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11-15-2010, 05:10 AM   #1
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K5 and AF accuracy with Sigma 17-70mm HSM lens

Freshly bought K5: fabulous except for a baffling inability to focus accurately with my Sigma 17-70 f2.8-4.0.

The lens, BTW, has worked perfectly on my K7.

Even with full -10 AF fine adjustment it still front focuses on the brand new K5 body.

My Pentax 55-300 lens focuses spot on.

Anybody else having similar problems, or do I have a K5 body with a problem?


Last edited by amvic; 11-15-2010 at 07:28 AM. Reason: additional information.
11-15-2010, 05:25 AM   #2
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QuoteOriginally posted by amvic Quote
Freshly bought K5: fabulous except for a baffling inability to focus accurately with my Sigma 17-70 f2.8-4.0.

Even with full -10 AF fine adjustment it still front focuses.

My Pentax 55-300 lens focuses spot on.

Anybody else having similar problems, or do I have a K5 body with a problem?
Pentax can't guarantee correct AF and SR work with 3rd part lenses. Sigma hacked Pentax's lenses protocol. No any legal licence.
11-15-2010, 06:36 AM   #3
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SIGnificant MAlfunction
11-15-2010, 06:46 AM   #4
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As of now, yes, SIGnificant MAlfuntion.

Which is a pity as on my K7 this particular Sigma lens was sharper than much of my Pentax glass.

11-15-2010, 06:46 AM   #5
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Yea, I would take the lens and have it calibrated. I had the problem with the 300f 2.8 lens. Sigma is a bit bad on QC. But if it is the lens. They will happily re calibrate it for you.

I picked mine up from Sigma service today. I put up the sigma focus chart on pbase if you want to check. I had the same front focus problem. Even with 10,,, still out of focus.
I had the original front focus charts but i accidently deleted them.

Wait i see it is ok with the k-7? that is a wierd one. Mine went bad on my k-7 and k-x both with the same result....

Sigma 300mm f2.8 tests. Photo Gallery by Gary at pbase.com

Last edited by garyk; 11-15-2010 at 06:53 AM.
11-15-2010, 07:44 AM   #6
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QuoteOriginally posted by garyk Quote
Yea, I would take the lens and have it calibrated. I had the problem with the 300f 2.8 lens. Sigma is a bit bad on QC.
But how it could be QC if it works properly with the K-7? Unless Pentax has changed something under the hood.
11-15-2010, 08:05 AM   #7
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QuoteOriginally posted by farhagh Quote
But how it could be QC if it works properly with the K-7? Unless Pentax has changed something under the hood.
I suppose that's my question: did Pentax change something under the hood, or do I need to exchange my new K5 body because there's something wrong with it?
11-15-2010, 08:25 AM   #8
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QuoteOriginally posted by amvic Quote
I suppose that's my question: did Pentax change something under the hood, or do I need to exchange my new K5 body because there's something wrong with it?
Most likely, there is nothing "different under the hood" (or anything wrong with your K-5) except that you have a new camera.

Every camera and lens is a consumer product, and manufacturing tolerances for them are not as tight as we'd like to think. The sensor on your K-7 is on a different plane (relative to the lens) than on your K-5 by some small amount. Apparently it exceeds what the AF Adjust can compensate for. Maybe the Sigma and K-7 were both "equally off" in the same direction?

If the max AF Adjust won't fix it, but your other lenses are sharp (w/ AF adjustments), then send the body and lens to Sigma for lens re-calibration. They can adjust the lens helicoid, not just the sensor. Or you can try a different K-5 or a different Sigma 17-70...

11-15-2010, 08:27 AM   #9
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I have the non HSM version of this lens. It did not require any AF adjustment in my K7 but -5 on my K5

Almost all my lenses that needed adjustment lost 5 pints negative.

So on my body at lease I show an across the board -5 adjustment.
11-15-2010, 08:34 AM   #10
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So in all of the cases above it's not about Sigma, neither the K-5. It could be any combination.
11-15-2010, 08:57 AM   #11
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QuoteOriginally posted by farhagh Quote
So in all of the cases above it's not about Sigma, neither the K-5. It could be any combination.
Exactly. Let's say his K7 was FF by 4 (not an unusual or unacceptable amount) and the Sigma lens was BF by 8. That would require an adjustment of 4. Now if the new K5 is BF by 4 (again, a fairly small error); that, combined with the Sigma's 8, would require an adjustment of 12, which is significant and beyond the camera's adjustment range.
11-15-2010, 02:32 PM   #12
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Hmmm. Lens calibration, sending away the camera, makes sense... but I'd much rather not.

Pity that the K5 won't do Spinal Tap AF fine tuning and go up past +/-10!

I just want to take pictures.

The K5 is everything the K7 was and a bunch of things it should have been... More than decent shots at 12800 ISO, excellent motion tracking, all kinds of menu tweaks that never made it into the final firmware for the K7.
11-19-2010, 10:53 AM   #13
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Curioser and curioser... Today I tried the same Sigma 17-70 on another K5 body and it's out by exactly the same amount.

Tried other Pentax lenses on my K5 and they are in perfect AF sync.

Perhaps the poster who suggested Pentax changed "something under the hood" is correct.

Am I the only one to experience this curious problem?
11-19-2010, 11:18 AM   #14
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I have the Sigma 17-70 HSM lens and have no FF or BF problems on my K5. I have not run tests on it but have shot with it quite a bit since I got the K5 with no apparent problems.

I found that all of my lenses that required adjustments on the K7 require no adjustments on the K5 body. Sigma 50-500 had a -8 adjustment on the K7 and needs nothing on the K5 and is in perfect focus. It is like having new lenses with this body.

Hope you can solve your problem with the 17-70 as it is a real nice range for walk around.
11-19-2010, 02:37 PM   #15
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QuoteOriginally posted by BrianK Quote
I have the Sigma 17-70 HSM lens and have no FF or BF problems on my K5. I have not run tests on it but have shot with it quite a bit since I got the K5 with no apparent problems.
Glad to hear that somebody has their Sigma 17-70 HSM working. As you say, BrianK, it's a great walkaround zoom.
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