Something I've learned living with the PL100-400 for 3 years (or so)

[Lindsay D]

Hmm, thoughts on what might be up with the 25mm and 42.5-45mm data relative to cine and your measurements? Roger's also published some wide angle and standard observations from 2012.

I didn't say he was wrong, it just isn't what I've experienced. Maybe the lenses he's been hiring out or testing have been from certain batches where there was an issue on the production line (it does happen, common with Canon kit lenses). There were apparently also some quality issues after the tsunami. It may also be lens specific too.

 

[archaeopteryx]

It may also be lens specific too.

That's rather what Roger indicates. My reading of his descriptions is

• normal variation: Olympus 45 1.2, Olympus 45 1.8, Voigtländer 45 0.95
• above normal variation: Olympus 25 1.8
• high variation: Olympus 25 1.2, Panasonic-Leica 25 1.2, Panasonic-Leica 42.5 1.2, Panasonic 42.5 1.7, Panasonic-Leica 45 2.8

However, while Roger mentions n < 10 because there isn't much m43 rental demand, he doesn't say what n is anywhere I've noticed. The implication, though, is it wouldn't take a particularly large additional n to add skill to the estimates. Hence the curiosity. Especially as the two substantial samples I'm aware of (yours and Roger's) appear to have opposite indications regarding Panasonic variability.

I've n = 1 for all my lenses and the only one of the above which I have is a PL 45 2.8, so unfortunately can't make much of an assessment from my own data. I can say among of the 29 or so lenses I've had over the years (I may be forgetting one or two) from 8 different manufacturers only one has had problems meriting a return.

 

[Lindsay D]

Quite interesting to see those lenses being the culprits. I'm wondering though, if they'd be the 'most hired' ones, vs the more affordable lenses (bar the oly 45.f1.8) so as you suggest it's hard to know if similar variance feedback might apply to any others in similar numbers. I also wonder if hired lenses get banged around more than privately owned lenses, so are more susceptible to the kind of knocks which I know from experience can cause softness and decentering. Assuming he's testing from his hire inventory?

I can't comment on those because the only one I've owned is the Oly 45 f1.8. I can't justify spending on the faster variants when that one is so good, and so cheap!

 

[Bidkev]

All this begs the question (s) why do some folk have "bad luck" with lenses and others don't, or are "very lucky" Online "professional testing" results are not "gospel". Just as individuals have a chance of getting a bad copy, so do those testing. Is that bad copy a result of lax quality control, or simply a characteristic of the lens? How much is user error no matter how careful the testing? Do we expect too much from our lenses? Does the "flaw" really matter in the real world of viewing an image as opposed to pixel peeping?

I'm pretty anal and view all my images at 100% before displaying anywhere but I freely admit that this isn't always necessary for the production of a "good" image. As I said earlier, I returned two copies of a samyang 8mm, the only lenses that I've ever had to return in 55yrs of buying and through all formats from medium down to m43.I bought samyang for m43 because I'd had a few previously that were amazingly sharp and I accepted the downside of extra weight (on m43) as worth it. Australia is a small market and I'm prepared to bet that despite being from different outlets, they were from the same batch. Both these lenses didn't need any pixel peeping to tell me that they were faulty as the fault was so glaringly obvious in that they simply didn't focus...........every shot was OOF!

There are also "human element (s)" in this lens v's that lens, or IQ assessment, which I've seen so many times before............blind faith...............penis envy FWOABW............total denial, and likely more human characteristics of photographers, that muddy the waters where gear is concerned.

Blind faith is where a photographer (invariably an amateur) puts absolute faith in his/her lens (invariably a pro model) and refuses to accept criticism of that lens or his/her images simply because he thinks his/her gear is "top notch" and his/her images are "sound". He/she is an advocate for his/her gear as opposed to a critic of his/her images.

Penis envy is where a photographer is envious of others' gear/format/images produced by that gear, and will defend to the hilt their choice rather than concede that someone else's gear, or images produced by that gear, have more merit than their own. Often, this photographer will secretly wish that they had more money to invest in larger format

Total denial. often hand in hand with penis envy, is where a photographer refuses to admit to self, or others, that they have made a bad choice in relation to what they expect from their gear or how they have applied it. It's hard to admit to a mistake and they go into "overdrive" attempting to convince others and self, that they have made the correct choice and invariably deny (to self) that their images may perhaps be lacking due to either their gear or technique.

I have hesitated many times, here, and on other forums, to state what I am about to, because 1. It may sound like I am blowing my own trumpet, but I assure you I am not, and am simply trying to illustrate my disagreement with some views expressed generally in threads of this kind, 2. I don't want anyone to think that I am referring to them and 3. Some may think that my arguments here are based on what I have just illustrated above..............but................... I call a spade a spade so here goes in the next para

Forget what lens rating sites claim (to an extent) The results refer to the lens(es) in their possession at that time and under their conditions. What matters is your lens, and your technique........and now the iffy bit, I have seem images on this forum and other forums, produced by pro grade glass and in some cases by pro photographers, used in debates such as this to illustrate how the IQ of their images are better than mine, or someone else's involved in the debate, who is seen as having "lesser" gear (in their eyes). I have seen many debates such as this and on only one occasion was I convinced by their images. In most cases, I personally, could see no merit in their argument based on their images and in many cases, quite frankly, their images, if I had taken them, would have gone straight to the recycle bin. You see the danger here? I am risking the old argument about "subjective" or whether I am fit to judge, but IQ isn't subjective, it's a fact and even where a lens has better IQ, if that IQ isn't being utilised due to poor technique, then where's the argument? The lens only produces better IQ if it is used to it's potential, and just how much IQ do you need?

I post all the above merely to support my love of this lens and to allay the fears of those thinking of making a huge financial investment (for them) who may be swayed by contra arguments. A lens is only as good as the hands it is placed in and I think, that any arguments with regards to "it's not as good as this lens or that lens" are invalid at this point because no images that I have seen so far, support that argument. Pointing to images in the direction of other sites and what photographers have produced with their lens, are also invalid (IMHO) as although many produce wonderful images with for example faster pro glass, those images cannot be assessed IQ wise as they are invariably not displayed at 100%, nor do they need to be because as I said above, just how good an IQ does a photographer need to produce images of merit?

Herendeth my rant

 

[Lindsay D]

There are good lenses and there are bad lenses. Some of the cheap kit DSLR lenses are diabolical, and the odd one is surprisingly good (Canon 18-55 STM, which I love). There's copy variance but at the end of the day those lenses just aren't very well calibrated or coated. Most professionally graded lenses will show far fewer of the anomalies we might see in consumer grade lenses. There are some consumer grade lenses however which are gems - and in my experience many if not most of those are micro 4/3 lenses.

Lens buyers need to understand that, but as you say it isn't a reason to negate the human element which is massive. I've been teaching and training photographers for a long time and I can say categorically that around 90% of my students will state, at the outset, that they believe there is something wrong with their camera or their lens. The biggest 'need' from the training is getting sharp images. Thus far I think I have only encountered one lens from my students which I think may have been faulty. The most frequent reason for why their images are not sharp is down to poor understanding of focusing. That can be all over the place, in fact they may have no real idea of where their point of focus even is. Then there are DOF misunderstandings, and the list goes on.

I met one guy last year, carrying the EM1ii, 300 f2.8 - telling me it was completely useless, no sharp images. And he has '20 years in the photographic industry'. Well the images I took were spot on, I was well impressed with his sh*tty camera and cr*ppy lens. His settings and technique were hair raising and no amount of explaining could sway him from his conclusions.

The knowledgeable or professional photographers I know don't seem to complain about their gear. They know if there's an issue, and they'll deal with it. And issues are thankfully quite rare.

When I start a lesson with a student who feels there is an issue with his or her equipment I will ask the student to show me how they shoot. Then I'll take a look at their settings. I will then take the same photograph with their equipment, using whichever settings I feel are appropriate. We can then compare the images back home on the computer. But sometimes the problems just 'are', an example being a very small target some distance away, very difficult for any camera or lens combination to accurately focus on (assuming the student has actually managed to focus on it and not something else entirely). Add to that the same problems that any lens will have under those conditions which will include atmospheric pollution, bad light and the student's tendency to savagely crop an image to the point where it is useless.

The lenses which are 'really bad' virtually across the board are the consumer grade superzooms such as those made by Tamron and Sony if it's based on a Tam design. They are much worse than the Canikon equivalents (which aren't great). The two worst lenses I've ever tested are the Tamron 16-300 and the Tamron 18-400. I tested two copies of each, both equally dire. I don't believe there are 'good copies' of the things. Yes, you'll get reasonably sharp images of objects within 8 feet providing you're within the first half of the zoom range, but beyond that things go downhill rapidly no matter how sound your technique. Yet there are people out there who claim those lenses are fantastic. Without any exceptions I can think of those people are the sector those lenses are aimed at - casual consumers who want to take informal travel snaps, with a generally low knowledge base.

Earlier today I was going through some photographs I'd taken a few years ago when I used Canon DSLR equipment and expensive L lenses. It reminded me of how we used to shoot back then. We would shoot multiples of the same scene or pose, and today I was reminded why. A proportion of the images would be soft, not due to photographer error but because some of those lenses aren't particularly reliable in terms of autofocus in anything other than ideal conditions. But we understood that, and we worked around it. We didn't blame the kit, we accepted its limitations. Nowadays, with my more recent mirrorless offerings that is largely unheard of (using broadly 'equivalent' lenses).

Anyway, that is a bit of the digression from the original topic but does serve up some background to ponder on.

The only area of variance I saw with the PL 100-400 lay with a stiff zoom ring, rather than the optics. Mine was a bit stiff at first, not helped by the fact I hadn't fully unlocked the lens (easily done).

 

[Ross the fiddler]

Not so lucky in Australia
View attachment 693903

Their service procedure with this lens concerns me too & why I'm waiting for the Olympus 150-400 F4-5.6 lens to come. It should have Sync IS as well to match with my E-M1 models. Unfortunately it will probably cost more than the PanaLeica though.

 

[archaeopteryx]

Assuming he's testing from his hire inventory?

Yup! Roger's most recent blog is actually on customer use of hired lenses; mostly they get good care and are optically stable. Optically degrading events like drops tend to get caught, readjusted, and retested. So Lens Rentals has a well curated data set that's scientifically quite desirable. Much more than I can say for lenses I've had as I've got nearly all of mine used and seldom have a clear sense of their history.

Lens Rentals offers 105 m43 mount lenses so there's probably not too much suitability for hire bias to their data in that sense, though they don't offer some common lenses such as the Olympus 9-18 and Panasonic 12-60 f/3.5-5.6. I would expect the volume of data is inevitably biased towards more commonly hired lenses. But Roger's rigorous enough it seems likely that's reasonably well controlled for.

That can be all over the place, in fact they may have no real idea of where their point of focus even is. Then there are DOF misunderstandings, and the list goes on.

Those are probably the two most common technical issues I've encountered as well, closely followed by understanding the limitations of stabilization and subject motion.

just how good an IQ does a photographer need to produce images of merit?

Well, sometimes they work rather well but I'll confess pinholes are usually a bit too fuzzy for me to find satisfying. I'm also kind of fond of lenses which open up to f/5.6 as that's often helpful in managing subject motion. But it's more of a nice to have; most of my large format work I couldn't have used faster than f/8 even if there'd been enough DoF that wide.

 

[Ziggy]

Interesting, but do Mu-43 lenses have more variation that other systems. I used to hear that Fuji had very good quality control but recently have seen some complaints about their 2.8 zooms having variations

Apparently, yes.
But others still have problems. The popular Nikon 200mm- 500mm was twice returned by a high profile US photographer before he got a sharp one.
I asked the online crowd I was going to buy mine from to do what tests they could after I'd paid for it but they refused .

 

[Ziggy]

Interesting, but very much counter to my own experience (that's shooting professionally with the system from the beginning, and testing more lenses than I can remember in the four years I was a brand ambassador at Olympus, when I also had as many Pana lenses in my bag). I'd cite Tamron APS-C followed by Canon APS-C and FX as the worst offenders. So far I've found Sony to be very similar to Canon (on the consumer front, but strong on the pro lineup).

What it boils down to is this - we should always test a lens when we get one. Easy for us to say, I'm used to doing it but newcomers don't even think to. And when they do, the testing protocols can be all over the place. An example being myself many years ago when I got a Canon 12-35L f2.8. Really soft on the edges wide open. So I exchanged it and the next one was the same. At that point I didn't even realise it was a 'normal' characteristic of that lens and it's rarely used at those settings in fact.

I understand that when Cicala does a lens test he aims to test 10 samples of each (of all brands) to average out the variation.

 

[wjiang]

I've never felt a need to micro adjust a lens, particularly when I'm using that lens on multiple bodies (which might not have any tweaking built in). I do pretty simply tests with a tripod, a spirit level and a brick wall. Then I shoot the same shots in a familiar testing environment. This means focus at infinity, then a long row of buildings, etc. Then I simply ask myself if I'd be happy making a large print and if or where any compromises might lie. We also have to be realistic - it's often not worth returning a lens for very minor 'issues'.

Were you using them for PDAF or CDAF? For CDAF there is never a need to adjust them.

 

[paul macro]

Their service procedure with this lens concerns me too & why I'm waiting for the Olympus 150-400 F4-5.6 lens to come. It should have Sync IS as well to match with my E-M1 models. Unfortunately it will probably cost more than the PanaLeica though.

Yes I would wait either until Panasonic make pro care available or Olympus release the 150-400mm which will probably be the better option especially if as sharp as other pro lenses the 100-400mm pl really was (is) the only option as a long tele zoom ,my lens hood screw has stripped on the 100-400mm and it is £125 to replace the lens hood alone ,heck mpb only offered me £280 for my 100-400mm and there is not a mark on it optically clean ,but because the lens action was stiff they claimed heavy use ,my lens zoom has always been stiff from new ? Not a good return

 

[paul macro]

All this begs the question (s) why do some folk have "bad luck" with lenses and others don't, or are "very lucky" Online "professional testing" results are not "gospel". Just as individuals have a chance of getting a bad copy, so do those testing. Is that bad copy a result of lax quality control, or simply a characteristic of the lens? How much is user error no matter how careful the testing? Do we expect too much from our lenses? Does the "flaw" really matter in the real world of viewing an image as opposed to pixel peeping?

I'm pretty anal and view all my images at 100% before displaying anywhere but I freely admit that this isn't always necessary for the production of a "good" image. As I said earlier, I returned two copies of a samyang 8mm, the only lenses that I've ever had to return in 55yrs of buying and through all formats from medium down to m43.I bought samyang for m43 because I'd had a few previously that were amazingly sharp and I accepted the downside of extra weight (on m43) as worth it. Australia is a small market and I'm prepared to bet that despite being from different outlets, they were from the same batch. Both these lenses didn't need any pixel peeping to tell me that they were faulty as the fault was so glaringly obvious in that they simply didn't focus...........every shot was OOF!

There are also "human element (s)" in this lens v's that lens, or IQ assessment, which I've seen so many times before............blind faith...............penis envy FWOABW............total denial, and likely more human characteristics of photographers, that muddy the waters where gear is concerned.

Blind faith is where a photographer (invariably an amateur) puts absolute faith in his/her lens (invariably a pro model) and refuses to accept criticism of that lens or his/her images simply because he thinks his/her gear is "top notch" and his/her images are "sound". He/she is an advocate for his/her gear as opposed to a critic of his/her images.

Penis envy is where a photographer is envious of others' gear/format/images produced by that gear, and will defend to the hilt their choice rather than concede that someone else's gear, or images produced by that gear, have more merit than their own. Often, this photographer will secretly wish that they had more money to invest in larger format

Total denial. often hand in hand with penis envy, is where a photographer refuses to admit to self, or others, that they have made a bad choice in relation to what they expect from their gear or how they have applied it. It's hard to admit to a mistake and they go into "overdrive" attempting to convince others and self, that they have made the correct choice and invariably deny (to self) that their images may perhaps be lacking due to either their gear or technique.

I have hesitated many times, here, and on other forums, to state what I am about to, because 1. It may sound like I am blowing my own trumpet, but I assure you I am not, and am simply trying to illustrate my disagreement with some views expressed generally in threads of this kind, 2. I don't want anyone to think that I am referring to them and 3. Some may think that my arguments here are based on what I have just illustrated above..............but................... I call a spade a spade so here goes in the next para

Forget what lens rating sites claim (to an extent) The results refer to the lens(es) in their possession at that time and under their conditions. What matters is your lens, and your technique........and now the iffy bit, I have seem images on this forum and other forums, produced by pro grade glass and in some cases by pro photographers, used in debates such as this to illustrate how the IQ of their images are better than mine, or someone else's involved in the debate, who is seen as having "lesser" gear (in their eyes). I have seen many debates such as this and on only one occasion was I convinced by their images. In most cases, I personally, could see no merit in their argument based on their images and in many cases, quite frankly, their images, if I had taken them, would have gone straight to the recycle bin. You see the danger here? I am risking the old argument about "subjective" or whether I am fit to judge, but IQ isn't subjective, it's a fact and even where a lens has better IQ, if that IQ isn't being utilised due to poor technique, then where's the argument? The lens only produces better IQ if it is used to it's potential, and just how much IQ do you need?

I post all the above merely to support my love of this lens and to allay the fears of those thinking of making a huge financial investment (for them) who may be swayed by contra arguments. A lens is only as good as the hands it is placed in and I think, that any arguments with regards to "it's not as good as this lens or that lens" are invalid at this point because no images that I have seen so far, support that argument. Pointing to images in the direction of other sites and what photographers have produced with their lens, are also invalid (IMHO) as although many produce wonderful images with for example faster pro glass, those images cannot be assessed IQ wise as they are invariably not displayed at 100%, nor do they need to be because as I said above, just how good an IQ does a photographer need to produce images of merit?

Herendeth my rant

I mostly buy s/h anyway my Olympus 300mm f4 was a s/h purchase with damage which was covered under Olympus pro care and repaired free of charge can not fault Olympus or the lens ,I have recently purchased the 200mm f2.8 pl s/h with out damage but for a lot less than new it looks like new to me I will do some varying lens tests I have hired the lens 4 times now so really time to buy I thought to myself .In fact with all the m43 dying business has really improved the s/h buying potential not so good if you are selling .

 

[Lindsay D]

Were you using them for PDAF or CDAF? For CDAF there is never a need to adjust them.

I have never felt a need to micro adjust a lens. That is irrespective of brand or focus technology.

I can't speak for anyone else, whose camera and lens combos might show the kind of variance which makes micro adjustment useful. But I do know a lot of photographers who have done this, either themselves or through a camera store/workshop and who haven't been happy. It's something I'd rather leave well alone unless I genuinely felt it 'necessary'. But as I said it's an individual thing and it's certainly beneficial for some.

 

[Bidkev]

I have never felt a need to micro adjust a lens. That is irrespective of brand or focus technology.

I can't speak for anyone else, whose camera and lens combos might show the kind of variance which makes micro adjustment useful. But I do know a lot of photographers who have done this, either themselves or through a camera store/workshop and who haven't been happy. It's something I'd rather leave well alone unless I genuinely felt it 'necessary'. But as I said it's an individual thing and it's certainly beneficial for some.

Fully agree Lindsay. < flame war coming> I liken advocates of it to those wine tasters who spit out disagreeable wines (which are acceptable to "the masses") who are akin to elevating themselves above others who haven't discovered the "finesse" of good wines aka micro adjustment. Yet again, the focus is on gear and not on the image. How the fnck did the "clsssics" in photography, before AF, ever become so designated? Odds are, they would never get designated nowadays with this preoccupation on IQ. technolgy, and not vision/artistic merit
I would think that with the added benefit of m43 DOF that a lens would have to be well off focus to warrant the tedious effort of micro adjusting. I'm fairly new to m43 relative to some here who may perhaps advocate micro adjusting, but even if I wasn't, my gut, aka instinct based on practical experience in the m43 theater so far, and my text book reading and pursuance of photography thus far,, that is, as opposed to internet "noise", would tell me that with the DOF that m43 provides, then to have to resort to micro adjusting and it's inherent dangers of "getting it all wrong and fncking it up" speaks either of pixel peeping to extreme, use of very slow pro lens apertures, which, in the "real world" of members who frequent forums, (good pros don't have much time to frequent), don't affect the amateur photographer, or else finally, it could be (rarely) a faulty lens.

Pursuance of (technical) perfection it may be, good or bad, but it doesn't make for art. Micro adjustment must attract lots of bucks to site owners/bloggers who have it picked up by google................how many of these "experts" actually have examples of their work (in full res) on their sites/blogs? Someone once said to me, and being a cynic it struck a chord, "If that music teacher is so good, why is she teaching and not in a band"?

One of the things that attracted me to m43, contrary to most who seem to have a preoccupation with bokeh, was it's DOF. Of course, I relate this mainly to wildlife photography and grabbed shots where you may not have the time to get the focus point on the eye, and not to someone who wants to photograph their dog, cat.or child indoors with a 1.2 where only the eyes are in focus and the rest of the face looks like it is in a steamy kitchen...................but sadly, an amateur using that gear that they have invested n heavily in, thinking it will result in better images, based on some blog, wonders why they have steamy kitchen?...............Could it be that perhaps it's because they get sucked in with gear and micro adjusting before they've even learnt the age old art of photography as it once was................ because they've bought pro gear and took advice from some blog/forum thread raising revenue from it's advocacy of gear hoping it would make them a better photographer and yet most of their image iare OOF...............Don't laugh.................OK, feel free if an advocate of micro adjusting, but don't lynch the messenger................it happens and I've seen it all before in so many threads. There's a reason why most universities here insist on students staring with film and recommend they buy into old systems. Full familiarity with DOF and what your lens is capable of, and adjusting aperure for same, can negate the need for micro adjusting particularly with m43 where DOF is wide.

Another rant over

 

[Lindsay D]

It’s not really about depth of field, it’s about where the precise point of focus lies relative to where the photographer has placed the point of focus. This is sometimes broadly referred to as front or back focusing, and that can be down to either the camera or the lens. We think of equipment calibration is being exact, but we know it often isn’t. The placement of the sensor may vary by a few microns from camera to camera as might the calibration of even a very well specified lens. The point of micro adjustment is to ‘marry’ your lens with your body, because your lens and your body might be different to someone else’s body and lens combination. Of course this also means that the same lens which is razor sharp on one EM1 for example, may not be as sharp on your spare EM1. It doesn’t mean that the camera of the lens is faulty, it is just the natural variation in normal tolerances.

There are problems inherent to micro-tuning your equipment and I would advise that a person only does this if they fully understand good technique and what micro-tuning actually does. A case in point is a student I had last year. He told me he was having a terrible time getting his lenses sharp and had tried various means of fine tuning them including a couple of well approved workshops. At the lesson it became apparent that the problem was more likely to be his technique, so no amount of tuning was going to help and was in fact making things worse.

 

[D7k1]

Real sampling has to be done on a random basis. You would need to have lenses from different parts of each production run as well as other production run to do a valid test. You also have to do sampling in numbers that are statistically relevant. And in the end correlation is not causation - ask Mrs. Clinton. If a statistical analysis is not peer reviewed be aware when considering its validity.

 

[cnyap]

I don't see how micro AF adjustment applies at all to m43. Micro-adjustment is for traditional/mirrored DLSRs which can have issues with the imaging sensor and/or focusing mechanism (which is located behind the mirror?) not being exactly positioned correctly, thus creating an "offset" between where the AF sensor focuses the image/lens onto versus where the sensor actually is located during manufacturing.
The beauty of m43 (and "live view" on DSLRs) is that the focusing sensor's location and image sensor's location are in the same exact location, thus there inherently cannot be any "offset" issue. A camera can still focus incorrectly, but it's not due to camera body alignment issues.

 

[Lindsay D]

I don't see how micro AF adjustment applies at all to m43. Micro-adjustment is for traditional/mirrored DLSRs which can have issues with the imaging sensor and/or focusing mechanism (which is located behind the mirror?) not being exactly positioned correctly, thus creating an "offset" between where the AF sensor focuses the image/lens onto versus where the sensor actually is located during manufacturing.
The beauty of m43 (and "live view" on DSLRs) is that the focusing sensor's location and image sensor's location are in the same exact location, thus there inherently cannot be any "offset" issue. A camera can still focus incorrectly, but it's not due to camera body alignment issues.

I think EM1ii allows micro adjustment with native u43 lenses and I think the older 4/3 lenses for PDAF. I'm not certain about it though because I don't have an EM1ii.

You can also send an older body and lens combo to a workshop to have them calibrated to 'match' if you're that enthusiastic about your tuning.

 

[cnyap]

Interesting but weird...I wonder what causes a focusing error that can be remedied via a fixed adjustment value. M1 has phase AF but it's on-sensor, so I can't think of what could go wrong there (other than random focus errors in both directions), and what would make a specific lens have a predictable/fixed amount of focusing offset....hmmm.

 

[Lindsay D]

Interesting but weird...I wonder what causes a focusing error that can be remedied via a fixed adjustment value. M1 has phase AF but it's on-sensor, so I can't think of what could go wrong there (other than random focus errors in both directions), and what would make a specific lens have a predictable/fixed amount of focusing offset....hmmm.

They're not actually focusing 'errors'. No production process is perfect though well specified cameras and lenses are set within certain tolerance values - over a range, not a fixed number. If your camera is at one end of the range and your lens the other the two together might not be as sharp as the next same camera and same lens. Micro tuning is merely a way of trying to get the 'best' case scenario.