[Jannis]

As I rebuilt my kit I have decided (after short bad experience with Sigma 18-35 1.8) to replace zoom in short focal lengths with primes.
I get HD DA21ltd for landscapes and I'm very satisfied. But I need to cover gap between 21mm and 50mm on my DA*50-135. So I decided to buy Sigma 30 1.4 A, for low-light capabilities (only fast pentax in that FL is FA limited 31, more than $1600 here) and maybe later DA40ltd or XS.

I have Sigma 30 1.4 A only 2 days, but I encounter strange AF behavior. Im not sure that it focuses same on different focusing distances.
It focuses differently under day light and under artifical. And it focuses diferently even under same condition - I made test with tripod - and from 10 tries only lets say 7 is precise, one almost and two off. I changed nothing. Only turn focusing ring and let AF done its job again... I'm confused. That confirms my thoughts from field - sometimes nice sharp shots, but often "some soft" and sometimes absolutly off. Also consider that if focus is not really nailed, aberation tends to show up and makes worse overall IQ.

What I shall do? Its brand new. It is possible that in repair center they can calibrate it to be precise? Or they can only calibrate it to focus right on every focusing distance, but they do nothing with this inconsistency?

Do you think, that I shall try to return it?
And buy 35ltd or 40ltd instead of it?

Thanks for your advices.

EDIT: I'm using k-5ii

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Last edited by Jannis; 10-05-2014 at 08:05 AM.

[panoguy]

What camera are you using?

The lens has no AF module to tell it what to focus on, only a motor and electrical contacts to take what the camera gives it and move the elements. Sometimes the lens takes the information and doesn't move the elements enough (the 18-35 has this problem sometimes), but most often it is the camera that is sending the "wrong information" (but again, the camera doesn't know what you want, only what is most contrasty and vertical in the scene).

Pentax cameras earlier than the K-3 all have focus points that are huge in comparison to other brands, too, so what you see as "to be focused on" and something nearby might be inside the large cross that the AF module uses to detect a focus point. Also, the little red squares in Pentax viewfinders are nowhere near the size of the focus zone, they are only indicators of which focus point is being used. Pentax still has a long way to go in AF development, even after the K-3 and its incredible improvements.

Next, if you go for a lens with a smaller aperture (f/2.8 vs f/1.4) and less rotation needed to get things in focus (DA40), then the AF will appear to be better, but that is just physics and mechanics helping the camera operator along. That said, an f/1.4 lens will help the AF in low light because it lets more light in, regardless of what aperture is used for the photo (AF is done with the aperture completely open).

EDIT: you're using a K-5ii, and the AF bothers you, get a DA40. It will "help" your camera along by being fast to move the elements, and the max aperture of f/2.8 has a larger in-focus area (less bokeh, OOF), so you are less likely to see softness when the camera focus on something other than what your brain wants. Stop it down to f/3.2 and you'll be quite happy. The DA35 macro is a bad choice if you want fast AF (long throw macro focusing mechanics), but it has probably the best optics of any Pentax lens, IMHO (having owned all the FA Ltds, and a bunch of DA and FA primes).

[Jannis]

What camera are you using?

Im using K-5ii.

Thanks for comments. But I know that AF sensor is larger than red square so I made test in condition that AF cannot be conused by anything in that area. I must say that no Pentax lens that I ever try do it. Only Sigma 18-35 behave same (but much worse).
Here is one sample. f1.4 / 1/640. This is crop from part just up from center. AF on center spot . And its evident, that focus is somewat behind the wall but ahead of fence. This is one of worse of my test shots. Often it is not as bad . It off only slightly- but it is even more dangerouse, becouse I cannot spot it on camera - and I have ugly suprise at home

[panoguy]

Got it. Thanks for the sample! AF is a tricky thing to design, with a complex system of factors in each camera, so the problems are tough to understand... too often people blame the camera or the lens before acknowledging that it could have been user error. In your case, you know what you're doing in your tests, so it is almost certainly the lens!

What is weird is that the EXIF says "focus point = distant" but the aberrations show that you are right, the focus is somewhere between the wall (purple CA) and the fence (green CA). At 30mm, it would be weird to see this as "distant" (near infinity). I'm not even sure the camera knows where the focus is!

If the lens needed to be calibrated to the camera (like most lenses), then the error would be consistent, and either Sigma could adjust the lens, or you could adjust the AF in your camera for that lens. However, if there is miscommunication between the lens and camera (what this looks like, but I'm thousands of km away from the it all), then the lens is faulty.

Will a different Sigma 30/1.4 fix the problem? I have no idea. Most likely, given the hints at your experience with the 18-35/1.8 (which I have, and love, but not on Pentax), you should return the 30mm and get either a used FA28mm (cheap, same focal length), or DA40 (sharp, fast, closer to 50mm). I have many Sigma lenses, and while they have their quirks, none are frustratingly bad. Of course, what I don't have is a 30/1.4 Art, or even the earlier version.

[Jannis]

Thanks for your thoughts. I appreciate your experienced perspective.
If it was consistency BF or FF problem - it was ok. I will calibrate my microadjustment in camera. But this is quite odd. It is just unreliable. I just done another test. Problem is - when I hit AF button it gets to certain af position. And (nothing changed) when I hit AF button again - it change its af position. And again and again. Usually it travels between two points (in about 0,5mm - 1mm distance on focus scale), which one is right and second is miss. Sometimes it gets absolutely way off, but not very often. And everytime is focus confirmed by beep and icon... I'm quite dissapointed - Just spent about half my sallary and cannot to get consistently sharp picture - so I apologies for my "grumbling".
i found this in rewiev here:

Before we move on, we unfortunately have to report some bad news about the autofocus performance of this lens. While using it in the field, in the studio, and in outdoor tests, the lens did not consistently deliver reliable results.
...
We suspect that the latter is more likely, as reviews of the Canon and Nikon versions of this lens have been favorable. Thus, a lens firmware update might solve the AF problem in the future.

I must say that this issue is persistent



I'm almost decided to return this lens (I'm affraid that it would be difficult, because they filled in warranty card - against my request). If they refuse, I must go thru 30 day warranty period...
In past I had opportunity to test 35ltd and I was very excited about it. What do you think about combo:
21ltd (I already have)
35ltd
40xs for fast AF needs? Here is 40XS about $190 cheaper then DA40.

?

[THoog]

What focus mode are you using? I had similar AF problems with the Art 30 on my K-5II and K-30 in auto-select mode, but it's fairly reliable in center-point and select-point mode. I prefer center-point focusing anyhow.

[Kozlok]

I have the old Sigma EX-DG. It focuses very reliably. Only available used now, though.

[Jannis]

What focus mode are you using? I had similar AF problems with the Art 30 on my K-5II and K-30 in auto-select mode, but it's fairly reliable in center-point and select-point mode. I prefer center-point focusing anyhow.

Center or select. I don't use auto.

[MadMathMind]

Also, the little red squares in Pentax viewfinders are nowhere near the size of the focus zone, they are only indicators of which focus point is being used.

Also, the "Use Expanded AF Area" is enabled by default, I believe. If it is checked, you can try unchecking it. I have not played with this setting personally, but it may help.

[VisualDarkness]

That really looks weird, maybe it would be worth to send it in to Pentax.

[Jannis]

That really looks weird, maybe it would be worth to send it in to Pentax.

But I have no problem with pentax lenses. Only with third party. To some degree with Tamron 17-50 (but there it is BF/FF cross focal lenghts) worst with Sigma 18-35 and now with this.

[Adam]

As I rebuilt my kit I have decided (after short bad experience with Sigma 18-35 1.8) to replace zoom in short focal lengths with primes.
I get HD DA21ltd for landscapes and I'm very satisfied. But I need to cover gap between 21mm and 50mm on my DA*50-135. So I decided to buy Sigma 30 1.4 A, for low-light capabilities (only fast pentax in that FL is FA limited 31, more than $1600 here) and maybe later DA40ltd or XS.

I have Sigma 30 1.4 A only 2 days, but I encounter strange AF behavior. Im not sure that it focuses same on different focusing distances.
It focuses differently under day light and under artifical. And it focuses diferently even under same condition - I made test with tripod - and from 10 tries only lets say 7 is precise, one almost and two off. I changed nothing. Only turn focusing ring and let AF done its job again... I'm confused. That confirms my thoughts from field - sometimes nice sharp shots, but often "some soft" and sometimes absolutly off. Also consider that if focus is not really nailed, aberation tends to show up and makes worse overall IQ.

What I shall do? Its brand new. It is possible that in repair center they can calibrate it to be precise? Or they can only calibrate it to focus right on every focusing distance, but they do nothing with this inconsistency?

Do you think, that I shall try to return it?
And buy 35ltd or 40ltd instead of it?

Thanks for your advices.

EDIT: I'm using k-5ii

I don't want to over-generalize, but this lens does have AF accuracy issues. The problem can be in part remedied using the Sigma USB dock, but I don't think there's a definitive solution. Sigma might be able to fix the issue via a lens firmware update.

I would recommend that you contact Sigma to see what your options are. If they don't have a fix or say the lens is working as intended, I would just return it and either get a 35mm F1.4 or one of the Limiteds.

[Jannis]

Thanks all for your help.
Lens is returned and I have time to think about wich lens will complement my ltd 21 in short to normal FL. I have in mind DA40 or 35ltd and I can't decide...