[rdj92807]

First Day Testing New Sig 18-35
I just received a brand new Sigma 18-35. Tested it carefully with K-01 versus Pentax 20-40 Ltd, Pentax 20/2.8 FA and Pentax 16-50 DA*. I have good, pretty centered copies of all these lenses. I tested at 2.8, 4, 5.6, 8 where possible.
Here goes:
- Sigma 18-35 is optically superb across the frame. Amazing. At 20, the Pentax 20-40 is very close but the Sigma beats it just slightly in the extreme corners. My Pentax 20-40 did very well at 20 2.8. At 35, the Sigma was just a little sharper than my Pentax 20-40 overall.
- My Pentax 20 2.8 FA is a great small lens, and is full frame. The Pentax matched the Sigma at all f stops except 2.8, where the Sigma has just slightly better contrast.
- The Sigma 18-35 and Pentax 16-50 were pretty much equal in the centers at 18 and 35, but the Sigma wins out at the edges especially from 2.8 to 4.0. By a significant margin.
- The Sigma underexposes at 35 1.8 for some reason with some vignetting. All other settings superb.
- The Sigma AF is so quiet it can hardly be heard, if at all, when the K-01 is taking video. Excellent.

Regarding autofocus, listen up. On the Pentax K-01 v1.05, this lens is fantastic. The CDAF is pretty fast, very quiet AND EXTREMELY ACCURATE. I had not one out of focus pics out of hundreds taken today, under all kinds of light and conditions. Bravo to the K-01 which will probably sell like crazy now. I love the K-01 even more after this test, as without it, Pentax fans could not really use this lens.

HOWVEVER, USING PDAF ON MY PENTAX K-5, I HAD THE SAME ISSUES AS ADAM. EXACTLY. A DISASTER. SOME PICS WERE IN FOCUS, OFTEN THE CLOSER FOCUS SHOTS AND MACRO, BUT GET ANYWHERE NEAR INFINITY AND THE FOCUS DOES NOT WORK RELIABLY AT ALL. PERIOD. I MEAN MOST WERE OUT OF FOCUS, NOT A SMALL PERCENTAGE, LIKE 85% BAD. LOW LIGHT MAKES IT WORSE. And I adjusted the fine AF in my Pentax K-5. This is not the issue or cure. And the infocus shots seem totally random.

So this is a great lens on the K-01, or using the clumsy live view focus on a Pentax DSLR (good luck), but forget using it with PDAF.....until maybe a firmware update? I hope Sigma goes back to work this problem, which is clearly in the lens software (not optics) and has been reported with users of some other brands as well. Corrections with the Sigma dock may help, but I kind of doubt it.

[seventysixersfan]

I just went back to re-read the autofocus section of the official PF review and there is a new note at the bottom that says:

"Editorial note: due to the surprisingly-poor autofocus results we obtained with our first two test copies of this lens, we will be re-evaluating and comparing two additional copies over the course of the coming weeks."

Very interested to see what they find out with the re-test using more copies...

[rawr]

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.

I also have that big Sigma 70-200, and it AF's wonderfully, which is why I was puzzled about the AF behaviour of my big Sigma 35 f1.4. I agree - it's not AF motor inertia per se. It's an issue with the camera PDAF system - maybe the AF sub-system optics, the algorithms, the tolerances, the AF sensitivity (particularly below f2.8), the interfaces to other camera systems.

It's a pity Falconeye isn't as active around here as he used to be. He did some great work debugging the K-5 (and D800's) AF quirks in his lab. Only when someone is able to mount a Sigma 18-35 (or 35/1.4) on a K-3 and test it like Falk is able to will we get some really solid answers to what is going on.

[Digitalis]

t's a pity Falconeye isn't as active around here as he used to be. He did some great work debugging the K-5 (and D800's) AF quirks in his lab

I agree - if anyone on here could get to the bottom of this issue it would be him. I'm way too busy for that kind of patient Teutonic testing.

I was puzzled about the AF behaviour of my big Sigma 35 f1.4. I agree - it's not AF motor inertia per se. It's an issue with the camera PDAF system - maybe the AF sub-system optics, the algorithms, the tolerances, the AF sensitivity

I wouldn't rule inertial wobble out just yet. If it is a systemic issue with the AF then why aren't more people complaining about the faster ART primes? It seems to me the 18-35mm f/1.8 art seems to experience more problems with AF accuracy than any of the other ART series lenses put together. Also the fact that this lens is not exactly parfocal and has a very short focus throw isn't helping matters either.

[MadMathMind]

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...

What camera and firmware are you using?

The review suggests that nothing is ever in focus at any f stop (seems hard at f/8 and 18mm, considering DoF is basically infinite). I think if you were experiencing anything like the review's encounters, you'd know it immediately.

I have found focus to miss a fair amount, but I don't think it's a lot more often than my FA50 or F50 f/1.7 miss. A trick I use with both is to hit the AF button multiple times at low f. With the FA, I can hear the screw drive move a bit even after the initial focus confirmation. Selecting the AF point myself seems to help too, but I haven't really tested auto selection much; I don't really trust it to give me what I want and at large apertures, that will result in horribly misfocused images.

[UncleVanya]

I wouldn't rule inertial wobble out just yet. If it is a systemic issue with the AF then why aren't more people complaining about the faster ART primes? It seems to me the 18-35mm f/1.8 art seems to experience more problems with AF accuracy than any of the other ART series lenses put together. Also the fact that this lens is not exactly parfocal and has a very short focus throw isn't helping matters either.

The review on this site mentioned similar problems with the 30mm 1.4 art but not the 35mm 1.4 art which could be sample variation, but with multiple lenses tested in this review...

---------- Post added 11-10-14 at 11:27 PM ----------

First Day Testing New Sig 18-35
HOWVEVER, USING PDAF ON MY PENTAX K-5, I HAD THE SAME ISSUES AS ADAM. EXACTLY. A DISASTER. SOME PICS WERE IN FOCUS, OFTEN THE CLOSER FOCUS SHOTS AND MACRO, BUT GET ANYWHERE NEAR INFINITY AND THE FOCUS DOES NOT WORK RELIABLY AT ALL. PERIOD. I MEAN MOST WERE OUT OF FOCUS, NOT A SMALL PERCENTAGE, LIKE 85% BAD. LOW LIGHT MAKES IT WORSE. And I adjusted the fine AF in my Pentax K-5. This is not the issue or cure. And the infocus shots seem totally random.

"And by never, we mean that not once in the entire combination of five camera bodies and two lenses and two USB docks did we get a single in-focus shot unless we switched to Live View. "

At least you beat the review... 85% failure is better than 100%.

[JinDesu]

I always thought the older Sigma 30 F1.4 had focus issues with regards to inconsistency as well. Maybe it's a certain type of optical formula with regards to how phase detect AF (specifically interaction of the lens transmitted image with the beam splitter in the PDAF mechanism)?

[Enrique S Toso]

Same problem in Nikon. Here the link to Nikon Rumors.

Sigma 18-35 1.8 Focus Problem - only in view finder focus - Other Manufacturers Discussions on Nikon Rumors Forum

Looks like it is "something" with newer cameras, and the last lenses coming out of the factory. All reviews made when the lens come to the market, doesnt have this problem.

This is just sad, since this lens was my last excuse to stay in APSC format .

Please Sigma, solve it before the end of the month!!

[Digitalis]

Please Sigma, solve it before the end of the month

not a chance.

[JimmyDranox]

This is just sad, since this lens was my last excuse to stay in APSC format .

Please Sigma, solve it before the end of the month!!

Translating the focal and aperture, Sigma 18-35mm F1.8 is equal to a 27-52.5mm F2.7 lens for FF. And I think that you can find some good 24-70mm F2.8 lenses, if you want to change to FF

[AtitG]

When pentax rated their PDAF sensors at 2.4. I guess they already knew that their manufacturing tolerances would only create a system that will work reliably with lenses 2.4 and slower. I am under the impression that the sigma lens will have focusing issues since all AF happens at 1.8, and it only closes its aperture after AF.

[UncleVanya]

When pentax rated their PDAF sensors at 2.4. I guess they already knew that their manufacturing tolerances would only create a system that will work reliably with lenses 2.4 and slower. I am under the impression that the sigma lens will have focusing issues since all AF happens at 1.8, and it only closes its aperture after AF.

But the sigma 30mm 1.4, Pentax FA 31mm 1.8, etc work fine.

---------- Post added 11-11-14 at 12:19 PM ----------

Translating the focal and aperture, Sigma 18-35mm F1.8 is equal to a 27-52.5mm F2.7 lens for FF. And I think that you can find some good 24-70mm F2.8 lenses, if you want to change to FF

The f stop conversion only applies to the depth of field as I understand it. The speed of the lens - the low light performance - it's the same at the same f stop without taking sensor size into account.

[rdj92807]

>HOWVEVER, USING PDAF ON MY PENTAX K-5, I HAD THE SAME ISSUES AS ADAM. EXACTLY. A DISASTER. SOME PICS WERE IN FOCUS, OFTEN THE CLOSER FOCUS SHOTS AND MACRO, BUT GET ANYWHERE NEAR INFINITY AND THE FOCUS DOES NOT WORK RELIABLY AT ALL. PERIOD. I MEAN MOST WERE OUT OF FOCUS, NOT A SMALL PERCENTAGE, LIKE 85% BAD. LOW LIGHT MAKES IT WORSE. And I adjusted the fine AF in my Pentax K-5. This is not the issue or cure. And the infocus shots seem totally random.<

OK, I have had several more days to experiment with my new Sigma 18-35 on my Pentax K-5. Good news. Clearly, this lens is VERY sensitive to FINE AF adjustment on the K-5. If this is off, the AF is really bad. I was able to take a lot of test shots, and readjusted the Fine AF settings on my K-5 after a lot of testing, and the keeper rate has improved dramatically. Even shooting at 1.8, using PDAF, I am getting pretty decent hit rates. It is still missing once in a while, but overall not too bad. I am verifying all this by shooting Live View and then PDAF for every shot.

So I am now inclined to keep the lens, get a dock, and finish adjusting settings even better. Clearly CDAF is a no-stress option (I love the K-01), and I may never totally trust PDAF on this lens, but I think it may work out after all. I would be very interested in hearing other owner experiences and I also wonder if Adam played with the Fine AF adjustments on the K-3s.

[Enrique S Toso]

Great news, this is what we all hope, that the problem can be solved with the USB dock and in camera AF settings.

I dont know why they guys in the PF review couldnt fix the issue with the dock.

Im buying it, plus a K3, and some cactus V6. Lets show the world you can be a pentaxian pro wedding ph in southamerica .

[rdj92807]

Hi Enrique. Please post your experience with this lens when you get it. Thanks and good luck.