[sharik]

Perhaps I should have posted in this thread earlier, but still this information may be useful to the others. I had my Sigma 18-35 (one of those copies affected by the AF problem) serviced by CR Kennedy under warranty in July 2015. I asked them to 1) service or replace the ring motor, and 2) calibrate the lens with a USB dock, and sent it together with my K3. I got it back in less than a week, and, although the service report is obscure as usual, I can tell that the lens was taken apart (it got a little play in the barrel that was not there before), and that it was calibrated (the settings of the camera have changed). Since this service, the AF problem has been much reduced, although it did not disappear completely. Out-of-focus images are about 10 times less frequent than before. I use it now like any other lens without AF problems, but still check the images for small AF errors... CDAF performance is flawless as before.

[Digitalis]

although the service report is obscure as usual, I can tell that the lens was taken apart (it got a little play in the barrel that was not there before)

This is why I'm not such a huge fan of CRK. Their reports no not include enough detail over what the problem was, what caused it, and what was done to fix it....they just list things that were replaced and how much they cost. Also, the barrel movement is troubling, as there shouldn't be any to begin with.

[Class A]

So those claiming, that the lens in question is completely unreliable when it comes to focusing are either doing something wrong, or their copies are defective.

I agree with you with the caveat that there appears to be an unfortunate high number of copies with at least some problem.

Whether it is fair to refer a percentage of failures that may be within in the normal range of a zoom that is this fast a "crippling issue" is another question, but it is clear that there are copies which require servicing.

Still, reports from people like you appear to be ignored for those who want to establish the notion that every user of this lens must be experiencing issues.

The attitude of (a relatively small number of) respective posters goes against explicit reports of people stating otherwise and against the presence of nice samples of the lens that are presented without comments such as "It took me 10 attempts to get that shot" (or similar).

Some posters will even go as far and take on absurd positions like referring to Sigma's USB dock "as a somewhat pathetic admission that they're clueless about how to make the AF of some of their fast, more complex lenses work on a wide range of bodies".

For some reason the inclusion of an AF micro-adjustment feature on cameras is not seen as a "somewhat pathetic admission that camera manufacturers are clueless about how to make AF work with a wide range of lenses".

I'm glad you are enjoying your lens and just wish others would not find it "increasingly hard to believe" that you do, since they are preventing at least some percentage of people from getting the same enjoyment, despite not being able to know whether their position of "the lens model as such is intrinsically flawed" is maintainable.

[Digitalis]

I agree with you with the caveat that there appears to be an unfortunate high number of copies with at least some problem.

And it isn't just a problem with Pentax DSLRs - Nikon and Canon bodies are also affected.

but it is clear that there are copies which require servicing.

The distressing part is that most of them are new lenses, practically straight out of the box. The Sigma lenses I have for other mounts, most of which were bought new (not just pentax KAF mount) have required minimal adjustments. Even other ART lenses have proven to be equally as reliable as brand name lenses*.

I'm glad you are enjoying your lens and just wish others would not find it "increasingly hard to believe" that you do, since they are preventing at least some percentage of people from getting the same enjoyment, despite not being able to know whether their position of "the lens model as such is intrinsically flawed" is maintainable.

Time will tell.

* At present. However in the case of the Sigma 18-35mm f/1.8 ART in KAF mount is the only copy I have, and it will be the last APS-C lens I will ever get.

---

Last edited by Digitalis; 12-23-2015 at 08:49 AM.

[sharik]

This is why I'm not such a huge fan of CRK. Their reports no not include enough detail over what the problem was, what caused it, and what was done to fix it....they just list things that were replaced and how much they cost. Also, the barrel movement is troubling, as there shouldn't be any to begin with.

I see your point and agree. As far as I remember, you mentioned earlier that you have different arrangements for repairs of your equipment. And the barrel movement felt strange and unpleasant, although I didn't notice any effect on performance. So, being an amateur with no formal photographic commitments , I'm prepared to put up with the limitations of CRK.

[Digitalis]

Well that is where I differ from many photographers on forum sites - I work commercially in photography. The equipment I work with has to be reliable and consistent - neither of those characteristics were apparent in the performance of my copy of the Sigma 18-35mm f/1.8 ART.

[UncleVanya]

Well that is where I differ from many photographers on forum sites - I work commercially in photography. The equipment I work with has to be reliable and consistent - neither of those characteristics were apparent in the performance of my copy of the Sigma 18-35mm f/1.8 ART.

I think this sums it up. As a hobbyist I can put up with more shortcomings than Digitalis can. So it stands to reason that others can also. Where I get freaked out is when there are people who initially think they have a working copy of the lens and think they know whats causing the problems for the shots that fail and then later come back dejected indicating that they thought that they had it - but it is less simple than they realized.

The lack of reasonably consistent focus across time for users is what disturbs me the most.

[MadMathMind]

I guess I'll send mine in for warranty service and see if it helps. I don't really trust the lens right now so I don't use it much. I can live without it for some time.

[JanG]

I've just bought this lens new from a well known retailer in Germany. At first I was extremely happy because I got quite a few super sharp photos with the lens even at 1.8 (the aperture I shot at almost exclusively of the past few days). So I think I can claim that I am not affected by the PDAF sharpness issue.
Yet ...
Yet, my lens seems to suffer from the unjustified focus confirmation issue. About one in five shots (26 in 128 shots) is blurry (without reason - like take three photos of the same subject in a slow series and you get 'click' super sharp, 'click' unusable, 'click' super sharp without moving the camera, focus confirmation acquired in all three cases) to the extent of not being usable (not even for small images by web standards). Now what would you recommend me to do?

Obviously, I got a lens that at least can PDAF perfectly with my K-3 and in very demanding light (like ISO 6400 with f/1.8 and 1/15s at 35 mm focal length in artificial light).
I really like the lens. And the next copy might well be worth. I think I could live with the 1 in 5 percentage - but it might affect shots I will miss bitterly.

I have about one more week to return the lens and step back from the purchase but in any case I would try a second copy.

Any recommendation? What would you do?

Thanks.

[Digitalis]

my lens seems to suffer from the unjustified focus confirmation issue. About one in five shots (26 in 128 shots) is blurry (without reason - like take three photos of the same subject in a slow series and you get 'click' super sharp, 'click' unusable, 'click' super sharp without moving the camera, focus confirmation acquired in all three cases) to the extent of not being usable (not even for small images by web standards). Now what would you recommend me to do?

There is nothing that can be done about that - AF calibration won't help, as the problem is random*. As far as I can tell it is a characteristic of this lens.The only effective solution is to stop the lens down to f/5.6~f11 and let DOF cover up the AF mistakes. I know you love f/1.8, but as you have discovered: this lens is unreliable with AF and the shallow DOF at that aperture just makes it obvious.

*and not just on pentax cameras either, Nikon and Canon are also affected buy this issue.

[MadMathMind]

Where I get freaked out is when there are people who initially think they have a working copy of the lens and think they know whats causing the problems for the shots that fail and then later come back dejected indicating that they thought that they had it - but it is less simple than they realized.

You just described me in this thread.

I can usually get shots that are ok at screen (25-35% size). If you pixel peep, however, it's clear the focus is wrong.

[Digitalis]

Within the first hour of working with this lens in the field, I knew something was rotten in the state of Denmark. Even on the focusing screen on my K5IIs I could tell the lens wasn't focusing properly, on the image review LCD I could also spot the decentering in my copy of the lens.

[Class A]

What would you do?

I feel that at the end of the day, it can only be your call.

Only you know whether you can live with the percentage of bad shots this copy of the lens is delivering.

Despite the fact that some people in this thread can now only report on the negative aspects of the lens, despite having substantially praised the lens for its optical performance potential in the past, there is no denying that outside adverse conditions, this lens can deliver superb results and that it may be acceptable to live with some uncertainty about focus accuracy. Note that the Sigma lens dock may help to address focus issues to some extent, but that an erratic component appears to remain even after calibration, at least for some copies (I am in no position to ascertain that all copies are affected by the erratic focus behavior, nor is anyone else here).

If I were you, I would try to come to an arrangement with the dealer to try another copy of the lens with the fallback option that you could get back your original copy in case it seems to be a perfect copy with the exception of the focusing issue. If you are lucky, then perhaps you get a copy whose focus failures are entirely within the normal range of Pentax PDAF performance (it is not perfect and for technical reasons absolutely cannot be perfect for lenses with an aperture wider than f/2.8 (<- the widest base of the PDAF system, with most focus areas only achieving f/5.6 precision).

For every person who claims my "so and so genuine Pentax lens never misfocusses, there is another person reporting the occasional misfocus result from wide aperture genuine Pentax lens). So don't expect 100% perfection. An 80% success rate is not what one would normally expect, though.

Would be good to be hearing back from you, should you decide to try another copy.

[UncleVanya]

Despite the fact that some people in this thread can now only report on the negative aspects of the lens, despite having substantially praised the lens for its optical performance potential in the past, there is no denying that outside adverse conditions, this lens can deliver superb results and that it may be acceptable to live with some uncertainty about focus accuracy. Note that the Sigma lens dock may help to address focus issues to some extent, but that an erratic component appears to remain even after calibration, at least for some copies (I am in no position to ascertain that all copies are affected by the erratic focus behavior, nor is anyone else here).

I would suggest that Digitalis initially had high hopes and thought the lens was fantastic optically but before the end of that very thread he was already calling AF into question and saying he could not recommend the lens to the casual user due to these issues. It is entirely certain that someone like him has a far greater set of experiences with various lenses and can provide a higher level of insight than the casual user.

[MadMathMind]

For every person who claims my "so and so genuine Pentax lens never misfocusses, there is another person reporting the occasional misfocus result from wide aperture genuine Pentax lens). So don't expect 100% perfection. An 80% success rate is not what one would normally expect, though.

I have recalibrated my FA limited. They don't miss too often now. But they do miss from time to time, particularly when the light is meh. The biggest difference is that when they do hit, the sharpness is off the charts.