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Thread started 30 Apr 2015 (Thursday) 15:09
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Baffling focus issue with Canon EF-S 17-55mm/2.8

 
gjl711
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Jun 27, 2015 13:24 |  #181

Archibald wrote in post #17612315 (external link)
....
When I do this with my 17-55mm, the results are chaotic. Sometimes the lens behaves perfectly without a flaw, and does so repeately. Other times the scale bounces around between the 3 meter mark and the infinity mark. Sometimes it even goes below 3 meters. I don't understand what factors cause it to behave or misbehave. ....

I have actually had a camera do thins in the past. Can't remember if it was my 40D or 50D but the symptom was exactly the same. The reason it does that is because the AF chip did not find a decent high contrast edge to use as the focus area. In my cameras case, it turned out that I had a hair on my secondary mirror and I guess it just so happen to land as to be obscuring the center focus point. I blew the hair away and the issue went away but in your case, there might be something different that is messing with the AF sensor not allowing it to lock on to a high contrast edge.


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Jun 27, 2015 13:30 |  #182

gjl711 wrote in post #17612428 (external link)
I have actually had a camera do thins in the past. Can't remember if it was my 40D or 50D but the symptom was exactly the same. The reason it does that is because the AF chip did not find a decent high contrast edge to use as the focus area. In my cameras case, it turned out that I had a hair on my secondary mirror and I guess it just so happen to land as to be obscuring the center focus point. I blew the hair away and the issue went away but in your case, there might be something different that is messing with the AF sensor not allowing it to lock on to a high contrast edge.

Canon has acknowledged to me privately that it is a known issue with this lens.


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Jun 27, 2015 15:53 |  #183

I don't understand the technology enough to understand why this would be a problem with the lens vs the body.

My assumption is that the lens has a motor that moves the focus element group back-and-forth in response to commands from the body. The processor in the body analyzes the image from the focus sensors and sends commands to the lens to focus in and out until the sharpest contrast is detected. The lens itself has no sensors and has no way of determining if something is in or out of focus, it can only adjust in response to signals from the body.

I guess it could be that the lens is not responding accurately to the body's signals. But I can't see how this would result in a shift in focus at the moment the photo was taken. Unless the body is attempting to fine-tune the focus at the moment you take the photo and the resolution of the focus motor is large enough that this results in a big swing in focal distance that doesn't get locked-on before the shutter trips. But I've never encountered this problem, and still would consider it a problem with the body (trying to refocus at the last moment) rather than the lens.

But if it is the lens then I count myself extremely fortunate that my lens works perfectly. I just looked at every single photo in my Lr catalog shot with this lens and see no evidence of any AF problems. But if my theory is correct, this because I'm using a 70D and not a 7D2.


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Jun 27, 2015 17:24 |  #184

I have to wonder if there is a sensor/part in the lens that has failed, or something that helps to set the focus where the camera says it is in focus.. otherwise, it doesn't make sense that there are good and bad copies of this lens. But then you would think canon could fix it. Very puzzling.


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Jun 27, 2015 18:26 |  #185

Archibald wrote in post #17612315 (external link)
I've discovered that it is possible to check the autofocus behavior of the 17-55mm/2.8 lens simply by noting what the focus scale is doing. So, you can tell ahead of time if the subject being focused will actually be in focus. It isn't even necessary to take a picture to see if the AF is behaving properly or acting up.

no foolin'? :D This is exactly why i said the below in bold:

Left Handed Brisket wrote in post #17570288 (external link)
i don't doubt your story, but if you want to document this problem you could set your camera on a tripod, center point only on a distant object that has nothing else around it that could throw off focus. Then take a cell phone video of the issue. Document the set up and you could probably even shoot the cell phone camera through the view finder to show the framing and the center point locking focus. Then aim the cell phone at the distance window and fire off 10 or 20 shots throwing off focus with your hand in front of the camera each time.

You should be able to easily show that the lens is indeed "grabbing" focus at vastly different distances, not reporting focus failure, and allowing the camera to release the shutter without focus. keeping the cell phone trained on the distance scale on the lens window is key. You might even rig up a tripod for the cell phone.

again, should you need to do this? no. but you're not going to get much traction with Canon without becoming a pain in the backside.


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Jun 27, 2015 18:37 |  #186

After reading thru this entire thread I went out and tested my copy of the 17-55. The Lens is Mated to my 60D. I took eight shots each at 17, 20, 24, 35, 55mm at f/2.8, ISO 200 and 800th-1000th sec. One shot, center AF, Hand held with Stabilizer On. It was Cloudy and the Test Target was a house a little over a 1/4 mile away. I had no OOF shots, all were sharp.

I don't doubt you one bit Archibald, I'm pretty sure my copy will not work on your 7DII. The only thing I can think of is the New and Old Tech are choking on each other. It's almost like there is a com issue but not enough to trigger a fault? ...............


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Jun 27, 2015 18:54 |  #187

ksbal wrote in post #17612632 (external link)
I have to wonder if there is a sensor/part in the lens that has failed, or something that helps to set the focus where the camera says it is in focus.. otherwise, it doesn't make sense that there are good and bad copies of this lens. But then you would think canon could fix it. Very puzzling.

Very puzzling indeed. There are no good and bad copies of the lens, they are all the same. Canon can't fix them because they all meet specifications; they are all working as designed. The very puzzling part is how inconsistent it is, not only from shot to shot, but from user to user.


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Jun 27, 2015 19:10 |  #188

Chief_10Beers wrote in post #17612698 (external link)
After reading thru this entire thread I went out and tested my copy of the 17-55. The Lens is Mated to my 60D. I took eight shots each at 17, 20, 24, 35, 55mm at f/2.8, ISO 200 and 800th-1000th sec. One shot, center AF, Hand held with Stabilizer On. It was Cloudy and the Test Target was a house a little over a 1/4 mile away. I had no OOF shots, all were sharp.

Good stuff, count yourself lucky.

I don't doubt you one bit Archibald, I'm pretty sure my copy will not work on your 7DII. The only thing I can think of is the New and Old Tech are choking on each other. It's almost like there is a com issue but not enough to trigger a fault? ...............

My 17-55mm also had AF problems with my old XT, XSi, 40D, and 7D.

Well, not all the time, though. Sometimes it worked great. Other times it would not.


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Jun 27, 2015 21:03 |  #189

Chief_10Beers wrote in post #17612698 (external link)
After reading thru this entire thread I went out and tested my copy of the 17-55. The Lens is Mated to my 60D. I took eight shots each at 17, 20, 24, 35, 55mm at f/2.8, ISO 200 and 800th-1000th sec. One shot, center AF, Hand held with Stabilizer On. It was Cloudy and the Test Target was a house a little over a 1/4 mile away. I had no OOF shots, all were sharp.

I don't doubt you one bit Archibald, I'm pretty sure my copy will not work on your 7DII. The only thing I can think of is the New and Old Tech are choking on each other. It's almost like there is a com issue but not enough to trigger a fault? ...............

Okay, I have to ask you if you checked the distance scale when you did your tests. You didn't mention, so I assume you didn't.

The distance scale provides a better way to check for AF errors. It avoids pixel-peeping, which everyone does differently, probably leading to different conclusions depending on the person. Checking the distance scale is less subjective.

You don't have to have your gear on a tripod for this. You can just look at the distance scale right after taking a shot. If you are shooting at something far away, the scale should be on infinity. If it is not, then the AF malfunctioned.

Now let's do a thought experiment. What would happen if you and I exchanged gear? You got my 7D2 and 17-55mm lens, and I got your 60D and your 17-55mm lens. Then we went out and did some shooting and ran some focus tests. What would we find?

I am almost for sure that you would find no problem with my gear, and that I would immediately see intermittent AF errors with your gear.

Obviously we can't do such a swap. But I did something similar to this about a month ago... I took my body and lens in to a reputable camera store here in town. Their help is knowledgeable, many are pro photographers. I told the guy there about the problem I was seeing with the 17-55mm lens. The two of us went outside and took a bunch of shots, and scrutinized the pics on the LCD. The AF errors were obvious! To me. The store guy shrugged them off, he scoffed even. He said that it was normal, all lenses are like that. Then he went about his business helping other customers.

Evidently most users of this lens are happy with the results. And that is in harmony with the opinion of the sales guy.

Some guys are not happy, though. Those guys have tried multiple copies of the lens and find the same unsatisfactory AF with all.

But the 17-55mm does have an AF problem, and other lenses like the 15-85mm, 18-55mm II, and 18-55mm STM don't. There is a demonstrable difference.

So that's my conclusion. It's all in the eye of the beholder.

If you like the results, carry on!


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Jun 27, 2015 21:07 |  #190

Left Handed Brisket wrote in post #17612684 (external link)
no foolin'? :D This is exactly why i said the below in bold:

again, should you need to do this? no. but you're not going to get much traction with Canon without becoming a pain in the backside.

Thanks, Left.

The critical piece of info that was needed for this was whether the focus distance scale corresponds reliably to the actual distance that the lens is focused on. If there was a defect in the glass of the lens, for instance, this would not be so. But there is good correspondence according to my tests. Accordingly we can trust the distance scale to indicate where the lens is focusing.


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Jun 28, 2015 00:59 |  #191

Again, if this was a defect with the LENS why would many others report zero problems? And again, the LENS cannot know whether something is in or out of focus because there is no focus sensor inside the lens. The lens can only do what the body tells it. If there is a problem it is that your BODY isn't compatible with the lens.

Canon confirmed to you that the lens is in-spec and fully functional. Why do you resist sending the body in with the lens for testing? Or did I miss that part? Because if Canon gets enough samples of the 7D2 having trouble with the 17-55 they can possibly analyze the problem and develop a fix in firmware.


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Jun 28, 2015 01:57 |  #192

bumpintheroad wrote in post #17612941 (external link)
Again, if this was a defect with the LENS why would many others report zero problems?

See three posts up.

And again, the LENS cannot know whether something is in or out of focus because there is no focus sensor inside the lens. The lens can only do what the body tells it. If there is a problem it is that your BODY isn't compatible with the lens.

Canon confirmed to you that the lens is in-spec and fully functional. Why do you resist sending the body in with the lens for testing? Or did I miss that part? Because if Canon gets enough samples of the 7D2 having trouble with the 17-55 they can possibly analyze the problem and develop a fix in firmware.

This is not a recent problem. It did not start with the 7D2.

It's a long thread, I know, but I did send the body in with the lens.

Indeed, the lens and body work together to achieve focus. As mentioned several times in this thread, the 17-55 has inconsistent focus, whereas other lenses do not. I don't know why. Canon says it is a known issue.

Maybe Canon could find a firmware solution, but after 9 years they have not.

My guess is that the inconsistent focus is caused by the coarse focus mechanism in the lens at long distances, together with the requirement for very fast focusing. From 3 meters to infinity is only 3 mm on the focus scale. When the lens receives the focus instruction from the body, the ultrasonic motor responds very fast but over a very small distance. This appears to result in mechanical overshoot at times.

As mentioned, it's only 3mm on the scale from 3m to inf. By comparison, it is 5mm from 1m to 3m, and 7.5mm from 0.5m to 1m. The closer distances focus reliably, the longer ones don't.

I'm not a lens engineer, so this is only speculation.


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Jul 07, 2015 15:53 |  #193

I'm currently going through the process of trying to find a decent copy of this lens. This is part of my Annual Drive to Find A Correctly-Assembled Wide-Mid Zoom. I've yet to find one that stacks up to my EF-S 18-55 IS (yes, the "kit" lens -- of which I suspect I might have a very good copy) in terms of good optical alignment or AF accuracy. Copy 1 was decentered; 2 had consistent front-focus; 3 was decentered; 4 was just optically awful, trounced at 55mm by my 55-250 STM.

The most recent, attempt 5, was decentered with unreliable focus around 35mm: about 60% of shots were front-focused. It had a particular aversion to green subjects. This again suggests botched optics. (As I understand it, phase detect AF relies on the body having a good understanding of a lens's spherical aberration.) Happened on both my 60D (centre AF point) and 7D (and yes, I do know what I'm doing). The 18-55 is spot on in the same circumstances.

The 17-55/2.8 is so often hailed as borderline miracle-glass, but for me so far it's been an expensive, time-wasting pain in the unmentionables. Sometimes I wonder if Canon is taking my £500 seriously: at that price damn right I'm going to be fussy. Yes it's a shame to have to swing the Sale of Goods Act Hammer so often and so hard, but none of the examples I've seen of this lens so far have been either of satisfactory quality or fit for purpose. I feel sorry for the retailers who, in the UK, have to take responsibility for these faulty copies.


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Jul 07, 2015 16:21 |  #194

Why dont you just get rid of it?
Save you a lot of grief!


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Jul 07, 2015 16:32 |  #195

I am still very perplexed by this thread. I have now found 4 friends/family/acquain​tances who have this lens and no one is able to reproduce the issue described in this thread either by taking the picture or by looking at the focus scale. Fact is that outside of this thread and one post Ken R. posted years ago, no one has heard of this issue. Try as I may, I just don't see it.


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Baffling focus issue with Canon EF-S 17-55mm/2.8
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