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11-25-2012, 04:56 PM   #1
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Strange AF behavior with Tamron 70-200 f2.8

I have tried around a dozen lenses in AF and MF with the K-01 in all lighting conditions and am generally pleased with all Pentax branded lenses, particulalry in MF mode with the focus peaking feature. Yesterday I tried the Tamron 70-200mm F/2.8 and it simply refused to autofocus correctly in any light. Today I tried it again with a series of controlled tests on a tripod in daylight and the thing refuses to autofocus correctly with the following strange behavior:

1. The camera drives the lens through its range and settles on a focus point, visual observation indicates that focus is at least close and in fact the focus peaking feature shows the subject is in focus.

2. The camera then goes into "Auto zoom" mode and shows a very badly out of focus target.

3. Capture the image and in fact it is out of focus.

What is going on here?

Turning OFF "Auto zoom" mode yields good focus and the image is captured in focus most of the time.

Turning "Auto zoom" back on and the image is out of focus again.

OK, what is happening? Listening carefully, the camera is telling the lens to adjust focus at the same time that the "Auto zoom" is engaged. Feel for it, the focus barrel moves at that precise instant. Turn off "Auto zoom" and no tactile feedback, no focus sound and no defocus of the image.

The lens works fine on my K-5 (which is in for service). I believe that I have uncovered a firmware bug because I can't figure out any other reason for it to do this, at least not repeatedly at that precise instant.

11-25-2012, 06:48 PM   #2
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It might be a bug, just like where the camera can freeze when using the FA 200mm macro.

Regardless, I don't think the K-01 is meant to AF well with long lenses! The AF algorithms could certainly use a little improvement.

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11-25-2012, 07:00 PM   #3
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Long lens use

I agree, long lenses are only relevant on the K-01 where it is used as a backup for a DSLR, otherwise it is a shorter range tool and a very good one.
11-27-2012, 04:17 AM   #4
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I experience the same with my tamron 70-200. I think the k-01 does not work so great with non Pentax zoom lenses in af mode

11-27-2012, 05:55 AM   #5
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That happens with my K5, K5IIs, K7 and K01. My Sigma HSM zooms also do that to varying degrees. IMHO, the aftermarket lenses don't play nice with Pentax live view period.

11-27-2012, 06:30 AM   #6
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Yep, got the same behavior on my K5-Tamron 70-200 combo...

To be precise, it looks like the camera overshoots and ends up slightly further than what it tried to achieve...
Indeed, you get front-focus if the lens was focused farther than your target when it started moving, and back-focus in the other case...
Maybe it's a inertia thing, with those heavy lenses...
11-27-2012, 12:22 PM   #7
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I don't think that it is an inertia issue since there is a definite separation between the two events and it only seems to happen when the auto zoom function kicks in. At first, I thought that the encoder might be loose and it was adjusting focus to keep the same encoder position (I assume a potentiometer but have never taken one of them apart). I design cameras and see that sort of thing from time to time on units in for repair. However it is so closely tied to the auto zoom function that it appears like a bug.

Here is a thought - is it attempting to take up backlash?

In the mean time, I'll just manually focus whenever I use that monster on the K-01. I would only use it in a backup situation as the smaller primes seem to be perfect for that camera for regular use.
12-10-2012, 06:21 PM   #8
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I'd just like to add that I have the same problem with my K-01 (firmware version 1.01) and Tamron 70-200 f/2.8. Same exact problem with my Tamron 24-135 (my walkaround zoom). But even in low light, I can put on my DA-L 55-300 and AF perfectly (even at 300mm). So it's not the focal length, and it's not a low light problem. It's just weird. Also, for me, AF autozoom has no effect one way or the other. Sigma 30 f/1.4 works fine. Sigma 10-20 f/4-5.6 is fine. Even my Sigma 170-500 nails focus hand-held with the K-01 in pretty low light. So it's not a simple 3rd-party problem. Absolutely nutty that the Tamron 70-200 f/2.8 won't autofocus.

Edit: My Tamron 28-75 f/2.8 also focuses fine.

Last edited by Designosophy; 12-11-2012 at 08:06 AM. Reason: Additonal info
12-17-2012, 01:12 PM   #9
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I'm thinking of getting a Tamaron 200mm or 300mm lense... Is this going to be a problem? I just got the K-01 and the Tamaron lenses seem to offer some nice specs for the money.

Looking at these 2 lenses.

Tamron AF 28mm-300mm f/3.5-6.3 XR DI Lens w/Hood f/Pentax #AF061P700 -USA- AF061P700

Tamron 18-200mm f/3.5-6.3 XR DI-II LD AF Zoom Lens AF014P700

Please help. How is the image quality with the Tamaron lenses on K-01. I just got the K-01 and if it doesn't work out I'm going to return to vendor.
12-17-2012, 08:40 PM   #10
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I have two Tamron lenses, the 70-200 f/2.8 and the 18-250 f/3.5-6.3, the 70-200 has the strange behavior with the autozoom turned on. The 18-250 doesn't have this problem and the image quality is fine. Autofocus is good with the slower lens, even indoors. The faster lens only has the problem with the autozoom, but it also has the quickshift manual focus so it can be focused properly, just manually.

I wouldn't shy away from the K-01.

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