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06-22-2009, 04:42 PM   #1
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Lack of AF on 18-250mm Tamron

I was mucking about in the back garden yesterday and found that my 18-250 entirely too often could not focus on, for example, a flower head. It was mid-afternoon with medium-grey clouds. Focus was almost impossible in the "shade" under a good-sized tree and only somewhat better in areas exposed directly to the sky. Sawed back and forth a few times and gave up - blinking away.

This is a good-sized garden with only a few trees, not a dense forest.

I've sometimes had that trouble in other outdoor situations with the lens, occasionally even in bright shade.

I was using A priority to keep the aperature as large as possible but it was generally 5.6 - 6.3.

I was assuming that it was the K10D but then I swapped the Tamrom for my new DA 55-300. Instant relief. At the same aperatures and focal length.

I've always assumed that AF speed was determined by both the camera and the lens but the ability to get focus at all is determined by the camera.

Any thoughts?? The 18-250 range is entirely too useful to give up and I don't see replacing the lens with one of the new versions if I'm considering a K-7. I'm certain I cannot afford both.

As always, thanks for your wisdom.

06-22-2009, 05:46 PM   #2
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Have you tried cleaning the contacts on the lens with a pencil eraser? Might be a bit dirty making for a bad contact with the body.
06-22-2009, 08:03 PM   #3
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A coupla thoughts. is light through your Tamron lens metering correctly. If you take photographs of the same trouble spot with the 18-250 and 55-300 at the same Aperture and shutter, are you getting about the same exposed images. If so, then it would appear that the metering circuit and equipment is doing just fine.

Does the 18-250 focus correctly when it has sufficient light. If you can find some light situation where the focus circuit works correctly, i think this is telling you that the contacts are working okay.

With adequate light, and using aperture control, AV or M i suppose, do all the aperture settings work ok with adequate light. To test this, i'm suggesting you go from large openings down to the smaller ones, with more than adequate lighting, to see if there is some mechanical problem with some of the aperture settings.

I assume you aren't using any filters with your 18-250. If you are, remove them for the above tests. I don't use any filters with my Tamron 18-250, except for a polarizer in glare conditions. A polarizer can cost you 2 stops of light. This could cost you some focusing problems in the situation you describe.

I would do the above checks with metering the same for both lenses, either center weighed or sector, but not spot metering.

If you find a problem with the above checking, i wouldn't hesitate to contact Tamron about the lens. I once contacted them from their website, after i had questions with their 1.4 TC and the 18-250 working together. I got back a very good reply by email that resolved my concerns.

About a month ago, i upgraded from the K10 to the K20. Doing photos in a local theater playhouse in rehearsals, a coupla days ago, i routinely shot at 800 ISO, 1100 and even once at 1600 using TAV. 18 out of 21 shots were acceptably focused and i suffered no situation where the camera would not focus.

What i'm trying to say is, that buying the K20D made the 18-250 feel like a much better lens to me. It focuses faster and the extra ISO enables shutter speed to support the longer FL. With the K10, i was trending more towards primes, now i'm finding myself increasing my use of Tamron. I think either the K20 or K7 will improve your satisfaction with this lens.
06-22-2009, 11:15 PM   #4
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You mention trying to focus on a single flower head - is it possible you weere simply closer than the minimum focus distance for the lens?

06-24-2009, 01:51 PM   #5
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Original Poster
Bimjo, I've had a number of very intermittent problems with this lens/body combination. I have cleaned the contacts regularly with erasers. I'll check into some other method, as well. I do worry that there is a communication problem (owing to some exposure nonsense) but it's so intermittent that I cannot simply try the lens with another body and duplicate the problems.

Phil, I did not remove both UV filters and then make the comparison again. Regarding a CP, I'll check but I feel confident that the Tammy did not have the CP in place. I rarely use it for all the reasons you can imagine. I don't have one (yet) for the Pentax.

Marc, I did make that mistake early on. But I've experienced troubles at 10+ meters.

Thanks all, I've research to do this weekend. I bought the lens new and have about four years left on the warranty. I'd have it back to Tamron in a hearbeat it would fail hard.

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