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12-05-2017, 07:27 AM   #1
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Should this qualify for a repair?

I've got a zoom lens that has significant front/back focus issues. It is entirely correctable, but for one issue. The extreme ends of the focal range are slightly front focused, while the middle of the range is significantly back focused. So when I correct for one, I mess the rest of the range up. This is noticeably impacting my event photography pictures, where plane of focus is slightly off target.

Is this something can be repaired by either precision or keh, and is it worth it, at a flat rate of $350?

thanks,
Imp

12-05-2017, 09:15 AM   #2
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It doesn't sound like it can be fixed.

Although you don't say what kind of lens it is, or what camera you're using it on, the problem mainly comes from the camera AF module not correctly perceiving correct focus. Spherical aberrations and other optical issues cause these problems.

Cameras with built-in focus correction ability presently only allow one correction per lens. So if you plug in a +10 correction for that lens, it will do that at all focal lengths. So, if you're in focus in the mid-range, you'll be wrong at the ends.

As far as I know, there's no adjustment to a lens that will fix your problem, aside from re-designing it.

To be certain that focus errors are the true cause of your woes, be sure to test it using live view at all focal lengths. If all images are precisely focused, then yes the phase detect AF module is getting it wrong when the mirror's down. If the lens is soft in the mid range no matter what, then that's a whole other issue.
12-05-2017, 09:33 AM - 1 Like   #3
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QuoteOriginally posted by Ontarian50 Quote
If the lens is soft in the mid range no matter what, then that's a whole other issue.
This last sentence is golden. If a lens is simply soft or if there are sufficient aberrations, even the best focus mechanisms will fail in some way.

As for whether the problem with Imp's lens can be fixed, that is hard to say. It is my understanding that PDAF front/back focus is often traceable to lens element alignment issues that may be addressable if a proper test harness is available and the design supports adjustment. The repair shop should be able to provide some guidance.


Steve
12-05-2017, 11:14 AM   #4
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QuoteOriginally posted by Imp Quote
I've got a zoom lens that has significant front/back focus issues.
Imp
It would help if you said what lens with what camera is having the issue.

Also, presuming this is a DSLR with live view, have you tried comparing the problem with and without using the live view for focusing....and if so, are you getting the same problem or is it improved?

12-09-2017, 12:59 PM   #5
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Thanks guys! It's a DA* 50-135 on my K-3. It needs the following calibration:

90-135 0
85-90mm -10
50-85 0

Last edited by Imp; 12-22-2017 at 04:10 PM.
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