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07-04-2011, 09:38 AM   #1
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Tamron 17-50 back focusing issue

So I've finally gotten a chance to give this lens a good run, and I find that 80-90% of the shots are back focused. No problem, I should be able to compensate for this on my K7, but my question is, when doing some setup / testing, is there a particular FL I should be setting up to? Should I do it around the middle range of the lens, or do it at both ends?

07-04-2011, 11:32 AM   #2
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You could test it at 17mm, then at around 35 and at 50. That should give you enough information how to adjust focus. If back focus is visible at all FLs, you shouldn't have any trouble adjusting the lens
07-04-2011, 05:32 PM   #3
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Played around with it tonight, and it's better than it was. Definitely closer than it was. Still seems to sometimes be slightly off, but not much. Seems to depend on what I'm aiming at, and distance. Just need to play some more and tweak.
07-04-2011, 10:34 PM   #4
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If you're not aiming at a well-designed focus test chart, there's a very good chance that what you're seeing isn't backfocus but simply the camera choosing to focus smewhere other than where you wanted because the focus sensors are too broad to allow you to pinpoint focus..

07-05-2011, 05:07 AM   #5
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QuoteOriginally posted by Marc Sabatella Quote
If you're not aiming at a well-designed focus test chart, there's a very good chance that what you're seeing isn't backfocus but simply the camera choosing to focus smewhere other than where you wanted because the focus sensors are too broad to allow you to pinpoint focus..
Very legitimate point, but when I was using it, I was doing head shots, and they were not in focus, but the wall behind them were. My other lenses doing basically same shots had no issues. During my adjustments, the focus did change between regular focusing and live view, and I read that's a good indication that the system is off. Now it doesn't do that. Just need to now do some more "real life" shooting to see how it is now.
07-05-2011, 09:28 AM   #6
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Exactly. A true backfocus issue would be a matter of focus being of by a couple of *millimeters* here or there. If you see focus being off by several feet, there is absolutely no possible way that's backfocus - it's the camera having chosen to focus on the wall. I'm still guessing you either didn't select the appropriate focus point, or that a portion of that wall was within range of the selected focus point, because the head wasn't large enough in the frame or you positioned the focus point to close to the edge of the head. but again, absolutely no possible way that this is backfocus.
07-05-2011, 12:23 PM   #7
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You make a point that I didn't realize about only being off by a few millimeters. I just find it strange that it is only happening with the one lens. Using center point only, focus on eye's and recompose. Not sure what to think now. I was shooting at F4, so it even left some room for error. I'll have to do some more testing I guess. Thanks for the info. Now I'm confused even more.
07-06-2011, 07:41 AM   #8
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If the face isn't a very large portion of the frame, then even if you focus on the eyes, the background woukd still be in ranfe of the focus sensor, which is much larger than the dot that may appear in the viewfinder. It's worthwhile to spend some time practicing focusing on a dot on on a piece of plain paper, with the dot just off center, to see just how far off center you can get and still be in range of the focus sensor.

07-08-2011, 01:26 AM   #9
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I have a back focus with tamron 17-50 on the Pentax K-7, but when I put it at Pentax KX then it's fine
07-08-2011, 01:27 AM   #10
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All other lenses on-k7 are fine only Tamron 17-50 has a back focus. On k-x is very sharp.
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