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07-09-2011, 09:15 PM   #1
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Tamron 28-75mm is front-focusing

I've been displeased with my tamron 28-75mm, I just couldn't produce sharp photos at f2.8 Well, I printed out a focus test chart, and low and behold, it's front focusing about 4mm. So my question is... how do I fix it?

I have a Kx with the original firmware (1.0).

Also, this pretty much makes the lens useless on my SF10, doesn't it?

Charles.

07-09-2011, 09:30 PM   #2
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Not necessarily useless on your film camera but unfortunately there isn't a solution to the FF on your K-x other than to have the pair sent in for calibration.
07-09-2011, 09:33 PM   #3
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Pentax Kx / Pentax K-x AF adjustment through undocumented "debug mode"
07-09-2011, 09:39 PM   #4
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Check that, it's not just the tammy. I've been going back through my lenses, and my 18-55 front focuses by the same 4mm, and my Sigma 28-105 UC-II front focuses by 6mm. My Vivitar f1.7 50mm manual, the focus confirmation also goes off at 4mm front-focused.

HOWEVER, my Tokina 35-70mm and my 55-300mm DAL are perfectly dialed in, and the SMC-F 35-70 is close, maybe 2mm at most front-focused.

Soooo, do I have a bad Kx then?

Charles.

07-09-2011, 10:47 PM   #5
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Well, checking the Tamron, it needed -200 adjustment. That seems like an awful lot. It needs more adjustment at 75mm than it needs at 28mm, and -200 pretty much makes all my other lenses soft, especially the 55-300 and both 35-70's. The 18-55 only needed -50 adjustment, and I don't much care about the other lenses.

So, do I have a dud with the Tamron then? Can it be calibrated? If so, by who? I bought it used.

Thanks.

Charles.
07-10-2011, 01:32 AM   #6
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I am interested in the answer to this post. I have a Kx and want a Tamron 28-75 but I'm a bit scared to buy used or imported for this reason, but retail in Aus is $600
07-10-2011, 10:53 AM   #7
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If you tested indoors, then I'd say there is nothing wrong - most cameras will FF under tungsten or other yellow light. Just part of the design of phase detect focus. Also, note that due to field curvature, it's also normal for edges of a focus test chart to appear to show FF.
07-10-2011, 03:19 PM   #8
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You know, I use this lens almost 100% indoors, in low light, wide open, so front focusing 6mm is rather a bad thing.

Charles.

07-11-2011, 06:12 AM   #9
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So I played with this in daylight, and while the lens front focuses less under natural light, it still front focuses. What's worse, it front focuses significantly more at 75mm than it does at 28mm. If I adjust so 75mm is correct, 28mm will be severely back-focused. If I adjust so 28mm is correct, 75mm still isn't correct. I emailed Tamron, hopefully I'll be able to get this repaired for not a whole lot of cash.

Charles.
07-11-2011, 08:56 AM   #10
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Hi Charles. I have the exact same problem with my Tamron 28-75. I tried 2 copies of Tamron 17-50 before and sent them both back and now I'm trying a second copy of Tamron 28-75 as well. ALL 4 lenses had the exact same problem: focus shift throughout the focal length. If I fix the front focus at the long end I get back focus at the wide end. It's hopeless. It's impossible to make the lens focus properly at both ends. After 4 lenses I was starting to think that maybe there's something wrong with my K-5. But I also own a K10 and the it behaves the same.

However, today I discovered something that might find us an answer. This focus problem only happens at f2.8-f4.5. As soon as I stop it down to f5.6 everything is in focus at both ends. And no, it's not the DOF. I noticed that focusing the lens at f5.6, locking the focus and then opening to f2.8(while the AF is locked) produces good focus all the time at both ends! Give it a try and let me know if it works for you too. I'm also curious what Tamron will have to say about it. Is this a camera or lens problem.......?
07-11-2011, 05:28 PM   #11
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Tamron does have a reputation for focusing problems. On the other hand the company offers 3 year warantees and is good about fixing lenses.
07-11-2011, 07:23 PM   #12
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Nicks, tried your method, but no dice. My lens front focuses no matter what the aperture is set to.

mistickcowboy, that's good to know... I emailed them friday but haven't heard back yet. Hoping they don't just ignore me....

Charles.
07-11-2011, 09:51 PM   #13
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Must be lucky with my Tamron 28-75

I must be lucky. 4 year old Tamron 28-75, been on MZ-s, K10, 2 K20s and now 2 K-5s. Never adjusted anything and I live on it. I do have a thumb callous from the AF button and use a single point focus and recompose a lot. 003 Cross CC Chatham St _IGP8773 | Flickr - Photo Sharing! From last Friday in Cary.
07-11-2011, 10:00 PM   #14
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QuoteOriginally posted by ChopperCharles Quote
Check that, it's not just the tammy. I've been going back through my lenses, and my 18-55 front focuses by the same 4mm, and my Sigma 28-105 UC-II front focuses by 6mm. My Vivitar f1.7 50mm manual, the focus confirmation also goes off at 4mm front-focused.

HOWEVER, my Tokina 35-70mm and my 55-300mm DAL are perfectly dialed in, and the SMC-F 35-70 is close, maybe 2mm at most front-focused.

Soooo, do I have a bad Kx then?

Charles.
I have the same issue with my K-x, anyone with more than a few lenses is going to strike the same problem. What I do to solve it is adjust the debug AF amount so that as many lenses as possible at least have the focus point within the DOF, even if some are slightly FF and some slightly BF. You just want to ensure that what you focus on with each lens ends up within the DOF.

For me I have 1 perhaps 2 which can't be adjusted to achieve this when they are wide open, not without throwing out too many others lenses. For those lenses I know I need to stop down or purposely fudge my focus point to achieve an in focus image.

My advice then to to adjust to compensate for a little bit of FF, but not enough to ruin your 55-300.

This is one good reason to buy a K7/K5/K20.
07-12-2011, 02:54 PM   #15
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Well, Tamron wants between $140 and $250 to adjust the lens. They say warranty is non-transferable and since I bought it used I'm shit out of luck. I pointed out that Tamron has a known history of focus problems with their lenses, and that their prices are beyond unreasonable to fix a manufacturing defect (I mean, I only paid $300 for the lens), but I don't expect they'll do right by me.

Can a local shop adjust the lens? Or can I do it myself?

Charles.
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