridiculous to walk around with a 2 mm hex wrench to adjust the range finder every time, when changing lenses, right?

 

For this kind of investment it should work flaweless, it least the focus part.

 

But still I am very happy with my files from the M8 and the great colors.

Look in the photoforum in Nature 2 pictures of tulips.

 

Happy shooting for every body.

 

Theo

[ho_co]

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Tim--

Once you adjust your rangefinder follower roller, you're on your own. That's not to say you didn't have to do it, but as you're aware, it is the link to all lenses, not just the one you're trying to 'fix.'

 

Perhaps one of your lenses is out of proper adjustment and another isn't.

 

Have you made the same test on the other M8? If you haven't tweaked its rangefinder coupling it might give you a better idea of which lens might have the problem.

 

Maybe Jamie is right and it's quite simply a rangefinder adjustment issue, coupled with the idea we've discussed previously and which Theo has just succinctly repeated, viz that film and digital see differently, and our mechanism for interpretation has changed with the move to digital.

 

I know, not much help. Damn, I wish you'd get just one lens about which you could rave!

 

--HC

[ho_co]

Theo--

You're right. Period.

 

Notice that Leica brought out a new 28mm with the M8, and told Tim that it showed no focus shift.

 

Notice that Leica told Tim that the 35/1.4 definitely had the 'problem' he had noticed, and that the 35/2 has similar behavior but to a lesser degree.

 

Notice EP's suggestion that Leica lenses are exactly in focus wide open but that the focus may shift slightly when stopping down.

 

As I said in a previous thread, this design feature of Leica lenses is one which gave them their outstanding quality on film because we couldn't see as accurately on film what was actually happening.

 

I think part of the solution is for us to realize that we have greater ability to deconstruct digital images than we had with film, and to use that capability carefully and think more about the print and less about the screen.

 

And I think part of the solution may be for Leica to make gradual revisions to their designs, recognizing the importance of the flatness of the sensor.

 

As I said above, I think that is a major part of the new 28/2.8 design. I would guess that they have taken this into consideration with the WATE and are incorporating it in their future designs.

 

But I think it's also up to us not to go off the deep end looking at our images on the screen. (And no, Tim, I'm not saying you're doing that. I think most of us are. We have the ability to enlarge to 100% or larger with a click of the mouse and we do it.)

 

In this case, though, Tim has an inconsistency that indicates there's something more behind the problem.

 

--HC

[newyorkone]

The images that I have seen from the new 28 elmarit are impressive indeed. I guess the reason for that is now becoming more clear.

[ho_co]

Steve--

Please don't think I know what I'm talking about.

 

When I use the new 28 as an example, it's just that.

 

I think each new design Leica comes up with is an improvement over the preceding one. I don't think the designers sat around and said, "Hey, with a flat sensor we better redesign our lenses."

 

Leica had a number of reasons for the new lens:

 

1) Previous lens was a pretty old design.

2) Old lens worked fine, but a new design could be better.

3) New design less expensive, good package lens for M8 and people new to M system.

4) New design more compact, gives more the feel of the 'classic M.'

5) Focal length a natural for standard lens on M8.

 

And of course, on #2, you could add the phrase "especially with the M8 sensor replacing film."

 

Or you could change that to "especially if it's specifically adapted for use with the concentric lenses of the M8 sensor."

 

In other words, what I said is pure speculation; Leica is aware of the advantages and disadvantages of various lens parameters. And with each new generation, their lenses improve. A small part of that may be the difference of changing from film to sensor.

 

Because you're right, it's a great little lens!

 

--HC

[charlesphoto99]

I'm noticing that the "hex" key fix may perhaps not be the fix that we all think it is. I noticed that adjusting the circular hex RF adjustment does make close to medium focus better but it throws off infinity focus on all of my lenses but most noticeably on the Noct.

 

No, it's not and Carsten first posted here about having to also adjust the near/far washer at the end of the rangefinder arm and I followed suit and have posted several times that this is the case. One cannot just adjust the infinity roller without also adjusting the near/far! There is a fine dance between the two. Leica adjusts the near/far at one meter, and then the infinity wheel at 10 meter and again at infinity. And for me it took a combination back and forth of both.

 

My 28 summicron is unsharp racked to infinity even though the rangefinder patch is aligned. Don Goldberg says it is fine. My other lenses are fine though the 35 Lux asph backfocuses from about f4 on. Not sure if there's any perfect solutions here. It's really a matter of old world mechanical linkages meeting new age high tech - there will be interface issues.

[bradreiman]

and on the other end of the spectrum, i adjustided my cam for infinity on the noctilux and every one of my lenses focusses perfectly now. noctilux,135/2.8,cv15,cv35,cv21,summarit 50/1.5, elmar 50 etc. all of my lenses show perfect overlap at infinity as well. some lenses may be out, some rangefinders may be out, but the m8 can and will focus perfectly with all lenses when adjusted properly.....b

[carstenw]

Tim,

 

The brass focusing cam on the lens can be moved with respect to the internal focusing mechanism, which would then shift the focus for that lens only. This is what needs to be done. If one place won't do it, I wouldn't personally accept no for an answer, but take it to the next place. There is a place in Holland and another in Germany where they do that sort of work, and both have good reputations. However, I think you might want to give Leica a crack at it first.

 

I would also make a print as large as you are likely to ever want, and assess that the unsharpness you see in full-size on-screen enlargements actually translates to the real world. If a large print won't show it, there isn't much point in worrying about it. At around 240 dpi, I am not confident that we can see these differences in prints.

[wattsy]

The other possibility is that the sensor hasn't been shimmed perfectly. If the mount to sensor/film plane distance is slightly out (remember the Hexar RF 'debate'), the focus errors will be most obvious with shorter focal length lenses. This may also be the reason why Charles' 28 'cron doesn't focus properly at infinity.

[LJL]

I am not sure, folks, but I do think that there are some things that work better than others. I am sitting here playing around with three very different lenses...Zeiss 25/2.8, CV 35/1.2 Nokton and Leica 75/2 AA. All are capable of delivering very good images. Out of the three, only the Leica lens has back focus issues, and they get progressively worse at greater distances until infinity. In other words, the lens does really well on close-up shooting of say 0.7-1.5m. As I start hitting 2-4m distance, I have to start compensating a bit by focusing closer. By the time I am at 5m distance, the focus is a good 0.3m BEHIND the subject. Usually not an issue at f5.6-8 or so, but disaster at f2, which is where the 75 Cron shines for me.

 

On the flip side, the Zeiss and CV lenses are dead on every time. No front or back focus issues at all that I can see, and the CV 35/1.2 is pretty unforgiving with respect to shallow DOF at f1.2.

 

I also have a CV 50/1.5 Nokton that is dead on accurate also. Only the 75 Cron, the Leica lens, gives me back focus issues on the M8. I am awaiting my Noctilux, and at this point, am fully expecting to have back focus issues with it also.

 

The theories about film versus sensor for permitting us to "pixel peep" may not be far off, but that still does not excuse the issue that some lenses may need adjustment. Leica states that both the rangefinder and the lenses are calibrated to be accurate at a common close distance of 0.7m, and I tend to believe that. I just am not sure why some tend to go so far out on longer distances.

 

I am not sure adjusting the in-camera cam is the way to go. I have been tempted, but will not do it, fearing it will throw things off for my other lenses that now work just fine. My first impression was that maybe the new "coded" lens mounts had something different with respect to thickness, how the notch was cut, etc., but honestly that is beyond me to expalin at this point.

 

There was one explanation posted a while back about the focus shift with aperture stop down, and that seems plausible if Leica sets the focus point for the red wavelenghts when shooting wide open. The blue wavelengths tend to focus further back and that seems to be more problematic when you stop down, which then reduces part of the image circle, throwing the blue further back. In theory, I can understand that, and with respect to film with layers, that may be more applicable, but I really think we are splitting microns here.

 

I am just learning to make the mental adjustment on my Leica lenses with respect to focus. In close they work great and are sharp as tacks. Moving away, there is a tendency for more back focus, and I do not understand why. Zeiss and CV do not have this issue, why Leica? What has changed? Just my thoughts here.

 

LJ

[charlesphoto99]

The other possibility is that the sensor hasn't been shimmed perfectly. If the mount to sensor/film plane distance is slightly out (remember the Hexar RF 'debate'), the focus errors will be most obvious with shorter focal length lenses. This may also be the reason why Charles' 28 'cron doesn't focus properly at infinity.

 

Hi Ian,

 

That would be nice reasoning if it weren't for the fact that all my other lenses (21,24,35,50,90 apo) focus real sharp at infinity, even my late model 135 tele-elmar (now sold) where I would think I would notice even more discrepancy than with a wide lens (methinks you have that backwards). Anyway, I think soon the whole lot will be trundled off to Leica, M8 and lenses, for coding and repair/replacement on body for a seperate issue. I'll have evrything adjusted then. I cross my fingers that it won't set off a whole 'nother round of problems though....

[tashley]

Thanks to all who have replied so far. Rather than post twenty replies, I'll try to encapsulate responses and answers in one reply.

 

Firstly, the main point about my OP is NOT that I have proven or indeed thoroughly investigated what is giving me soft focus on the 28 cron. Regular participants in these threads know that I've had issues with 35 luxes and Crons and I have more or less tested them to death, as I will do with the 28 when I get back to my other lenses, tripods and second body.

 

The main point I was making was that my 'trusted lens' (a 50 lux that has perfect focus at all apertures and distances on both my M8 bodies) is the lens I used for the Allen key experiement. With it, a very distant object is perfectly aligned in the RF when the lens is set to infinity. However with the 28 cron, this is not the case: the distant object has a slight double when seen through the rangefinder.

 

In other words, there is some interaction between the lens and the camera that is not constant from lens to lens: something that causes the rangefinder to be correctly calibrated with one lens and not with the other.

 

I now need to see if the 28 cron is the odd one out here, by double checking all my other glass to see if any other lenses cause the RF to go out of whack.

 

So I DO have a lens to rave about - my 50 lux. My 90 F4 macro is also pretty cool, I have hyper sharp shots from it. I exclude the CV15 because it isn't coupled. The WATE has also been good, though not had enough outings to really be sure.

 

Til I get to test more fully, here's a couple of samples to be going on with. Both are f5.6, 1000 second, ISO 160. The first is RF focussed on the blue banner with the phone number on it, the second is pulled to focus a little closer.

 

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

 

 

Now come two 100% crops from my roof both at F4. First the 50 lux - and yes I know that it gets in closer and therefore shows more detail but nonetheless it is clearly in excellent sharp focus - whereas the second shot from the 28 cron is not. The 50 lux shot (first) is resized (i.e. downrezed)

 

It is worth adding that though the first set of examples show that the 28 cron CAN produce excellent sharp results, the full sized image of the shot from my roof is OOF throughout. I suspect it is in some way focussing past infinity (if that's possible!) and that this is to do with the way it is interacting with the rangefinder.

 

In any event, these shots are soft enough for it to show up in a print. I have not printed one yet but I am quite quite confident of this since it is evident at 'fit to screen' size on a 17" laptop.

 

 

 

In summary: I do not think that this is another example of 'wide asph lens showing focus shift with stopdown' because the effect appears so far to be uniform across the field, and is very similar at f2 thru f8 (I've not tested further). I DO think that some lenses coming out of Solms are not adjusted properly.

 

I will do all this properly over the weekend with tripods and second bodies and so on. What fun!

 

Tim

[tashley]

Hi HC!

 

I just wanted you to know that I'm not hyper-critical, by saying that I do have glass that makes me rave! It was precisely the experience of getting such nice shots from the 50 lux in Venice (and other lenses too) that opened my eyes to just how amazing the M8 with good glass can be when everything is adjusted correctly. So now of course, whenever I get new glass, I take it for a spin to see if it measures up. I am very demanding, I know, but it's clear that the gear can deliver when it's well made and works together properly.

 

I'm going to try all this in rigourous detail with the other body and other glass over the weekend but for now all I have with me is one body and two lenses.

 

I so want this 28 cron sorted out. Walking around with it today convinced me that it is the best focal length for a one lens outing, and I have seen amazing examples of what it can do.

 

Best

 

Tim

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[ho_co]

Tim--

I deleted my intermediate response because it seemed a bit self-righteous.

 

I think the only thing you enjoyed about the previous set of lens tests was Barcelona!

 

There's clearly a problem here with one of the lenses, but possibly also with the body. I'm just appalled that these problems keep turning up in your arsenal.

 

You're not "hyper-critical." As you said, you are rigorous and you need to be able to count on your equipment. (And I just noticed that already in your first post you said you had only the one body with you. Just getting ahead of myself again. )

 

The problem with a matter such as this is that it brushes on so many related issues as well--film vs digital, screen vs print, focus shift when stopping down, rangefinder adjustment vs lens adjustment etc--all of which have been mentioned by various ones of us in the thread. They are all valid concerns, and need to be kept in mind. But IMHO this is at a minimum a lens problem. The shots from the 28/2 are noticeably soft. When you make the same test with the other body, we'll have a much better idea whether this body is involved as well.

 

To extend your metaphor--first we became aware that the plot was thickening; now the soup seems to be thickening as well.

 

Dunno what to say except to wish you good luck and pleasant testing this weekend...

 

--HC

[californiajay]

I'm noticing that the "hex" key fix may perhaps not be the fix that we all think it is. I noticed that adjusting the circular hex RF adjustment does make close to medium focus better but it throws off infinity focus on all of my lenses but most noticeably on the Noct.

 

I fear that adjusting this part may be detrimental in the long run and that there is much more work involved to getting things in alignment that we had hoped.

 

I tried to adjust things back to what the M8 was out of the box last night in hopes of fixing infinity focus and will just compensate by focusing closer for the time being. When I get some spare time and can part with the camera for a little while I'm going to personally drop off the camera and lenses in NJ and hope that Leica can sort things out for good.

 

 

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.