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Old Jun 21, 2009, 4:27 PM   #1
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Default Pentax DA 17-70 autofocus issues

I picked up the DA 17-70 on Friday, and have been trying it out over the last few days. Overall, I'm impressed with the lens, and at lower focal lengths it really shines.

Problem is that at or close to 70mm, the autofocus behaves erratically. This leads to numerous blurry pictures, particularly (& obviously) at f4. I'd guess the miss rate is close to 50%. I'd like to keep the lens, so I'm debating whether or not to send it back to B&H for a replacement, or contact Pentax for repairs. Any thoughts?

- Arndt

(Two postscripts:

1 - messing (extensively!) with the auto focus adjustment on my K20D doesn't improve the situation.

2 - comparing it to my Tamron 70-300 (at 70, also at f4), the autofocus gets it right every time.)
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Old Jun 21, 2009, 5:24 PM   #2
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Hi Arndt,

It's always easier to make some guesses at a possible cause for a problem with some examples, but I'll give you a few outside possibilities.

DOF is always considerably thinner with tele FLs than wide, so focus errors show up more easily, especially wide open. It's a good idea to get familiar with the focus sensor's area to ensure that the AF point is actually "looking" at the right area. A good DOF chart will show that at 30 ft, 17mm f4 will give reasonable focus for anything from 8.5 ft to ∞, but at the same distance, 70mm f4 will give reasonable focus from only about 26 to 35 ft -- a big difference -- and any movement of the camera in holding while the AF is working can lead to less consistent accuracy in focusing.

A second outside possibility in a handholding situation with SR on is that you might not be waiting for SR confirmation before releasing the shutter. Hand shake is much more evident at tele FLs than wide, so full actuation of the SR system is more important. If you always wait for the little hand icon to appear before tripping the shutter, then you can be assured that you're getting the full benefit of SR.

-- Another possible SR related problem might be leaving SR on when using a tripod. The effects would be much less at wide FLs as opposed to tele FLs, so what you could be experiencing is SR induced shake instead of focusing problems.

Realize that I'm not being critical of your technique as I have no reason to question it -- just offering some possibilities how the DA17-70 might show these problems without the lens actually being at fault. You need to eliminate these as possible causes to be able to determine whether the lens needs to be sent back for factory adjustment, or you'll be wasting both your time and theirs.

Scott

Scott
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Old Jun 21, 2009, 9:39 PM   #3
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Thanks Scott,

Yes, one of the things I was looking for in replies was something I might have overlooked. Unfortunately, I'm afraid I covered your suggestions.

I wasn't looking for problems with the lens, but the blurriness was pretty evident almost immediately, which got me investigating. After narrowing things down (using a tripod, SR off, etc.), an autofocus issue is the only thing left I could come up with. I should have mentioned that manual focus generally works quite well at 70, though it's a bit of a pain since the focus ring on this lens has so little travel (about 1/6 of a turn).

But if you or anyone else has any other ideas, I'd be grateful,

- Arndt
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Old Jun 22, 2009, 9:14 AM   #4
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Morning Ardnt, 1/6 of a turn on the focus ring is all you have? I just looked at a number of pictures of the lens and your right, the full range of focus is about 1/6 of the lens' circumference. I do not have a copy of the 17-70, but the inability to focus at a particular focal length is just not right. I would return it and see if the store has another copy. Then try it out in the store first before taking it home. There has to be something out of alignment that progressively gets worse as you travel through the various focal lengths.

It sounds like you have a bad copy....
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Old Jun 22, 2009, 11:29 AM   #5
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If you can manually focus on a broad, flat subject like a brick wall with its grid of mortar, and yet not autofocus on the same subject, then the lens has a problem and should be returned and/or exchanged for another (assuming of course that your camera autofocuses correctly with other lenses). If you send it for repair, it could malfunction again, or else be replaced with someone else's repaired lens - all that warranties promise is repair or replacement with a refurbished product (only if a refurb is not available would you get a new one if they are too backlogged to repair yours quickly) . You paid for a new one and you should have a new one - you don't want someone else's problem.
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Old Jun 22, 2009, 5:24 PM   #6
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Thanks everyone. I've contacted B&H and will be exchanging the lens.

Hope the next one works as advertised.

- Arndt
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