Sigma 18-35 f/1.8 Art Focus Issues Thread

MadMathMind · 11-12-2014, 05:37 PM

Originally posted by 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.

Large apertures are always better for autofocus systems, as there's more light available. As pointed out, the FA Limiteds don't have major issues like this and those are all 1.8 (or 1.9 for the FA43) as well.

As I pointed out above, the f-rating has more to do with the limitations of precision of the AF system than anything else. The system just can't guarantee that it can focus to within the DoF of the low f-stop so it can't guarantee that it will focus correctly. Something in the image should be in focus, just not probably what you aimed at.

**Digitalis** · 11-12-2014, 06:42 PM

Originally posted by MadMathMind

Large apertures are always better for autofocus systems, as there's more light available. As pointed out, the FA Limiteds don't have major issues like this and those are all 1.8

Usually large apertures are good for AF systems however in the case of an f/1.8 zoom it doesn't seem to help at all. The FA limited lenses aren't zooms, primes are better suited to the AF calibrations on Pentax DSLRS - the Sigma lens dock allows for AF adjustments at different focal lengths

rawr · 11-13-2014, 05:25 PM

Originally posted by MadMathMind

Large apertures are always better for autofocus systems ...

Not always. While large apertures may pass more light, they can often deliver other problems - including greater softness and CA. All of which won't make AF better.

The corrective optics within the AF module of the camera can try and deal with some of those problems, of course. But it would be better if they weren't there in the first place. Pic related - the K-3's AF module.

The Sigma 18-35 seems to be well corrected for most forms of CA and is pretty sharp wide-open, so the issue of lens softness and CA disturbing AF may not apply to it that much.

MadMathMind · 11-13-2014, 05:37 PM - **1 Like**

I am starting to see the issues with focus. I don't see them past 2.8, probably because there's enough DoF to cover the misses. Even at f/2.2 or f/2.5, I don't see a huge percentage of misses, at least not terribly more so than what I saw with my F50 f/1.7 (the FA50 f/1.7 seems to be quite an improvement, though). I just never really shot it much at f/1.8 until this past week, where I used it quite a bit more.

Still, I have the dock coming from Japan now, so I can use it to take a look at my lens. I definitely get a lot of in focus shots even at f/1.8, so my issue isn't nearly as bad as what the review says, but it is a high percentage of misses. Maybe half?

Stagnant · 11-14-2014, 10:33 AM

I had focus issues at F4. It was not front or back focus. The whole scene was just a tiny bit out of focus. Strangely enough, recently I had almost no issues with focusing.

Download the images to see them at 100%.

**Dice** · 11-14-2014, 01:20 PM

Originally posted by Stagnant

I had focus issues at F4. It was not front or back focus. The whole scene was just a tiny bit out of focus. Strangely enough, recently I had almost no issues with focusing.

Download the images to see them at 100%.

The images you've posted and viewed at 100% appear to show unusual focus across the images. IMGP7844 appears to have front focused by quite a bit as the water in front of the ship is in crisper focus that than the ship. And the right side seems softer than the left side. IMGP7766 is harder to decipher. The sharper center is to be expected for most lenses though the differences on the sides would be something to explore. The right side seems to be weaker than the left. You might want to do some more testing to see if you have a decentering issue.

Stagnant · 11-14-2014, 02:34 PM

Originally posted by Dice

The images you've posted and viewed at 100% appear to show unusual focus across the images. IMGP7844 appears to have front focused by quite a bit as the water in front of the ship is in crisper focus that than the ship. And the right side seems softer than the left side. IMGP7766 is harder to decipher. The sharper center is to be expected for most lenses though the differences on the sides would be something to explore. The right side seems to be weaker than the left. You might want to do some more testing to see if you have a decentering issue.

Speaking about focusing issues, I found it to be more reliable at F1.8 than at F4. Haven't experience focus quirks recently.
I did notice some decentering on my copy. I had no chance to get it serviced, because I needed it badly.

**Digitalis** · 11-14-2014, 05:34 PM

Originally posted by Dice

You might want to do some more testing to see if you have a decentering issue.

I had the same problem - the de-centering was slight and was exacerbated by the AF inaccuracies of the lens, that the official repairer in my country refused to do anything to my lens. They "tested" my 18-35mm f/1.8 on a pentax K7 handheld at f/8 ISO 400. Sigma QC strikes again

The bottom line is, that there are things that can go wrong with the production of these lenses.

s76 · 11-16-2014, 01:23 AM

Originally posted by Digitalis

Usually large apertures are good for AF systems however in the case of an f/1.8 zoom it doesn't seem to help at all. The FA limited lenses aren't zooms, primes are better suited to the AF calibrations on Pentax DSLRS - the Sigma lens dock allows for AF adjustments at different focal lengths

Hi Digitalis.

I have now calibrated three first distance with good results. What distance did you use for last point? Marking is near 2m, but how it will work to 10 or 50 meters?

Sami

**Digitalis** · 11-16-2014, 02:14 AM

Originally posted by s76

I have now calibrated three first distance with good results. What distance did you use for last point? Marking is near 2m, but how it will work to 10 or 50 meters?

I used a high precision calibration chart to find the point of optimal focus at all the set distances. if you get the calibration right at 2m and infinity everything between those points should be reasonably accurate.

s76 · 11-16-2014, 05:04 AM

Originally posted by Digitalis

I used a high precision calibration chart to find the point of optimal focus at all the set distances. if you get the calibration right at 2m and infinity everything between those points should be reasonably accurate.

Thanks!

May I ask how did you infinity calibration test. I am wondering about distance, or did you use multiple distances. Or is it better to do with comparing af with live view af? My lens is now good to 2 meters, but infinity focus is not so good.

I know that it's hard to focus infinity right at f1.8 for lens, but I have taken great and sharp infinity photos with live view. I just want to do same via view finder.

Sami

Enrique S Toso · 11-26-2014, 07:49 PM

Any news with the af issue? Is it a hardware production problem? Where can i test and buy this lens in fort worth or Dallas region in Texas?

Ty!

**Digitalis** · 11-27-2014, 09:07 AM

Originally posted by s76

Or is it better to do with comparing af with live view af? My lens is now good to 2 meters, but infinity focus is not so good.

That would be one way of doing it. I actually did it because in AF mode the lens would undershoot infinity, it was really simple of figuring out how much I needed to correct it*.

Originally posted by Enrique S Toso

Any news with the af issue? Is it a hardware production problem?

The focus issues still persist, no hardware or software solution has been offered by sigma for any lens mount.

So my advice is: if you wish to buy this lens I strongly recommend getting the lens dock along with it.

* I live near the beach, so infinity focus subjects are very easy to find.

Enrique S Toso · 12-23-2014, 05:03 AM

Sorry for insist...but...any news or rumor about a solution for this problem?? Ty!!

**Digitalis** · 12-23-2014, 05:30 AM

Nope - Sigma hasn't made any statements regarding focus accuracy and this particular lens.

11-13-2014, 05:25 PM	#63
rawr Veteran Member Join Date: Jun 2009 Posts: 11,913	Originally posted by MadMathMind Large apertures are always better for autofocus systems ... Not always. While large apertures may pass more light, they can often deliver other problems - including greater softness and CA. All of which won't make AF better. The corrective optics within the AF module of the camera can try and deal with some of those problems, of course. But it would be better if they weren't there in the first place. Pic related - the K-3's AF module. The Sigma 18-35 seems to be well corrected for most forms of CA and is pretty sharp wide-open, so the issue of lens softness and CA disturbing AF may not apply to it that much. Last edited by rawr; 11-13-2014 at 05:36 PM.

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11-12-2014, 05:37 PM	#61
MadMathMind Veteran Member Join Date: Dec 2013 Location: Houston, TX Photos: Gallery Posts: 1,717 Original Poster	Originally posted by 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. Large apertures are always better for autofocus systems, as there's more light available. As pointed out, the FA Limiteds don't have major issues like this and those are all 1.8 (or 1.9 for the FA43) as well. As I pointed out above, the f-rating has more to do with the limitations of precision of the AF system than anything else. The system just can't guarantee that it can focus to within the DoF of the low f-stop so it can't guarantee that it will focus correctly. Something in the image should be in focus, just not probably what you aimed at.

11-12-2014, 06:42 PM	#62
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	Originally posted by MadMathMind Large apertures are always better for autofocus systems, as there's more light available. As pointed out, the FA Limiteds don't have major issues like this and those are all 1.8 Usually large apertures are good for AF systems however in the case of an f/1.8 zoom it doesn't seem to help at all. The FA limited lenses aren't zooms, primes are better suited to the AF calibrations on Pentax DSLRS - the Sigma lens dock allows for AF adjustments at different focal lengths

11-13-2014, 05:37 PM - 1 Like	#64
MadMathMind Veteran Member Join Date: Dec 2013 Location: Houston, TX Photos: Gallery Posts: 1,717 Original Poster	I am starting to see the issues with focus. I don't see them past 2.8, probably because there's enough DoF to cover the misses. Even at f/2.2 or f/2.5, I don't see a huge percentage of misses, at least not terribly more so than what I saw with my F50 f/1.7 (the FA50 f/1.7 seems to be quite an improvement, though). I just never really shot it much at f/1.8 until this past week, where I used it quite a bit more. Still, I have the dock coming from Japan now, so I can use it to take a look at my lens. I definitely get a lot of in focus shots even at f/1.8, so my issue isn't nearly as bad as what the review says, but it is a high percentage of misses. Maybe half?

11-14-2014, 10:33 AM	#65
Stagnant Veteran Member Join Date: Dec 2013 Photos: Gallery Posts: 603	I had focus issues at F4. It was not front or back focus. The whole scene was just a tiny bit out of focus. Strangely enough, recently I had almost no issues with focusing. Download the images to see them at 100%.

11-14-2014, 01:20 PM	#66
Dice Site Supporter Join Date: Feb 2008 Location: Emeryville, CA Posts: 1,122	Originally posted by Stagnant I had focus issues at F4. It was not front or back focus. The whole scene was just a tiny bit out of focus. Strangely enough, recently I had almost no issues with focusing. Download the images to see them at 100%. The images you've posted and viewed at 100% appear to show unusual focus across the images. IMGP7844 appears to have front focused by quite a bit as the water in front of the ship is in crisper focus that than the ship. And the right side seems softer than the left side. IMGP7766 is harder to decipher. The sharper center is to be expected for most lenses though the differences on the sides would be something to explore. The right side seems to be weaker than the left. You might want to do some more testing to see if you have a decentering issue.

11-14-2014, 02:34 PM	#67
Stagnant Veteran Member Join Date: Dec 2013 Photos: Gallery Posts: 603	Originally posted by Dice The images you've posted and viewed at 100% appear to show unusual focus across the images. IMGP7844 appears to have front focused by quite a bit as the water in front of the ship is in crisper focus that than the ship. And the right side seems softer than the left side. IMGP7766 is harder to decipher. The sharper center is to be expected for most lenses though the differences on the sides would be something to explore. The right side seems to be weaker than the left. You might want to do some more testing to see if you have a decentering issue. Speaking about focusing issues, I found it to be more reliable at F1.8 than at F4. Haven't experience focus quirks recently. I did notice some decentering on my copy. I had no chance to get it serviced, because I needed it badly.

11-14-2014, 05:34 PM	#68
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	Originally posted by Dice You might want to do some more testing to see if you have a decentering issue. I had the same problem - the de-centering was slight and was exacerbated by the AF inaccuracies of the lens, that the official repairer in my country refused to do anything to my lens. They "tested" my 18-35mm f/1.8 on a pentax K7 handheld at f/8 ISO 400. Sigma QC strikes again The bottom line is, that there are things that can go wrong with the production of these lenses.

11-16-2014, 01:23 AM	#69
s76 Junior Member Join Date: Aug 2012 Location: Nokia, Finland Posts: 25	Originally posted by Digitalis Usually large apertures are good for AF systems however in the case of an f/1.8 zoom it doesn't seem to help at all. The FA limited lenses aren't zooms, primes are better suited to the AF calibrations on Pentax DSLRS - the Sigma lens dock allows for AF adjustments at different focal lengths Hi Digitalis. I have now calibrated three first distance with good results. What distance did you use for last point? Marking is near 2m, but how it will work to 10 or 50 meters? Sami

11-16-2014, 02:14 AM	#70
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	Originally posted by s76 I have now calibrated three first distance with good results. What distance did you use for last point? Marking is near 2m, but how it will work to 10 or 50 meters? I used a high precision calibration chart to find the point of optimal focus at all the set distances. if you get the calibration right at 2m and infinity everything between those points should be reasonably accurate.

11-16-2014, 05:04 AM	#71
s76 Junior Member Join Date: Aug 2012 Location: Nokia, Finland Posts: 25	Originally posted by Digitalis I used a high precision calibration chart to find the point of optimal focus at all the set distances. if you get the calibration right at 2m and infinity everything between those points should be reasonably accurate. Thanks! May I ask how did you infinity calibration test. I am wondering about distance, or did you use multiple distances. Or is it better to do with comparing af with live view af? My lens is now good to 2 meters, but infinity focus is not so good. I know that it's hard to focus infinity right at f1.8 for lens, but I have taken great and sharp infinity photos with live view. I just want to do same via view finder. Sami

11-26-2014, 07:49 PM	#72
Enrique S Toso Senior Member Join Date: Jan 2014 Location: Mendoza - Argentina Posts: 141	Any news with the af issue? Is it a hardware production problem? Where can i test and buy this lens in fort worth or Dallas region in Texas? Ty!

11-27-2014, 09:07 AM	#73
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	Originally posted by s76 Or is it better to do with comparing af with live view af? My lens is now good to 2 meters, but infinity focus is not so good. That would be one way of doing it. I actually did it because in AF mode the lens would undershoot infinity, it was really simple of figuring out how much I needed to correct it. Originally posted by Enrique S Toso Any news with the af issue? Is it a hardware production problem? The focus issues still persist, no hardware or software solution has been offered by sigma for any lens mount. So my advice is: if you wish to buy this lens I strongly* recommend getting the lens dock along with it. * I live near the beach, so infinity focus subjects are very easy to find.

12-23-2014, 05:03 AM	#74
Enrique S Toso Senior Member Join Date: Jan 2014 Location: Mendoza - Argentina Posts: 141	Sorry for insist...but...any news or rumor about a solution for this problem?? Ty!!

12-23-2014, 05:30 AM	#75
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	Nope - Sigma hasn't made any statements regarding focus accuracy and this particular lens.