[Class A]

In other words pentax DSLRs seem to be uniquely unsuited to lenses with hypersonic focusing due to the lower inherent AF tolerances caused by the hypersonic AF drive design.

Reading speculations like this (panoguy had a similar theory) -- with all due respect -- I cannot help but think that their authors have very little knowledge about how a closed feedback loop works and about the mechanical precision required to obtain proper AF.

No one seems to acknowledge that even the PF copies of the lens worked perfectly with CDAF. In other words, the lens will focus correctly, if it receives the right signals from the camera. This, with a high probability, rules out mechanical issues. It is conceivable that the camera would stress the lens differently when using PDAF compared to CDAF, but this does not make much sense in engineering terms and I'm rather sure that any answer to this puzzle will have nothing to do with the lens motor or its associated mechanics. Nor with any lens characteristics assumed by Pentax cameras.

It is unfortunate that the review did not elaborate on the nature of the errors (consistent vs erratic, dependent on aperture, focal length, subject distance or not). With more information -- and frankly with such a devastating judgement regarding the AF performance that already made many readers cross off the lens from their lists, I would have expected a more detailed analysis -- we would be able to better speculate.

I'd be very surprised, though, if the cause of the issues would be anything else but a failure in the correct AF adjustments to be applied by the camera per shooting scenario. Whether there was potential to alleviate the issues by using the dock accordingly, I cannot say. It is possible that only servicing (involving optical alignment and/or exchanging PC boards within the lens) can fix severe AF issues.

Let's hope we'll be hearing from Sigma some day.

[Class A]

Interesting fact: LensRentals is offering the Sigma 18-35/1.8 for Canon for renting.

This implies they must be in possession of multiple copies that AF satisfactorily with Canon bodies. If there were constant complaints about the lens, LensRentals wouldn't offer it anymore.

This suggests to me that this lens does not have an intrinsic AF issue.

[panoguy]

Reading speculations like this (panoguy had a similar theory) -- with all due respect -- I cannot help but think that their authors have very little knowledge about how a closed feedback loop works and about the mechanical precision required to obtain proper AF.

With all due respect, the same applies to your knowledge of a lens that you do not demonstrably own or have even used. Really, the most apparent feedback loop here is a knee-jerk dismissiveness toward anyone who disagrees with your (numbingly repeated) assertions that Adam & Alex have done something horribly wrong with their negative review of the Sigma 18-35mm AF performance.

(Helpful reminder: it's a review on the internet... Sigma isn't going to fail and no Pentaxians are going to suffer from worse photos due to a few words.)

I'm frankly surprised by all of this, Class A, as you are typically one of the more knowledgeable and even-handed gearheads on this forum. Why the aggressive (and counterproductive) defense of Sigma in areas you have no experience with? I'm actually curious... though it does make me wonder if the Coriolis effect applies to draining credibility.

This suggests to me that this lens does not have an intrinsic AF issue.

And now a straw man... jeez, just give it up already!

Boriscleto'd!

[Rondec]

Reading speculations like this (panoguy had a similar theory) -- with all due respect -- I cannot help but think that their authors have very little knowledge about how a closed feedback loop works and about the mechanical precision required to obtain proper AF.

No one seems to acknowledge that even the PF copies of the lens worked perfectly with CDAF. In other words, the lens will focus correctly, if it receives the right signals from the camera. This, with a high probability, rules out mechanical issues. It is conceivable that the camera would stress the lens differently when using PDAF compared to CDAF, but this does not make much sense in engineering terms and I'm rather sure that any answer to this puzzle will have nothing to do with the lens motor or its associated mechanics. Nor with any lens characteristics assumed by Pentax cameras.

It is unfortunate that the review did not elaborate on the nature of the errors (consistent vs erratic, dependent on aperture, focal length, subject distance or not). With more information -- and frankly with such a devastating judgement regarding the AF performance that already made many readers cross off the lens from their lists, I would have expected a more detailed analysis -- we would be able to better speculate.

I'd be very surprised, though, if the cause of the issues would be anything else but a failure in the correct AF adjustments to be applied by the camera per shooting scenario. Whether there was potential to alleviate the issues by using the dock accordingly, I cannot say. It is possible that only servicing (involving optical alignment and/or exchanging PC boards within the lens) can fix severe AF issues.

Let's hope we'll be hearing from Sigma some day.

Let's just say that it is a problem for Sigma. How big is uncertain, but the fact that the lens will work with CD AF doesn't change the usability of the lens on a traditional SLR, where 90 plus percent of the photos are taken with PDAF.

I'm not totally certain, Class A, why you are supporting Sigma so strongly. This thread, from what I can see, is not to bash Sigma, but to try quantify the possible issue with the lens, much in the same way that threads have been introduced in the past to quantify problems folks have had with K3 lock ups or, SDM failures.

Doug has said that the lens he got from Sigma was not focusing correctly and had decentering issues and he had it personally fixed after Sigma refused to do so, saying that it was up to spec already.

[Digitalis]

No one seems to acknowledge that even the PF copies of the lens worked perfectly with CDAF

CDAF uses slower movelemts from the lens AF motor.

very little knowledge about how a closed feedback loop works and about the mechanical precision required to obtain proper AF

I'm well aware of how closed loops work in the realm of AF mechanics. Those comments that I mentioned was purely speculative and based on conjecture - and without the aid of morning coffee.

the fact that you own so many Sigma lenses appears to be testimony to the fact that in terms of performance, according to your assessment, some Sigma lenses can compete with any other brand out there.

if you want to also ignore the difficulties I had with acquiring good copies of each lens. The Sigma 18-35mm f/1.8 still has focusing issues 9 and it was originally de-centered). The sigma 100-300mm f/4 APO EX flares easily and it has no focus limiter, the Sigma 180mm f/3.5 APO EX has flare issues, and isn't critically sharp at any aperture with focus distances greater than 15m. The Sigma 8-16mm f/4.5-5.6 has flare issues and has difficulty acquiring focus in back-lit situations.

yeah, sigma lenses are a huge success.....

[JimmyDranox]

yeah, sigma lenses are a huge success.....

Yes, Sigma lenses are a huge success because they cover the holes in the Pentax line.

But returning to the problem of focusing issues, there are some things that I consider that are misunderstood.

First. I don't think that inertia can be a problem. My 70-200mm 2.8 HSM Sigma is big, almost twice as heavy that 18-35mm 1.8. And it has no focusing problems, either in PDAF or CDAF.

Second. If a lens focus corectly with CDAF and not with PDAF, the logical explanation is that the problem is in the PDAF system, not in the lens. Let's remember that every camera manufacturer specifies that only a part of focusing areas work with F2.8 lenses, all other areas works with F5.6 lenses.

As I've remember I never saw a camera with focusing areas specified to work at F1.4 or 1.8. So, the manufacturers knows that PDAF system is reliable for fast lenses only until F2.8. And every lens is looking for focus wide opened. Sometime, some of manufacturers are saying that those areas permit focusing with lenses F2.8 or faster. Because they knew the inside of the PDAF system, and they can mach the faster lenses they make, to work in acceptable terms with their camera. But this is not valid for third party lenses. And I said ''acceptable'', because, in real word every lens, and especially the fast ones can miss the focus sometime.

So, I don't think that we can blame this Sigma lens because it has some inherent problems. Nor Alex or Adam for a review which reveals those fails. I knew that a series of tests to search exactly what is happening, and when and why is extremely time consuming, and frankly, I don;t remember seeing such comprehensive tests in a review, at least in the photo world.

[UncleVanya]

Other links to focus issues:
Need the speed? Sigma 18-35mm F1.8 DC HSM in-depth review: Digital Photography Review
http://forums.whirlpool.net.au/archive/2178744
Sigma 18-35 1.8 Focus Problem - only in view finder focus - Other Manufacturers Discussions on Nikon Rumors Forum
https://groups.google.com/forum/#!topic/rec.photo.digital.slr-systems/YL0cEwIqTqY

I had different hits previously but can't pull them up. What's interesting is to read also about the 35 f/1.4 and see where many compain of PDAF issues with D70's and other systems at anything faster than f/2.8 - in the Pentax world that's not what has been observed from a number of FA lenses (85/1.4, 31/1.8, etc) for many years. However I guess it may be relevant - are there any zooms with better than f/2.8 that are commonly used in PDAF mode reliably on Pentax systems? (I am not baiting - I honestly can't think of any zooms that fast besides this one.)

[Class A]

if you want to also ignore the difficulties I had with acquiring good copies of each lens. ...yeah, sigma lenses are a huge success.....

And yet you are keeping all five of your copies despite your assertion that cost is not an issue.

Why, one wonders, given that you are also using Leica glass, etc. and rarely miss an opportunity to tell people how inferior Sigma lenses are. If you had gotten rid of all of your Sigma lenses, I could understand that you wanted to warn other people about them. But apparently, no money in the world can replace them with something better, so why the constant complaints?

[Class A]

Yes, Sigma lenses are a huge success because they cover the holes in the Pentax line.

+1

Because they knew the inside of the PDAF system, and they can mach the faster lenses they make, to work in acceptable terms with their camera. But this is not valid for third party lenses.

Up until this point, your post was spot on.

I don't understand, though, what makes you think that third-party lenses are disadvantaged. I don't see how it would be possible for some lenses to be better tuned to PDAF systems than others.

[Class A]

I'm not totally certain, Class A, why you are supporting Sigma so strongly.

Two reasons:

1. Out of principle: I'm all for calling a spade a spade and if Sigma really has a huge problem then they should not be spared the respective criticism, but before reliable proof has been obtained that there is more at play than a few off-kilter copies, no one should claim that the PDAF of this lens is unusable. A review should assess the merit of a working copy, not report about faults of individual copies or even potentially user error. What if DPReview had received a K-7 with sensor stains and had given it a low IQ rating? What if DPReview had received a K-5 with a mirror flap issue and had rated it to be "unreliable"?
   
   It is one thing to state "It seems advisable to buy from a place with a good return policy since it may not be easy to obtain a fault free copy straight away" and another thing to state "PDFA is unusable with this lens". Even the first cautionary warning should, IMHO, only be issued if one has convinced oneself that one was not just very unlucky to receive two faulty copies.
   
   
2. Sigma is important for Pentax: They are currently filling gaps in the Pentax lens line up and provide important alternatives. If Sigma decides to drop Pentax, there isn't much left to Pentax for me. All their recent lenses were, AFAIC, uninteresting or overpriced. Typically, they are both. The cameras are still great but you also need glass for them and I wouldn't feel comfortable to recommend Pentax to anyone on the basis of them having to hunt the used market for interesting glass. Some people may think something like the DA40/2.8 is great. I don't belong to them and if Pentax is just "small and light (and slow, and optically so-so)" then I'm not interested anymore.
   
   I've seen many "OK, I was interested in this lens, but if it cannot AF, I'm out"-type responses to the review and if the damages to the sales reach a level that causes Sigma to withdraw from Pentax completely -- they already only offer a subset of their lenses in K-mount -- and it turns out that we were only dealing with a bad batch of lenses, i.e., no systematic problem then Pentaxians will be shorter of choice for no good reason.
   
   It has been a sad affair to walk around Photokina, talking to Zeiss, Voigtlander, Tamron, etc. about their Pentax support. They all left Pentax. I don't think this is the time to be unkind to Sigma without being 100% sure that there is a systematic problem.

I could be wrong and the reviewer may have done enough to obtain 100% proof. Just from what he has written, it does not seem like it.

I feel the correct way to handle the situation would have been to further investigate regarding the cause of the issues. I feel the review lacks a component that explains of what exact nature the AF errors are and how the reviewer attempted to address them.

If it was felt that the public should be warned before further investigations are finished, then the AF rating should have been suspended, with a comment about the ongoing investigations. I remain unconvinced that enough has been done to justify a devastating AF rating that has been putting off many potential buyers.

Doug has said that the lens he got from Sigma was not focusing correctly and had decentering issues and he had it personally fixed after Sigma refused to do so, saying that it was up to spec already.

To be precise, Doug was dealing with a distributor (not Sigma directly). Furthermore, he did not try to push the issue with the distributor, apparently because he knows this other facility he trusts and that could fix his lens to his satisfaction. Decentering with lenses unfortunately happens and could well impede on AF performance.

[Class A]

With all due respect, the same applies to your knowledge of a lens that you do not demonstrably own or have even used.

I do not need to have a copy of the lens in order to know when not enough has been done/published in order to justify a devastating review for a lens. What good is all the other praise of the lens, if the bottom line is "This is a manual and/or CDAF lens."? How many people will buy the lens nevertheless? Or are you counting on the idea that people will not give much credibility to a PF review and buy the lens all the same?

(Helpful reminder: it's a review on the internet... Sigma isn't going to fail and no Pentaxians are going to suffer from worse photos due to a few words.)

Sigma is already suffering from a bad reputation acquired quite some time ago because some people are incapable of learning that Sigma has changed in their approach (away from cheap and inferior glass towards high-end glass) and keep repeating the same old Sigma bashing.

Pentax must be pretty insignificant to Sigma already because

1. they only offer a small subset of their lenses in K-mount, and
2. it took ages until the K-mount version of the 18-35/1.8 was offered.

I fear it won't take that much damage to sales for Sigma to completely give up their K-mount support.

So first I believe the "few words" are going to make a difference and if you don't believe they will make a difference why write them in the first place?

I'm frankly surprised by all of this, Class A, as you are typically one of the more knowledgeable and even-handed gearheads on this forum.

I appreciate the compliment.

Please see above and my previous post as to why I disagree with the premature downgrading of the 18-35/1.8 due to believing that criticism should be expressed precisely (if there is a QC issue, it should not be cast as an AF issue) and requires solid support.

Why the aggressive (and counterproductive) defense of Sigma in areas you have no experience with?

I never meant to be aggressive and disagree with this assessment.

I decidedly disagree with unfounded criticism because I care about the brand targeted.

Since the discussion started to get personal (-> ad hominem), I will now attempt to withdraw from it. If I won't comment anymore, it means I have been successful to resist and the absence of any responses should not be construed as either consent or disagreement. I hope that Adam and Alex can shed further light on this matter and that we will be hearing from Sigma about potential explanations.

[JimmyDranox]

+1


Up until this point, your post was spot on.

I don't understand, though, what makes you think that third-party lenses are disadvantaged. I don't see how it would be possible for some lenses to be better tuned to PDAF systems than others.

What makes me think about this? Simple. I'm not an engineer, but after more than 15 years of fighting with amateur astronomical telescopes, I know a little about optics, and refractors behavior. And I look to the facts. Fast lenses from Canon works better than Sigma in terms of AF. Same for Nikon. In my mind, this means that they know what to put, and where, and what not, to gain the maximum signal exactly where their PDAF systems are more sensible. And when someone makes a reverse engineering, even a different coating can change things in an optical system. Of course, I don't have raw data (if I would have, tomorrow morning the guys from Sigma or Tamron could be at my door)

What will be if Sigma will decide to cut production of all lenses in Pentax mount? Sure, will be a very bad news for many Pentax users, and even for Ricoh in the short term. Not long ago we all saw some threads around here, about some Sigma lenses which was discontinued for Pentax. And I don't remember someone being happy about this. Or I remember?? Never mind.

This dispute reminds me about something. About a scale. About what I care. And I care about:

1. Photography
2. Video
3. Pentax.

And as much I like Pentax cameras, and I like a lot (until now), Pentax can never go higher than the third place. Maybe for others, the scale looks different, but is everybody's choice.

[Digitalis]

I disagree with the premature downgrading of the 18-35/1.8 due to believing that criticism should be expressed precisely (if there is a QC issue, it should not be cast as an AF issue) and requires solid support.

Do you have a copy of the Sigma 18-35mm f/1.8 ART?

[MadMathMind]

What's interesting is to read also about the 35 f/1.4 and see where many compain of PDAF issues with D70's and other systems at anything faster than f/2.8 - in the Pentax world that's not what has been observed from a number of FA lenses (85/1.4, 31/1.8, etc) for many years. However I guess it may be relevant - are there any zooms with better than f/2.8 that are commonly used in PDAF mode reliably on Pentax systems? (I am not baiting - I honestly can't think of any zooms that fast besides this one.)

The limitation of f stop and PDAF has more to do with depth of field. No autofocus system is perfect. It may focus a bit behind or in front of what you aim at. When you have enough DoF, this doesn't matter and everything is as you want. It's a matter of precision. For example, at 80mm, for a subject 8' away, at f/1.4 you have only 2" of DoF. At f/2.8, it doubles to 4". It's not a lot, but if the autofocus is only accurate to within 2", then you can't call it reliable at f/1.4.

Of course, the DoF changes a lot with focal length, but I'm sure so does the PDAF precision. At short focal lengths, the phase is harder to detect (smaller regions to focus on, finite precision of sensors), but it matters less because you have a lot more DoF. For a subject 8' away. At f1.4 and 24mm, you has nearly 2' of DoF; at f/2.8, it's slightly over 4'.

And the Limited lenses do miss focus from time to time when wide open. It's not pandemic and varies from case to case, but it's pretty easy to get out of focus shots below f/2.8. Again, it's precision. Something is in focus, just not what I want because the AF region isn't so tiny and at low f stop and high focal length, there's very little depth of field. Miss by an inch and everything is out of focus,


Of course, this is not what's being reported with the Sigma lens, but just general information.

[Malak]

I just picked up my example if the 18-35 yesterday... I've only had a short time to mess around with it, but I feel that the af is pretty inconsistent...

That said, it's no where near as bad as the in depth review seems to suggest... Once I've had more time to experiment, I'll comment more...