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Gear Talk => Lenses => Topic started by: Grummbeerbauer on August 25, 2013, 01:30:01 PM

Title: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Grummbeerbauer on August 25, 2013, 01:30:01 PM
Hi all,

I currently have an EOS 7D with a number of lenses, mainly Canon mid-range stuff (24-105L IS, 70-200L F4 IS, 17-55 2.8 IS, 100L 2.8 IS Macro, 85 1.8 ) and two Sigmas (30 1.4 EX (i.e., the old one), and the 10-20 3.5-5.6 EX).
I had always wanted a short FL prime lens which ended me up with the Sigma 30 1.4, but its lousy AF (both regarding precision and consistency) and soft images below f2.0 make me use it only on rare occasions where I can live with a low number of keepers.

I had been eying the new 18-35 1.8 since it was announced, even more so when the first rave reviews came in. Prime like quality and aperture in a zoom, simply amazing. Getting what basically seems to be a "variable FL prime lens" ;) I pulled the trigger when there was a good deal on Amazon recently (710€).

When it arrived, I was thrilled by the lens' haptics and feeling for quality, which is better than any of the Canon L lenses I own and just light years away from the Sigma 30 1.4 EX with its delicate "crinkle" finish and scratchy AF ring.

Focal-length-wise, while the 18-35 is definitely not an "always on lens", I found it to be very useful for a variety of situations.

I had no intentions to shoot any test charts, so I started testing it mainly indoors in mixed light with mainly static real-life subjects, which is my primary use case for such a lens. To my great disappointment, this particular copy on my particular body appears to exhibit a considerable frontfocus. So I printed an autofocus test chart (the one found at http://www.klein-bild.de/focus.htm (http://www.klein-bild.de/focus.htm) ) and did some test shots at 35mm and varying subject distances. I always set focus back to MFD or infinity before autofocusing again and did three test shots at each distance and AF adjustment setting. For each subject distance, I concluded the series with a shot focused manually with live view for comparison. 
The good news is that different from my 30 1.4 EX, autofocus was very consistent. But that's where the good news ended already.

It turned out that both at around 1 meter and at 0.5 meter subject distance, I needed to dial in a whooping +20 to get about the same level of sharpness that I got from MF (see screenshots below, the legend is in German, "Motivabstand" meaning "subject distance"). While I would not like the idea of having to do such a huge adjustment, I guess I could live with that.
However, at 0.35 meter subject distance, I got the best results at an adjustment setting somewhere between 0 to 10. That meant that at the +20 setting that I would need for the longer subject distances, I would be in the "barely acceptable" range of sharpness. Anyway, at that point it became pretty clear that even for a single  focal length, the single adjustment value my body offers me would not suffice.
Being tired of test shots for the moment, I then did some additional research and found the "dot tune" focus adjustment method (check out this video: DotTune: Autofocus fine tuning in under 5 minutes (http://www.youtube.com/watch?v=7zE50jCUPhM#ws) ), which seemed to make a lot of sense and promises much faster adjustments and does not require taking hundreds of test shots.
I tried it out at 35mm and 1,75m subject distance (as recommended for dot tune, 50 times FL). I found the range where I got a stable focus confirm to be between -3 and +16, which would indicate a correction setting of +6 or +7.
The big surprise was at 18mm and 0,9m subject, the range of stable focus confirmation only started (!) at a correction value of +17.
I could visually confirm both correction values with actual test shots.
A quick test on my trusty 450D basically confirmed the results on the 7D: Considerable front focus at 35mm which gets better at shorter subject distance, and a plain massive front focus at 18mm.

tl;dr

I found that the Sigma's AF is consistent at a certain focal length and subject distance, but would require different correction values for different FL/SD combinations. This gives me some hope that I could fix it with the Sigma USB Dock and some patience, but I am just not sure if I am willing to accept this.

So what should I do?
Keep it and get the USB dock? I have a week before my "no questions asked" return period expires.
Or send it back for good?
Any comments or similar experiences welcome.

Regards

Grummbeerbauer

Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: neuroanatomist on August 25, 2013, 02:46:32 PM
I don't fully believe the 'dot tune' method, I'd recommend FoCal or at least commercial tool like LensAlign.  In particular, contrast detect AF on the 7D has been shown to lack precision, so as the basis got 'dot tune' I wouldn't rely on it.

Regardless, it's certainly possible for a lens to require a different AFMA at the MFD vs. a reasonable distance away (although 20 units is a big difference!).  Then again, AF issues are Sigma's Achilles' heel. However, you might consider the Sigma Dock (which I'd assume is compatible with this lens, but I haven't checked).  Unlike the AFMA on the body, with their dock you can set different AFMA values for different distances, right in the lens (a great innovation by Sigma, IMO).
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: horshack on August 25, 2013, 03:35:58 PM
I don't fully believe the 'dot tune' method, I'd recommend FoCal or at least commercial tool like LensAlign.  In particular, contrast detect AF on the 7D has been shown to lack precision, so as the basis got 'dot tune' I wouldn't rely on it.

Contrast detect AF is not the basis of DotTune. As described in the video, either CDAF or manual focusing can be used, whichever method the photographer finds more convenient and/or accurate.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Policar on August 25, 2013, 04:29:27 PM
I have the same problem with mine on the 7D and Rebel XT.

Oddly, it's fine on the 5D Mark III.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Grummbeerbauer on August 25, 2013, 04:50:45 PM
I don't fully believe the 'dot tune' method, I'd recommend FoCal or at least commercial tool like LensAlign.  In particular, contrast detect AF on the 7D has been shown to lack precision, so as the basis got 'dot tune' I wouldn't rely on it.

I am also not fully convinced of the dot tune method... after all, in my case I had a range 20 (from -3 to +16) in which I got a stable focus confirmation... that is half of the AF adjustment range! And that was at 35mm and f1.8, i.e., the narrowest possible DoF with that lens.

Regardless, it's certainly possible for a lens to require a different AFMA at the MFD vs. a reasonable distance away (although 20 units is a big difference!).  Then again, AF issues are Sigma's Achilles' heel. However, you might consider the Sigma Dock (which I'd assume is compatible with this lens, but I haven't checked). 

I am indeed about two clicks away from ordering the USB dock, because I would really want this lens to work for me. Seems that for this lens, the Sigma software allows adjustments at 18, 24, 28 and 35mm and for subject distances of 0.28m, 0.35m, 0.5m and infinity, see the screenshot in the DPReview here http://www.dpreview.com/articles/4406844237/sigma-usb-dock-quick-review/2. (http://www.dpreview.com/articles/4406844237/sigma-usb-dock-quick-review/2.)
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Grummbeerbauer on August 25, 2013, 04:52:23 PM
In particular, contrast detect AF on the 7D has been shown to lack precision, so as the basis got 'dot tune' I wouldn't rely on it.

Contrast detect AF is not the basis of DotTune. As described in the video, either CDAF or manual focusing can be used, whichever method the photographer finds more convenient and/or accurate.

I used CDAF for initial focus and did some manual fine tuning. In any case, the precision was excellent.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Grummbeerbauer on August 25, 2013, 05:05:38 PM
I have the same problem with mine on the 7D and Rebel XT.

Oddly, it's fine on the 5D Mark III.

I presume that with "same problem" you are referring to the general need for different AF adjustments settings?

Interestingly, Roger Cicala at lensrentals.com had a four-part series of articles on autofocus precision, starting with http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy (http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy) (links to the subsequent articles are at the bottom). Very interesting read in general. In articles 3a and 3b he also found out that newer Canon bodies (from 5DIII onward) if combined with newer lenses focus considerably more accurately than older bodies (including the 7D).
That sort of confirms my own experience:I have never been too satisfied with the AF precision of my 7d anyway, it has always been hit and miss (could of course also be my technique, but what technique can be messed up when setting up the camera on a tripod and point it at a non-moving high contrast target?).

I think I will have to do some shots at daylight this week to see if the type of lighting also plays a role. Although a prime lens like this is of little use if it doesn't AF well in low or artificial resp. mixed light.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: horshack on August 25, 2013, 05:15:17 PM
I am also not fully convinced of the dot tune method... after all, in my case I had a range 20 (from -3 to +16) in which I got a stable focus confirmation... that is half of the AF adjustment range! And that was at 35mm and f1.8, i.e., the narrowest possible DoF with that lens.

The width of the confirmation range has no correlation to the accuracy of the VF confirmation since it's the midpoint of that range which is used for DotTune. The width of the range is intentionally padded by camera firmware to make it easier for those who rely on the VF confirmation for manual focusing - a narrower range would make it difficult if not impossible to quickly MF, particularly on lenses with short focus ring throws. Firmware applies padding in equal amounts to both sides of the tightest point of phase detection, thus the midpoint of that range is the apex of focus. On Canon bodies the amount of padding is the same for both AF and MF modes; on Nikon bodies the firmware pads the range by 2x when in MF to further assist those acquiring focus to the dot, which is why the video tells Nikon users to keep the lens in AF mode to assure that the confirmed range fits within the +/- 20 AF tuning range.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Policar on August 25, 2013, 06:22:52 PM
I have the same problem with mine on the 7D and Rebel XT.

Oddly, it's fine on the 5D Mark III.

I presume that with "same problem" you are referring to the general need for different AF adjustments settings?

Interestingly, Roger Cicala at lensrentals.com had a four-part series of articles on autofocus precision, starting with http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy (http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy) (links to the subsequent articles are at the bottom). Very interesting read in general. In articles 3a and 3b he also found out that newer Canon bodies (from 5DIII onward) if combined with newer lenses focus considerably more accurately than older bodies (including the 7D).
That sort of confirms my own experience:I have never been too satisfied with the AF precision of my 7d anyway, it has always been hit and miss (could of course also be my technique, but what technique can be messed up when setting up the camera on a tripod and point it at a non-moving high contrast target?).

I think I will have to do some shots at daylight this week to see if the type of lighting also plays a role. Although a prime lens like this is of little use if it doesn't AF well in low or artificial resp. mixed light.

By same problem I just mean it doesn't focus accurately or consistently; I think it's front focus. (I use the lens primarily for video.)

Fwiw, the 11-16mm Tokina I own similarly is less accurate on the 7D than on other cameras I've used it on.

Of course the one FF zoom I own that has focus issues (17-40mm L) focuses great on everything but the 5D Mark III. So does the Sigma 50mm f1.4 focus poorly on the 5D. Figures. :(
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Grummbeerbauer on August 27, 2013, 06:16:26 PM
The width of the confirmation range has no correlation to the accuracy of the VF confirmation since it's the midpoint of that range which is used for DotTune. The width of the range is intentionally padded by camera firmware to make it easier for those who rely on the VF confirmation for manual focusing - a narrower range would make it difficult if not impossible to quickly MF, particularly on lenses with short focus ring throws. Firmware applies padding in equal amounts to both sides of the tightest point of phase detection, thus the midpoint of that range is the apex of focus. On Canon bodies the amount of padding is the same for both AF and MF modes; on Nikon bodies the firmware pads the range by 2x when in MF to further assist those acquiring focus to the dot, which is why the video tells Nikon users to keep the lens in AF mode to assure that the confirmed range fits within the +/- 20 AF tuning range.

Interesting info, thanks for that.
It would be great if one could choose less padding for MF in some custom setting.
While I am spelling out wishes: It would be great if all lenses offered adjustment options like the new Sigma lenses and if it could be done on camera (maybe via USB connection to have the better UI of a desktop application).

And it would be even better if we could just set up camera and test target, type in the measures subject distance, and have the software trigger test shots (one done with CDAF for comparison), adjust AF settings in between, and figure out the optimal AF adjustment settings all by itself.  8)

Of course this can only be done by a camera system manufacturer, not for third party lenses... well, unless Sigma gets in contact with the MagicLantern guys to add their own Sigma auto-AF adjust plugin. If ML can more or less directly access the EF mount contacts to implement Sigma's protocol for doch-2-lens communication in software.

Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Grummbeerbauer on August 27, 2013, 06:27:30 PM
To finish the story of my copy of the 18-35: I asked a colleague (semi-pro photographer with lots of experience, not a pixel-peeping gearhead ;) ) to bring his backup 60D for testing. We tried it on both bodies in the same light on different targets at different subject distances and this basically confirmed what I observed on my 7D and on my 450D: At 35mm it is OK at close subject distances, way of at longer subject distances, and off by a mile at 18mm.

BTW: He had his Canon 50 1.4 on the 60D instead of a lens cap, so I tried it on my 7D: Focus was spot on. So it is definitely not an iffy AF on my 7D body.

Since I am not willing to bet on being able to fix these issues with the Sigma dock & software (and spending several hours of my precious free time in the process), the lens is going back to Amazon tomorrow. Since I am not yet willing to totally give up on this lens model, I asked for a replacement. Maybe I have better luck this time.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Pi on August 27, 2013, 07:43:58 PM
To finish the story of my copy of the 18-35: I asked a colleague (semi-pro photographer with lots of experience, not a pixel-peeping gearhead ;) ) to bring his backup 60D for testing. We tried it on both bodies in the same light on different targets at different subject distances and this basically confirmed what I observed on my 7D and on my 450D: At 35mm it is OK at close subject distances, way of at longer subject distances, and off by a mile at 18mm.

This is a proof that you have a genuine Sigma lens. I have experienced this with the few Sigma lenses I tried.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: kwirk on September 21, 2013, 09:43:32 PM
Registered here to post in this thread.  I have the exact same problems with my Sigma.  I tried resetting my 7D, deleted all my settings, everything.  This is my second copy of the Sigma already, the first copy had the same issues.  I'm going to buy the dock this time and see if that fixes it.  This lens is amazing when it works, so I don't want to give up on that.  I already lost $60 shipping my first copy back, which I could have used to buy the dock in the first place, so I figure I might as well just do it.  Might be useful if they update the firmware anyway.  I thought I was the only person having this issue.  Phew.  I also tried it on my 500D and it definitely has the same problem.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 22, 2013, 12:10:36 AM
i bought one a couple weeks ago and have the same problems it was front focusing on the wide end at close distance by about 6 or 8 inches but if you adjust for that with in camera afma then its way off at longer distance. its dead nuts on with the 70d and live view but i don't like having to use it like that. i have a usb dock but it will take a lot of fiddling to get it anywhere close to where it should be out of the box. i bought mine from digital rev and exchanging it is turning out to be a hassle so i am hoping there will be a firmware update. until then i would not recommend buying one unless you want to use it in live view mode or want to spend a lot of time trying to get the af right. 
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: kwirk on September 22, 2013, 08:28:23 AM
i bought one a couple weeks ago and have the same problems it was front focusing on the wide end at close distance by about 6 or 8 inches but if you adjust for that with in camera afma then its way off at longer distance. its dead nuts on with the 70d and live view but i don't like having to use it like that. i have a usb dock but it will take a lot of fiddling to get it anywhere close to where it should be out of the box. i bought mine from digital rev and exchanging it is turning out to be a hassle so i am hoping there will be a firmware update. until then i would not recommend buying one unless you want to use it in live view mode or want to spend a lot of time trying to get the af right.
So you have the USB dock?  Does the lens work now?
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 22, 2013, 05:22:34 PM
yes i have the dock and i have it to where when its on its on but it is still inconsistent especially when the light gets dim. i wouldn't go through all this bother if it wasn't such a great lens optically i will try and post some test shots, they are all in camera jpeg wide open center point focused. 
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 22, 2013, 05:26:02 PM
another
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 22, 2013, 05:27:25 PM
another
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 22, 2013, 05:28:45 PM
another
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 22, 2013, 05:30:25 PM
and another
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Kris J on September 27, 2013, 03:56:32 AM
Hi guys,

I also recently acquired this objective for my new 70D.

Out of the box, without pixel peeping, it seemed OK. However, upon close inspection of pictures taken it displayed considerable front focusing across the zoom range and at all subject distances...

I bought it together with the Dock so I did some quick calibration using the chart found here:

http://www.bobatkins.com/photography/technical/focus_testing.html (http://www.bobatkins.com/photography/technical/focus_testing.html)

Using a tripod of course and all other precautions, I tweaked the on camera MFA value until the image was perfectly focused for each focal length and subject distance (except for infinity). I thus checked 18, 20, 24, 28 and 35 at 0.28, 0.35 and 0.5 m, writing down MFA values for each.

I next fed these values into the Sigma Dock software and taking random pictures around the house, the objective seems pretty damned good for close range work now. Probably the MFA values could use some additional tweaking since I am an amateur, I do not have a well lit studio to create the perfect conditions for lens calibration but it is certainly good enough and for critical things there's always LiveView (which is superfast on the 70D anyway).

However, I then set out to test infinity... Having no large studio, I set out to just take pictures of license plates on the parked cars outside. I compared live view shots (using the zoom function there) with what I could get using the viewfinder.

This turned out to indicate massive from focus at all focal lengths.

Using the dock again, I fed values in the +15 range for all focal lengts at infinity. This drastically improved performance using the view finder on far away objects. However, there is a catch: the sigma dock has following subject distances in the correction table: 0.28, 0.35, 0.5 and finally infinity.

I found that, setting rather large MFA for infinity resulted in noticable influence for shots taken at 1 to 2 meters. If the MFA is high enough to get sharp long range shots, the shots at 1-2 meters would now be back focused, particularly at 35mm :s

I therefore resorted to dailing back the MFA at infinity for 35mm and using the on body MFA if I want to take long range shots to add some additional + correction. Like that I seem to get pretty good pictures most of the time but it certainly requires thinking and the objective sometimes still misses in low light. I guess there might be occasions where I miss a more candid shot because of all this.

From what I can tell, the objective is really nice, it certainly merits the rave reviews it is getting, it is really that sharp when in focus. Most people might not even notice slight focus inconsistencies but I m probably a stickler for details so I wanted it spot on and getting it there seems like a bit of a challenge... I think you cannot buy this thing without the dock.

By the way, for setting good infinity MFA values, what would any of you recommend? Is there a more reliable way than what I did? Focusing with viewfinder at 18mm on long range details is a bit tricky anyway because things get a bit tiny so it might be hard to tell if a soft image in these conditions is caused by poor technique or a bad setting...
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Pi on September 27, 2013, 10:26:57 AM
You would think that when you buy a lens, you also pay the manufacturer to tune the AF.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Kris J on September 27, 2013, 10:40:45 AM
Quote
You would think that when you buy a lens, you also pay the manufacturer to tune the AF.

You would think so, but then again, I don't know about your personal experience. However, from what I see with gear of some friends or gear that is used in my workplace to take photographs (including some 5DMIIIs with canon objectives), I think AF inconstancies are quite common.

Perhaps they stand out more on optics that are limited by the glass itself, such as the Sigma?

Also, did you read this?:

http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy (http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy)

Also, what is the alternative for this objective? The 17-55 f2.8 USM? It's more expensive and from what I can glean from the reviews, features more distortions at both ends of the range. What happens if AF performance is bad on your body? You send it back to have it calibrated? Maybe in the US, where shipping things is more common, this is an option but here in my part of Europe its even impossible to get a decent comment from any manufacturer Canon, Sigma or otherwise...

So yes, Having a lens that can be calibrated at home is nice to have...
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Pi on September 27, 2013, 10:54:13 AM
Quote
You would think that when you buy a lens, you also pay the manufacturer to tune the AF.

You would think so, but then again, I don't know about your personal experience. However, from what I see with gear of some friends or gear that is used in my workplace to take photographs (including some 5DMIIIs with canon objectives), I think AF inconstancies are quite common.

There are common but most of the time they are not nearly as bad as what you describe. Front/back focusing changing with the focus distance is relatively rare with Canon (I did experience it once, and the lens went to Canon the next day) but quite common with Sigma, both based on my experience and on posts in this thread. It is basically Sigma's signature - a way to recognize a Sigma lens with closed eyes.  :) Such a problem cannot be fixed with in camera MA. DPR reports that they did not have much success with the dock either.

Quote
Also, what is the alternative for this objective? The 17-55 f2.8 USM? It's more expensive and from what I can glean from the reviews, features more distortions at both ends of the range.

Actually, yes, in my experience. Never had an AF problem with that lens. You do not see the distortions (which are strong, indeed), since LR corrects them before you notice them. Also, it has what is considered a normal zoom range; a bit limited at the wide end by today's standards but then so is the Sigma's.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 27, 2013, 06:17:32 PM
at first it was front focusing by about 6 or 8 inches at close distances on the  wide end, that is pretty easy to check but its longer distance shots that are difficult to check.  this is what i had to do to mine, at first i tried to fix it with the in camera afma and that didn't work. i am not trying to bash them, its a fantastic lens optically and its built great, i just really don't understand how they can't do a better job of getting it to focus right? at first i thought that maybe because i was using it on a new camera (70d) that was the issue? but i tried it on my 40d and same thing? i know they have to reverse engineer their products and all that but they must have a few canon cameras around the shop they could check this with before they go out the door? do they have this same problem on their own bodies?   

p.s. at first i tried to get it close with the in camera afma and then fine tune it with the dock, don't try that, you will have an even bigger confusing mess.
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: tracingspirals on September 28, 2013, 09:47:37 AM
Hey I recently bought this lens and I registered here cuz my lens has been suffering from this problem too!

I did a semi scientific test of measuring the focus accuracy of the lens at 18mm, 24mm and 35mm focal lengths and at 113cm and 64cm focal distances. I also used the Canon 50mm f1.8 as a control lens. Each image was focused at the '0 line' on the scale. Here are the results:

18mm at 113cm:
(http://i.imgur.com/Uxcr0Ukh.jpg)
It doesn't even focus on the chart!

24mm at 113cm:
(http://i.imgur.com/XqpkSe1h.jpg)
Still a little off.

35mm at 113cm:
(http://i.imgur.com/OG40ayxh.jpg)
It's still focusing more than 3cm in front of the focus point.

Canon 50mm at 113cm:
(http://i.imgur.com/Dr3tscFh.jpg)
Surprisingly this is off by >3cm too!

Sigma 18mm at 64cm:
(http://i.imgur.com/YHO7Rfkh.jpg)
At least it's focusing on the chart.

24mm at 64cm:
(http://i.imgur.com/7erjDtOh.jpg)
Front focusing by about 1-2cm

35mm at 64cm:
(http://i.imgur.com/Nm35D8oh.jpg)
Front focusing by about 1cm

Canon 50mm at 64cm:
(http://i.imgur.com/mRE6c7Oh.jpg)
Off by only 1-2mm which is quite acceptable!


So now I have two options.
1. Return the lens back to the shop i bought it from and hope the replacement lens focuses better. Or
2. Buy the USB dock and have a go at fixing the focusing issue myself. Though with the focusing so bad at 18mm I don't know if the dock can fix it...

What do you guys think is the best option to go with here??
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 28, 2013, 10:52:13 AM
Hey I recently bought this lens and I registered here cuz my lens has been suffering from this problem too!

So now I have two options.
1. Return the lens back to the shop i bought it from and hope the replacement lens focuses better. Or
2. Buy the USB dock and have a go at fixing the focusing issue myself. Though with the focusing so bad at 18mm I don't know if the dock can fix it...

What do you guys think is the best option to go with here??

i would get the dock, i did and it worked on mine which was just as bad. i think this lens is the best normal zoom for aps-c on par with ef-s 60 for sharpness. i hate to say it but if you get an exchange i think you will have the same issues, maybe not as bad. maybe there are those who got this lens and it focuses fine but they are not posting about it? 
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: tracingspirals on September 28, 2013, 10:55:16 AM

i would get the dock, i did and it worked on mine which was just as bad. i think this lens is the best normal zoom for aps-c on par with ef-s 60 for sharpness. i hate to say it but if you get an exchange i think you will have the same issues, maybe not as bad. maybe there are those who got this lens and it focuses fine but they are not posting about it? 

Well where I am the lens seems to be selling like hot cakes so there are plenty of people buying this lens yet not everyone seems to be complaining about the focusing issue so maybe only the people who have faulty lenses are posting about it...
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: Mt Spokane Photography on September 28, 2013, 12:08:32 PM
Hey I recently bought this lens and I registered here cuz my lens has been suffering from this problem too!

I did a semi scientific test of measuring the focus accuracy of the lens at 18mm, 24mm and 35mm focal lengths and at 113cm and 64cm focal distances. I also used the Canon 50mm f1.8 as a control lens. Each image was focused at the '0 line' on the scale. Here are the results:


So now I have two options.
1. Return the lens back to the shop i bought it from and hope the replacement lens focuses better. Or
2. Buy the USB dock and have a go at fixing the focusing issue myself. Though with the focusing so bad at 18mm I don't know if the dock can fix it...

What do you guys think is the best option to go with here??
The test you ran is one that beginners use, and its a big mistake to expect that it means anything at all..
 
The reason is that the focus system likes horizontal lines and could lock onto anyone of them without your knowing it.  If your camera is not aligned with a high degree of accuracy, it will be even worse.  The pro test fixtures and methods were developed after trying all the slanted ruler methods and finding that they were not worth the trouble.
 
You must use a test method where the camera is forced to lock onto a known spot.  The professional methods do not try to lock focus onto a slanted surface, its just too unreliable.  Even with good systems, there is variability in a lens from shot to shot, so results are determined over many shots, resetting focus to the same starting point each time, and making sure the chart and camera sensor are perfectly aligned..
 
It is pretty easy to build a do it yourself fixture that will have the lens focus on a known spot, Neuro has posted one many times.
 
Try using Reikan Focal or spring for one of the commercial test fixtures. 
 
Here is the Lens Align fixture, you focus on the flat surface and view the slanted ruler to determine the AF accuracy.  There is a hole that you view a second hole that is set back.  When the sensor and the target are aligned squarely, you can see through both holes.  The back hole has red around it to help you get it right.
 
(http://www.mount-spokane-photography.com/Photography/Lens-Align-II/i-nLJmqx2/0/L/35mmL%20contrast%20Detect-1-L.jpg)
 
 
Focal also has a software tool that helps you align the target.  It produces a chart when finished that lets you see the effects of AFMA graphically, as well as saving the images of each shot.  I always do a lens AFMA on a new camera.  focal gives the shutter actuations from the camera as well.  Only 23 when I started this test.  This was a almost new 16-35 lens, and its exceptionally accurate.  Notice that one  point is way off.  That's typical, every lens will do this for one reason or another.
 
(http://www.mount-spokane-photography.com/Photography/FoCal/i-xJRx48k/0/L/120926_143504_FullyAuto_5Dmk3_62024006599_EF16-35mm%20f_2.8L%20II%20USM_16mm_Page_2-L.jpg)
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: candc on September 28, 2013, 12:41:49 PM
this is the diy setup i was using, you definitely do not want to focus on an angled surface
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: tracingspirals on September 28, 2013, 01:17:59 PM
Hmm, well when focusing on random objects at inifinity distance the lens is way off the mark too (which defeats the whole sharpness of the lens if it can't focus properly!). Even at f2.8 objects at infinity weren't in sharp focus. The only way of achieving sharp focus in this lens appears to be by using the live view focus which really isn't practical for everyday shooting.

I'm gonna return the lens on monday and make sure to test the replacement lens before buying it!
Title: Re: Frontfocusing Sigma 18-35 1.8 on 7D
Post by: justnice on October 28, 2013, 12:46:48 AM
Recently I also have the issue of Sigma 18-35mm lens cannot produce sharp image with my Canon 70D. I also try to perform the micro adjustment in the camera itself but still not very happy with the result might be due to the camera micro adjustment only have two setting per lens (W or T) therefore in some situation the output picture still not sharp enough. Until I purchased the Sigma USB dock to perform the micro adjustment now i am happy with the output for the lens :)

The USB dock allow you to adjust your lens with different focus length (if your lens is a zoom lens) and also based on the object distance away from the camera. You can see the image below showing the before (top portion) and after (bottom portion) effect of the fine tuning.

I had attached two jpg file, one is the before & after of the fine tune result and the other is the fine tune setting using the Sigma application with the USB dock. You can see the difference between the before and after fine tune really have a lot of difference in term of its sharpness. Hope this will help those who face the same issue as me. :)