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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 20 Sep 2006 (Wednesday) 04:09
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24-70L Back From Canon Repair

 
David ­ S
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Sep 20, 2006 04:09 |  #1

Sorry for the millionth thread on the 24-70L. Anyways, I just got my 24-70L back from Canon where they adjusted the lens to the best focus point. I still have a couple of issues which I think are related:

1) On the tele end, the lens focuses pretty good, however, I am still having bad back-focus on the wide end. I have to either manually focus, or I have to lock focus at 70mm and then zoom back to 24mm to get a sharp shot at the wide end.

2) Also, another issue is when I look at the distance window on the lens, it never stays in the same spot when I focus on the same object on the wide end. It jumps around quite a bit but still always back focused. So shooting in the same condition can result in varying amount of back focused shots. On the tele end, it's fairly consistent.

I don't suspect it to be the 30D as the 70-200L F4 works perfectly and focuses fine in the same condition.

Now, I wouldn't care too much, but I recently started this exact test after I noticed many of my real world shots being back focused.

Any suggstions?

Please see pictures below for examples.

1st Picture: Controlled testing. 100% crop. The left image is badly back focused and the right shot is with me manually focusing it.

2nd Picture: Dog Shot: As you can see this was center focus on one shot. The image is at 24mm and is badly back focused.


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René ­ Damkot
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Sep 20, 2006 04:16 |  #2

The second one is probabely user error. Been there, done that.
The AF field is bigger (quite a bit) then the red rectangle in the VF, so it picked the background.
There's an image on the net, I'll see if I can find it...

Edit: Here ya go (10D):

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO

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lensview
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Sep 20, 2006 04:37 as a reply to  @ René Damkot's post |  #3
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The box shots + your description of the test would indicate to me that the lens has a focus calibration drift.
With the dog shot, Rene may have a point....the amount of backfocus seems excessive. With AF sensor point placement one really needs to be conservative, not expecting too much from it in the sense of small area discrimination power.


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David ­ S
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Sep 20, 2006 04:37 |  #4

Thanks for the reply. I didn't realize the AF field was that much bigger.

Any suggestions or comments on the first pic? The actual AF field is within the lens box.


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homera1
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Sep 20, 2006 04:39 |  #5

Hi DavidS,
I had exactly this same problem with my 30D and the 24-70L. I was constantly thinking I had a problem with either my lens or my 30D. I wanted to send them both off to Canon but didn't get round to it. I experienced a similar problem with my 70-200L only not as pronounced which made me think it was the camera.

I then had the chance to get a 5D (swap plus cash for 30D) so I jumped at the chance. It is much better now, but there is still a small problem with the 24-70L at the open end and I sometimes have to manually focus it. I have put this down to the AF not being able to deal with all situations.

I have concluded that there are situations that the 24-70L can't deal with, but I also concluded that my 30D had back focus issues with AF. I read a lot on the internet about it and although Canon won;t admit it, it would appear that Quality control on the 30D isn;t as high as one would think and some units go out with back focus. The only advice I can give is to send the 30D to a Dealer under warranty and get them to calibrate the focusing... I know this isn't what you wanted to hear, but was the only conclusion I cold come to...... Do a search on the net for 30D focus issues....


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David ­ S
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Sep 20, 2006 04:46 |  #6

I did see some threads on 30D focusing problems, but is it really a camera issue when I don't have issues with the 70-200?


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homera1
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Sep 20, 2006 05:07 |  #7

It is difficult to say, but my experience was that I only noticed it with the 24-70 and thought the 70-200 was fine... but when I did some tests with my 30D and these two lens I found they both had the same problem. Only far less pronounced on the 70-200. I put this down the the characteristics of the lens.

It may pay you to down load one of the focus test charts from the web. This is the one I used.
http://focustestchart.​com/chart.html (external link)

at least then you could work out if the problems were with the camera or the lens.


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David ­ S
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Sep 20, 2006 13:44 |  #8

That is the weird thing. I have done focus charts and they look good. So I am bit confused if it is the camera or lens.


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davidfig
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Sep 20, 2006 13:50 |  #9

David S wrote in post #2012975 (external link)
That is the weird thing. I have done focus charts and they look good. So I am bit confused if it is the camera or lens.

My recommendation is before you consider it a lens or camera problem, assume its an operator problem.

During your tests was the camera on a tripod? Did you pre-focus far then focus, followed by a test of focus near then focus on subject to take picture. Figuring out if the lens or camera have a focus problem is not an easy task.


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David ­ S
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Sep 20, 2006 13:56 |  #10

Yup. As an engineer, I try to eliminate as many variables as possible. For the controlled test, I had it on tripod, mirror lock up with timer. I pre-focused far, macro, and in between.


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steved110
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Sep 20, 2006 16:27 |  #11

I would send that lens back again, you don't spend that much cash fopr alens that won't AF on its own box! I can accept that the dog picture relates to the AF sensor size, but there's no excuse for the box.

Sorry you're having all this hassle, I hope it gets sorted asap.


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David ­ S
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Sep 20, 2006 17:09 as a reply to  @ steved110's post |  #12

I am going to test the lens on my friends 20Ds tomorrow to see if is my body or lens.

Also, I just got off the phone with Canon and they told me something pretty disturbing. When I sent the lens 2 weeks ago, I sent them pictures and a description of the testing procedure and observations. The canon rep. on the phone pulled my file and read some of the notes filled out by the Irvine center. It looks like there wasn't enought thorough testing done on my lens, and also, the notes read high probability of customer resending the lens again. WTF?

Now they want me to send the lens again knowing there is a high chance I would. Now, I bought this camera and lens at this time because I am getting married in 1 month, and I am trying to document the engagement and all the events leading to the wedding.

I already had to use my 70-200 for an event last weekend in low light situation. That really sucked. Now, I am going to be without my lens for another 2 weeks which is not going to be possible as I have other wedding events upcoming this and next weekend. Sorry for the long rant.:(


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noname
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Sep 20, 2006 19:26 |  #13

i'm pretty sure that, on the 24-70L, you can't zoom tele to focus then zoom out because of how the lens is designed. I don't have any links to prove this.. and i don't have my camera and lens to test this out.


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Tom ­ W
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Sep 20, 2006 19:52 |  #14

René Damkot wrote in post #2011297 (external link)
The second one is probabely user error. Been there, done that.
The AF field is bigger (quite a bit) then the red rectangle in the VF, so it picked the background.
There's an image on the net, I'll see if I can find it...

Edit: Here ya go (10D):
IMAGE NOT FOUND
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO

Here's what you're looking for (my slightly edited version of Chuck Westfall's diagram):

IMAGE: http://www.pbase.com/photosbytom/image/42250385.jpg

Anyway, the initial image of the box indicates a significant focus error, assuming that all other factors were controlled (used tripod, remote release, etc.). If that's a consistent test result, I'd get hold of Canon and send it back. How does the camera perform with other lenses?

As for the dog image, it's quite easy for the AF sensor to grab focus on the high-contrast background object given the size of the sensor compared to the indicator in the viewfinder. I wouldn't use this one image as a good indicator of focus error. This is something that is sometimes difficult to see in the 30D viewfinder, but it's a situation to be watching for when you shoot.

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SkipD
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Sep 20, 2006 19:58 |  #15

noname wrote in post #2014471 (external link)
i'm pretty sure that, on the 24-70L, you can't zoom tele to focus then zoom out because of how the lens is designed. I don't have any links to prove this.. and i don't have my camera and lens to test this out.

I find that if I change the focal length of my 24-70, the focus will need tweaking. It doesn't change much, but it is noticeable - even through the viewfinder.


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