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Author Topic: Frontfocusing Sigma 18-35 1.8 on 7D  (Read 11497 times)

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #15 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. 

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #15 on: September 22, 2013, 05:22:34 PM »

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #16 on: September 22, 2013, 05:26:02 PM »
another

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #17 on: September 22, 2013, 05:27:25 PM »
another

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #18 on: September 22, 2013, 05:28:45 PM »
another

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #19 on: September 22, 2013, 05:30:25 PM »
and another

Kris J

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #20 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

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...
« Last Edit: September 27, 2013, 04:01:12 AM by Kris J »

Pi

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #21 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.

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #21 on: September 27, 2013, 10:26:57 AM »

Kris J

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #22 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

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...

Pi

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #23 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.
« Last Edit: September 27, 2013, 10:56:49 AM by Pi »

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #24 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.
« Last Edit: September 27, 2013, 06:29:42 PM by candc »

tracingspirals

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #25 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:

It doesn't even focus on the chart!

24mm at 113cm:

Still a little off.

35mm at 113cm:

It's still focusing more than 3cm in front of the focus point.

Canon 50mm at 113cm:

Surprisingly this is off by >3cm too!

Sigma 18mm at 64cm:

At least it's focusing on the chart.

24mm at 64cm:

Front focusing by about 1-2cm

35mm at 64cm:

Front focusing by about 1cm

Canon 50mm at 64cm:

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??

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #26 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? 

tracingspirals

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #27 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...

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #27 on: September 28, 2013, 10:55:16 AM »

Mt Spokane Photography

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #28 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.
 

 
 
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.
 

candc

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #29 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

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Re: Frontfocusing Sigma 18-35 1.8 on 7D
« Reply #29 on: September 28, 2013, 12:41:49 PM »