[jeff knight]

Some time ago I bought a used Tamron 17-50 at a really good price. Right away I noticed that it focused inaccurately at all wider settings, however it focused perfectly at 50mm. So I have to zoom in to lock focus and then zoom back out to get accurately focused images. This is a minor inconvenience shooting objects or landscapes, but a royal pain shooting models in the studio. I miss a lot of good shots. My question is: Is this a lens problem, or could there be some kind of compatibility problem which might involve my K20D? Should I send the lens in for repair? I've attached a jpeg at actual pixels illustrating the problem. Often the disparity is worse than pictured. I would be thankful for any information or advice.

Attached Images

 

[Adam]

I'd suspect that the lens is damages, especially since you said you got it used. I'd see if I couldn't return it.

[NoMaD_PS]

Sorry Jeff, I can't help you. But I have the same problem.

I've got the DA 16-45 recently and it focuses right at 45mm but at 16 mm tends to focus on a backstage or infinity.

Lately I had the same issues with F 35-70 at 35mm.

Could it be my K10? Or the lens? Or is it a common problem at shorter end?

[jeff knight]

Could it be my K10? Or the lens? Or is it a common problem at shorter end?

Yes NoMaD, those are the big questions I have. My DA12-24 focuses absolutely accurately at all times.

I think most everybody here is tired of discussing focus issues with Pentax bodies. It's obviously been Pentax's Achilles heel, before the K-7 at least,

and a real sore subject. I just know some lenses work, others don't on my body. I just bought the 16-45 myself. If it has the

same problems as my Tamron I guess I'll know what's what. Thanks for making me feel less alone.

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Last edited by jeff knight; 07-26-2010 at 08:55 AM.

[Jewelltrail]

jeff knight Some time ago I bought a used Tamron 17-50 at a really good price. Right away I noticed that it focused inaccurately at all wider settings, however it focused perfectly at 50mm. So I have to zoom in to lock focus and then zoom back out to get accurately focused images. This is a minor inconvenience shooting objects or landscapes, but a royal pain shooting models in the studio. I miss a lot of good shots. My question is: Is this a lens problem, or could there be some kind of compatibility problem which might involve my K20D? Should I send the lens in for repair? I've attached a jpeg at actual pixels illustrating the problem. Often the disparity is worse than pictured. I would be thankful for any information or advice.

Jeff, I have been shooting for over 2.5 years with this lens and the K20d--I am very happy with the combination. A question for you?

Are the shots above cropped at all?

[jeff knight]

Jeff, I have been shooting for over 2.5 years with this lens and the K20d--I am very happy with the combination. A question for you?

Are the shots above cropped at all?

Yes, they are crops at actual pixels. As I stated, often the out of focus quality is
much, much worse than what is shown on the left here. For what its worth I
think the Tamron is a truly spectacular lens. I could easily live with the bug mine has,
but it just isn't practical to zoom in and lock focus, then zoom out to compose,
while shooting fast posing models. I have to limit myself to very static, (posey) poses.
I'm just not that fast. It's sad but I may end up having to sell this truly fine lens
super-cheap.

[Aegon]

Before you sell it for "super cheap", contact Tamron and see if they'll warranty repair it (probably not) or charge you a fee to have it adjusted. You might have to send your body in too.

It might be worth your time and money. Certainly worth an email.

[jeff knight]

Before you sell it for "super cheap", contact Tamron and see if they'll warranty repair it (probably not) or charge you a fee to have it adjusted. You might have to send your body in too.

It might be worth your time and money. Certainly worth an email.

Thank you. That sounds like very prudent practical advice, but it's been my experience that Tamron repair is not cheap. I'm going to replace it with a Pentax lens.

[Marc Sabatella]

Would be interesting to see the non-cropped shots (resized). It's certainly *possible* you are simply seeing the results of the fact that the actual focus sensor is much larger than the focus indicator. At wide angles especially, this often creates situations where the object you are aiming at isn't the only thing within range of the focus sensor, and it unfortunately picks something else within that range to focus on. By zooming in, you solve that problem by making the subject big enough to cover the fullarea of the focus sensor, removing the ambiguity.

[jeff knight]

Would be interesting to see the non-cropped shots (resized). It's certainly *possible* you are simply seeing the results of the fact that the actual focus sensor is much larger than the focus indicator. At wide angles especially, this often creates situations where the object you are aiming at isn't the only thing within range of the focus sensor, and it unfortunately picks something else within that range to focus on. By zooming in, you solve that problem by making the subject big enough to cover the full area of the focus sensor, removing the ambiguity.

Marc, I know you are one of the keenest minds on this forum. I understand exactly what you are saying and it's a totally valid question. But with this lens: if I shoot a flat brick wall at 20ft at 50mm f5.6 it will be in extremely sharp focus. If I refocus at 20ft. at 20mm f5.6 it will be unacceptably soft. If I zoom into the same brick wall at 50mm and lock focus, then zoom back to 20mm and then take the same shot it will be tack sharp. The ironic thing is that I am not a mad pixel-peeper. I am only looking for relatively consistent and acceptable results. I really love this lens but I must reluctantly conclude that either Tamron QC is totally screwed, or else my camera is. I certainly hope it's the former! I strongly suspect that Adam is right about this lens.

By the way, I bought this lens used from B&H. Should have returned it early on but it was sooo... good despite it's flaws.

[Jewelltrail]

jeff knight: By the way, I bought this lens used from B&H. Should have returned it early on but it was sooo... good despite it's flaws.

Jeff, B & H is excellent, often going beyond expectations of its customers. I would call them and explain. Also, Tamron gives a 7-year warranty with this lens--it has not been out in Pentax mount for 3 yet. I'll bet someone there will step up and help you out if you give it a try.

If not, sell it to me cheap.

[Jewelltrail]

Marc Sabatella: Would be interesting to see the non-cropped shots (resized).

Agreed.

[Marc Sabatella]

Does sound like he's already accounted for the effect I was describing, though.

[jeff knight]

Does sound like he's already accounted for the effect I was describing, though.

Thanks everyone for your valuable input. I am certainly not dismissing anybody's advice,
despite my instinct to replace the lens.

I made this post in hopes of getting some intelligent feedback and that's exactly
what you gave me. No exceptions. I really appreciate it.

[Jewelltrail]

Marc Sabatella: Does sound like he's already accounted for the effect I was describing, though.

I do not doubt that--as an owner of the lens, I probably have more interest than others in this thread--so seeing original compostion would be interesting.

Good luck to you Jeff.