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09-27-2015, 10:35 AM   #1
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AF issues w/ Sigma 18-35mm

This weekend I was shooting from the top of my SUV. I had my K5 II mounted on a Gigapan with a Sigma 18-35. My back tripod leg was on the small section where the trunk folds up so of course when I decided I needed something from the trunk the whole set up came tumbling down, landing hard on the concrete from about 12 feet up. Lesson learned.

Basically the Gigapan walked away with some scrapes. The K5 was toast due to a cracked sensor of all things. But, it is the Sigma that I am worried about the most. Besides a large gash on the rim surrounding the lens, it appeared to be unscathed and working properly as first glance.

After I mounted it to my backup K5 I have noticed that it will AF fine when I am in Liveview, but when I am not in LV it front focuses pretty badly. So much so that I cannot correct for it even with the AF fine adjustments.

I am not completely aware how AF works and how cameras communicate with the lens but I am trying to figure out if it is simply the communication between them or if somehow there is some loose element in the lens that may be causing this issue? It confuses me why it will focus fine when if LV though. I generally do not use LV and am trying to figure out if the lens needs to be replaced. I am still shocked it survived the fall at all but besides the focus issue it seems to have not lost any sharpness.

Also, I have no experience with the Sigma lens dock and this may be a question for Sigma, but would this be a potential fix?

09-27-2015, 11:33 AM   #2
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The 18-35mm's quirky PDAF performance is more or less a known issue. While some users have reported no trouble with the lens, others have found viewfinder focusing virtually unusable. More details can be found in our review and a follow-up test:
Sigma 18-35mm F1.8 DC HSM "Art" Review - Autofocus | PentaxForums.com Reviews

The lens dock may be able to fix the problem, at least at certain focal lengths. It lets you dial in adjustments at 4 different zoom settings and 4 different distances between minimum and infinity.

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09-27-2015, 11:37 AM   #3
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I've got a whole thread about this.

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/277499-sigm...es-thread.html
09-27-2015, 01:23 PM   #4
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Thanks guys, I am reading that thread now.

The focus used to be spot on, however, and I had absolutely no issue at all for the few months I have owned it. The issue only started after the falls which I find curious.

09-27-2015, 07:27 PM   #5
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QuoteOriginally posted by Adam Quote
It lets you dial in adjustments at 4 different zoom settings and 4 different distances between minimum and infinity.
though most of the issues occur between 2m and infinity, an area where there is no ability to apply AF adjustment.

09-28-2015, 01:30 PM   #6
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QuoteOriginally posted by Digitalis Quote
though most of the issues occur between 2m and infinity, an area where there is no ability to apply AF adjustment.
Ah, looks like I won't waste my money on the dock then. My focus issues are exactly in line with what is described in that thread. I still find it odd that PDAF with this lens has worked flawlessly up until the fall and now I maybe get 1 in 20 that focused properly. I guess I will have to remember to use CDAF from now on
09-28-2015, 04:47 PM   #7
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QuoteOriginally posted by turff Quote
I still find it odd that PDAF with this lens has worked flawlessly up until the fall and now I maybe get 1 in 20 that focused properly.
I have had AF issues right from the start, when I bought my copy of the Sigma 18-35mm f/1.8 ART - there were several copies in the store and I had to pick the one that at least seemed to have the fewest problems.Optically it is an excellent lens, easily one of the best lenses money can buy for APS-C format, but the AF issues cripple it.

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