[cali92rs]

Sorry to hear of your woes, but if you didn't read the pentax forums review, or my comparative review of the Sigma 18-35mm Vs the FA31 You would have known what to expect. There are some people who own copies of this lens that allegedly do not have any issues with using this lens with PDAF - call me prejudiced but with the increasing evidence of AF issues and their prevalence outside Pentax brand cameras I'm finding these positive reports extremely hard to believe.



It really can't be just a coincidence that Nikon* and Canon* photographers are experiencing identical issues with PDAF with this lens as well. Sigma photographers are being suspiciously quiet about it.

*And these people were using current camera bodies with advanced AF systems like the Canon 7DII and Nikon D7100.

Oh i read the reviews, but some folks have said that they had better luck with the K-3 (II) and therefore i thought to give it a shot....but I bought from a reputable dealer with a great return policy, just in case. I certainly wasn't going to buy the lens used.

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Last edited by cali92rs; 11-28-2015 at 11:38 PM.

[Pouldu]

I have to say that I was also attracted by it but I finally moved toward the 16-85mm WR, which is by its name weather resistant. The sigma is not. This was an important point for me. Moreover, I sold my Sigma 35mm f1,4 due to autofocus inconsistency. I was disappointed to see that 1 shot over 3 didn't nail focus as expected. Sure the IQ is terrific, but the 16-85 is really good, even if it's a zoom.

[biz-engineer]

Maybe, the same lens would works on a different body.

[Heie]

Man, and I was RAKED over the coals for my AF findings when I conducted the PF in-depth review.

Sorry to hear about your experience

-Heie

[Digitalis]

I was RAKED over the coals for my AF findings when I conducted the PF in-depth review.

It wasn't any easier for me when I pointed out the crippling AF issues with this particular lens.There are certain members on this forum who seem to take an almost fervent stance that sigma products aren't to blame for the issues that users are reporting, that the camera bodies are to blame.

[rawr]

I sold my Sigma 35mm f1,4 due to autofocus inconsistency. I was disappointed to see that 1 shot over 3 didn't nail focus as expected.

Glad to see I'm not alone with the Sigma 35 1.4, and it's terrible AF strike rate. I may do the same as you.

Or maybe I'll wait until the FF arrives, and give the lens one more chance to perform reliably.

[audiobomber]

Cali92rs, have you considered the Sigma 17-50mm f2.8? I find the colours very like my 16-45mm, exposure is more accurate, AF is silent and needed no adjustment, useable at f/2.8, sharper across the frame than the DA 16-85 or DA*16-50, considerably smaller than the Sigma 18-135. My only beefs are the focus and zoom rings that turn the wrong way and the lack of QS, otherwise I am pretty thrilled with mine.

[Outis]

It wasn't any easier for me when I pointed out the crippling AF issues with this particular lens.There are certain members on this forum who seem to take an almost fervent stance that sigma products aren't to blame for the issues that users are reporting, that the camera bodies are to blame.

I'm just curious how it could be the case that the camera bodies are to blame-- the lens had to be designed to work with the camera bodies, no?

[reeftool]

It wasn't any easier for me when I pointed out the crippling AF issues with this particular lens.There are certain members on this forum who seem to take an almost fervent stance that sigma products aren't to blame for the issues that users are reporting, that the camera bodies are to blame.

Long before this lens was ever available for Pentax, there were AF issues reported, especially with Canon bodies. While I don't remember the exact numbers, the very first review I read said that at f/1.8, the percentage of acceptably in focus shots was very low and I think well under 50%. I suspect that the people who are having no issues with this lens are probably landscape shooters who are happily shooting away at f/8. It is a very sharp lens stopped down and the focal length range make it ideal for landscape shooting. Either that or they have luckily beat the odds and got a good one.

[Edgar_in_Indy]

I'm just curious how it could be the case that the camera bodies are to blame-- the lens had to be designed to work with the camera bodies, no?

Well, in theory the camera body is the one in charge of the AF. It tells the lens where to focus. The PDAF sensor in the camera body is supposed to report when focus is achieved. In the case of the 18-35mm, I'm not sure how the PDAF system could randomly be mis-reporting that the image is in focus, but apparently there is some kind of problem somewhere.

[Digitalis]

The PDAF sensor in the camera body is supposed to report when focus is achieved. In the case of the 18-35mm, I'm not sure how the PDAF system could randomly be mis-reporting that the image is in focus, but apparently there is some kind of problem somewhere.

The problem being is that when brand name lenses, even other third party lenses from the same manufacturer focus perfectly fine is indicative that the lens itself is at fault.

[silver_dragon]

Long before this lens was ever available for Pentax, there were AF issues reported, especially with Canon bodies. While I don't remember the exact numbers, the very first review I read said that at f/1.8, the percentage of acceptably in focus shots was very low and I think well under 50%. I suspect that the people who are having no issues with this lens are probably landscape shooters who are happily shooting away at f/8. It is a very sharp lens stopped down and the focal length range make it ideal for landscape shooting. Either that or they have luckily beat the odds and got a good one.

My copy sucks with focusing at F1.8, it appears to be fine at any other aperture however. Its no big deal for me being that I mainly use that lens for car shows/meets. When i do wish to use it wide open flicking over to live view isnt really a hassle at all.

[gfurm]

There are some people who own copies of this lens that allegedly do not have any issues with using this lens with PDAF - call me prejudiced but with the increasing evidence of AF issues and their prevalence outside Pentax brand cameras I'm finding these positive reports extremely hard to believe.
0.

Why do you find it so hard to belive?
Mine really focuses OK, I don't have the dock and I didn't replace it 5 times. It just works. Maybe I was lucky?
One of the first quick and dirty tests: handheld, available light, f1.8, focused just above A using PDAF.
https://flic.kr/p/xRtmoi

It's just like Pentax SDM lenses. Would you say they're all lemons and will fail after 3 months? After all there are people who had them fail after few months so all SDM lenses must be duds following that logic. Do you find it hard to belive anyone got working SDM lens with all the reports about failures all over internet? And if not can you eplain why?

[sTi]

I think there are two possible problems here at work:

1. PDAF sensors cannot see how the focus actually is at f/1.8. Older cameras like K-5 have f/5.6 AF sensors, newer like K-5 II and K-3 have f/2.8 sensors (at least in the centre). If the lens in question exhibits focus shift (point of perfect focus shifts when stopping down) the camera has to guess where the focus will be at 1.8. The LensROM may(!) contain focus adjustment offsets that tell the camera in which direction and how far to compensate the focus to work around this problem based on the aperture used. I am not entirely sure, but I think if such a problem were present it would show itself in wrong (but consistent) focus.
2. Sloppy lens manufacturing. Background: I once tested three DA*55/1.4 lenses (which are also infamous for focus problems!) side-by-side with the same camera (a K-5 at the time). Two lenses showed erratic focus behavior that could not sensibly be corrected. The third lens focused perfectly everytime in any condition. I kept the third one (of course!), and it still keeps focusing perfectly with two other cameras (2x K-5 II). I suspect if lenses are put together with too little care to keep tolerances small (distance between elements, centering, tilted elements) you can run into situations where its behavior becomes unpredictable, especially when considering that lens elements are moved in relation to each other when focusing or zooming.

It's probably hard enough to build a fast f/1.4 or 1.8 prime lens that focuses reliably with PDAF systems. If you make a zoom of it, add reverse-engineered focusing algorithms and are not 100% perfect in assembling the lens (and who could pay for such rigorous manufacturing and quality control?), you are simply asking for trouble...

[Edgar_in_Indy]

The problem being is that when brand name lenses, even other third party lenses from the same manufacturer focus perfectly fine is indicative that the lens itself is at fault.

I get that. I was specifically responding to the person who was wondering why somebody would blame the body. I still find the problem baffling, considering how AF works in a DSLR.

I think there are two possible problems here at work:

1. PDAF sensors cannot see how the focus actually is at f/1.8. Older cameras like K-5 have f/5.6 AF sensors, newer like K-5 II and K-3 have f/2.8 sensors (at least in the centre). If the lens in question exhibits focus shift (point of perfect focus shifts when stopping down) the camera has to guess where the focus will be at 1.8. The LensROM may(!) contain focus adjustment offsets that tell the camera in which direction and how far to compensate the focus to work around this problem based on the aperture used. I am not entirely sure, but I think if such a problem were present it would show itself in wrong (but consistent) focus.
2. Sloppy lens manufacturing. Background: I once tested three DA*55/1.4 lenses (which are also infamous for focus problems!) side-by-side with the same camera (a K-5 at the time). Two lenses showed erratic focus behavior that could not sensibly be corrected. The third lens focused perfectly everytime in any condition. I kept the third one (of course!), and it still keeps focusing perfectly with two other cameras (2x K-5 II). I suspect if lenses are put together with too little care to keep tolerances small (distance between elements, centering, tilted elements) you can run into situations where its behavior becomes unpredictable, especially when considering that lens elements are moved in relation to each other when focusing or zooming.

It's probably hard enough to build a fast f/1.4 or 1.8 prime lens that focuses reliably with PDAF systems. If you make a zoom of it, add reverse-engineered focusing algorithms and are not 100% perfect in assembling the lens (and who could pay for such rigorous manufacturing and quality control?), you are simply asking for trouble...

I would love to see Pentax release a really nice K-mount mirrorless camera, which would do away with most of the PDAF pitfalls. However, from what I've heard, it would require lenses specifically designed for CDAF in order to get good focus speed, so it would probably never happen.