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Archive 2016 · Decentering symptoms?

  
 
herion
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p.1 #1 · p.1 #1 · Decentering symptoms?


I just received a (very, very) lightly used 70-200 f/2.8L IS II lens, and while I was checking it out, I noticed a bit of softness at the 70mm end at infinity focus.

I have a 40D (yeah, yeah... I know... I'm going to repeat the process with my 70D ) - center point AF. I selected a bush half-a-block down from me. The 200mm pic was crazy sharp at 100%, but the 70mm at 100% was less so - other spots in front of and behind the focus point looked great (but still not as sharp as the 200mm). A car parked behind an iron fence was razor sharp at 200mm and again, less so at 70mm - again at infinity focus.

Pics taken at distances closer than infinity at 70mm look perfect across the frame.

Is this a symptom of decentering? I'm the first to admit that it could be camera shake as I'm not used to handling this heavy a lens. I'm a CPS Gold member and I haven't used up my CMS credits, so would it be wise to send it in with some pics?

I will also post some examples tonight/tomorrow for you to view.

Thanks in advance!




Aug 09, 2016 at 09:50 AM
jcolwell
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p.1 #2 · p.1 #2 · Decentering symptoms?


Decentering usually describes unsymmetric sharpness; typically with one side or one corner of the image being noticeably softer than the others.


Aug 09, 2016 at 09:55 AM
form
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p.1 #3 · p.1 #3 · Decentering symptoms?


Agree with above. That's how I've seen it. If there are softness issues at specific distances it may be the focus adjustment of the lens or another element misalignment.

Using live view, try to get the area sharp by manually focusing. If it never gets sharp, it's probably a lens fault. If it does get sharp, it's probably a focus issue. If your camera has AF fine tuning you can try that. Otherwise...it has to go to Canon for calibration.

Speaking from lots of experience...if you EVER purchase a used lens and have any kind of issue related to IQ or other functional oddities that seem even slightly questionable...RETURN IT before you get stuck with the repair bill.



Aug 09, 2016 at 10:16 AM
Sy Sez
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p.1 #4 · p.1 #4 · Decentering symptoms?


There's a fairly easy way to test for Decentering:

https://www.lensrentals.com/blog/2012/05/testing-for-a-decentered-lens-an-old-technique-gets-a-makeover/



Aug 09, 2016 at 11:14 AM
molson
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p.1 #5 · p.1 #5 · Decentering symptoms?


jcolwell wrote:
Decentering usually describes unsymmetric sharpness; typically with one side or one corner of the image being noticeably softer than the others.


For a definition and examples, look in the dictionary under "Sony"...



Aug 09, 2016 at 11:44 AM
garyvot
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p.1 #6 · p.1 #6 · Decentering symptoms?


That sounds like a focus calibration issue to me. It is possible for zoom lenses to focus asymmetrically at the long and short ends of the zoom range with specific camera bodies.

Decentering would cause one edge (but not the others) to be less sharp when focused at a common point in the middle of the frame.

Your 70D, being a newer body, should have the ability to apply micro focus adjust independently for the long and wide ends of the zoom. This should easily resolve your issue.

The easiest way I have found to determine a good MFA with a newer body is to use 10X Live View magnification in combination with Quick Focus (which uses the in body phase detect AF) to check focus accuracy with various targets. I'll usually shoot something with high contrast on an angle so that I can easily see if the lens is front or back focusing, then dial in a likely correction amount, and repeat. Once I feel it's dialed in, I'll shoot a series of other targets and confirm by looking at the files on my computer (I shoot RAW to do this which shows the best detail). It sounds convoluted, but it only takes a few minutes once you get the hang of it.



Aug 09, 2016 at 11:46 AM
herion
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p.1 #7 · p.1 #7 · Decentering symptoms?


Thanks all! I *know* the folks at FM are definitely less snarky and more helpful than DPR. I will try with my 70D tonight and see what happens.


Aug 09, 2016 at 02:50 PM
herion
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p.1 #8 · p.1 #8 · Decentering symptoms?


Update: With the 70D, I had similar issues at the 70mm end - AT FIRST. 85, 100, 135, 200 all looked perfect. Live view was better at 70mm, but still not the quality of the 85,100, 135, 200. Then repeated the experiment on a tripod. 70mm was perfect.

My shutter speed was between 1/200 and 1/350.

Then I tried it again... really concentrating NOT to move at all. PERFECT. It's definitely my technique; I'm not used to the weight of the lens yet, which means the solution to my issue is the same as to the question, "How do you get to Carnegie Hall?" - PRACTICE, PRACTICE, PRACTICE!

Again - thanks to everyone for the advice and suggestions!






Aug 09, 2016 at 06:31 PM
Milan Hutera
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p.1 #9 · p.1 #9 · Decentering symptoms?


What you're describing might be a focusing issue with your particular body - the 40D. As you write in your original post - the focus at 70mm AND infinity appears to be off, while the focus at 70mm closer than infinity seems ok. Unfortunately you cannot fix this yourself as Canon does not allow the MFA at various focusing distances, only the wide and tele ends of a zoom lens. You can set the MFA depending on focusing distance only on recent Sigma lenses using the dock.

You should try the lens on your other body and see if the issue remains or not. Then you can either send the lens and 40D to Canon for calibration or use the newer body, if it performs to your expectations.



Aug 09, 2016 at 06:41 PM
herion
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p.1 #10 · p.1 #10 · Decentering symptoms?


At some point, I may send the lens in for a checkup, but for now, I'm going to focus on my steadiness technique with both the 40D and the 70D.


Aug 09, 2016 at 07:24 PM
herion
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p.1 #11 · p.1 #11 · Decentering symptoms?


Well, the universe was telling me to send the lens in... I had read some of the techniques for better holding tele lenses and wanted to try it out. Worked GREAT with my 70-300L, but back to my 70-200 ... meh. When I looked at some less-than-infinity images at 70mm as well, they weren't as sharp as they should have been either.

Then I wanted to try it on the tripod again. The thumbscrew on the B(w) ring popped out and wouldn't re-engage to tighten the mount onto the lens.

So - the lens, clamp and a CD of sample images were sent to CPS in Jamesburg. I'm crossing my fingers that it's a simple recalibration issue.

I'll give you folks an update when I hear from them.




Aug 11, 2016 at 07:01 PM
dmcphoto
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p.1 #12 · p.1 #12 · Decentering symptoms?


herion wrote:
I just received a (very, very) lightly used 70-200 f/2.8L IS II lens, and while I was checking it out, I noticed a bit of softness at the 70mm end at infinity focus.

I have a 40D (yeah, yeah... I know... I'm going to repeat the process with my 70D ) - center point AF. I selected a bush half-a-block down from me. The 200mm pic was crazy sharp at 100%, but the 70mm at 100% was less so - other spots in front of and behind the focus point looked great (but still not as sharp as the 200mm).
...Show more

I had a 70-200 f/4 with identical symptoms. I sent it to Canon. They said "Your product has been examined and it was found that the part did not operate properly causing the focus to operate improperly. The part was replaced. Product functions were confirmed.”

I'm not sure exactly what was at fault, but it worked fine after that. I checked centering per the lens rentals method and that's fine too.



Aug 11, 2016 at 07:44 PM
EB-1
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p.1 #13 · p.1 #13 · Decentering symptoms?


dmcphoto wrote:
I had a 70-200 f/4 with identical symptoms. I sent it to Canon. They said "Your product has been examined and it was found that the part did not operate properly causing the focus to operate improperly. The part was replaced. Product functions were confirmed.”


That's pretty much what Canon always says. I think they have a drop-down response menu.

EBH




Aug 11, 2016 at 07:50 PM
herion
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p.1 #14 · p.1 #14 · Decentering symptoms?


CPS got my lens today (should have been delivered on Friday, but I'm sure the poor little UPS guy didn't feel like delivering it - I got an exception notice at 3:25pm that no one was available for signature - and CPS Jamesburg is open until 4:30pm... )

Anyway - there *was* some type of mechanical deficit and it's being repaired. Should have it back by the end of the week. I've also contacted the camera store I got it from and I'm going to very strongly ask for a "Hey, we f**k'ed up" store credit.

So ends my adventures in non-FM used lens purchases....



Aug 15, 2016 at 04:31 PM
herion
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p.1 #15 · p.1 #15 · Decentering symptoms?


FINAL UPDATE:

Lens back from CPS and now it is RAZOR-SHARP across the entire range, across all my bodies. I also took the chance and contacted the camera store where I purchased it used. They are giving me a credit equal to the CPS and shipping costs!!! (Which will, no doubt, go towards either the upcoming Tamron 10-24 VC SP, the Canon 24-105 II or the 6D II !!!)

As my grandma used to say, "You don't ask, you don't get!"



Aug 20, 2016 at 04:48 PM
jcolwell
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p.1 #16 · p.1 #16 · Decentering symptoms?





Aug 20, 2016 at 04:56 PM





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