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03-23-2011, 12:47 AM   #1
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K-x AF issues with Sigma 17-70 mm

Some time ago I bought a Sigma 17-70 mm f/2.8-4.0 as a walkabout lens for my K-x.
However, the Sigma sometimes has problems focusing, in low light it is much worse then the kit lens.

So I abandoned the Sigma and replaced it with a Tamron 17-50 mm f/2.8, which seems to focus like a dream. It's just as realiable as the kit lens, and also is good in low light.

Is this some "known issue" with the K-x, or could it just be my copy of the Sigma?


Last edited by drakkar; 03-23-2011 at 02:36 AM.
03-23-2011, 05:28 AM   #2
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I also have Sigma 17-70 but cannot compare to kit lens or Tamron.

Yes, It has sometimes (very limited) problem to focus in low light but I would not contribute this to be a Sigma problem. Maybe I do not have enough experience with other lenses.
I would say it is rather aperture related.

Comparing to some other Pentax lens, Sigma 17-70 seems very fast in AF.

However, I am also thinking to switch to the same Tamron as you have mentioned
03-23-2011, 05:40 AM   #3
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The Sigma was noticeable worse than the 18-55 kit lens, while the with the Tamron I haven't noticed problems at all.
Btw I also like the compactness and low weight of the 17-50 Tamron. And I got f/2.8 all the way, which I feel mote than counters the smaller range.
03-23-2011, 07:57 AM   #4
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Well, yes I have to admit that even before buying Sigma I was thinking about Tamron. Once I bought the Sigma lens I was already questioning the full range F/2.8.

Technically I have preferred Sigma because of OOF and 50 vs. 70mm. Practically I have bought it because it was available while Tamron not. Tamron was even cheaper.
Yes size of Tamron would be also nice.

But again I would not say that the focusing is much worst than with e.g. DA 55-300

03-23-2011, 08:00 AM   #5
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Recently on dpreview is someone complaining (unsharp, loud, not focusing) about Tamron.
03-23-2011, 11:43 AM   #6
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Hi drakkar,

I have the Sigma 17-70 f2.8-4.5 (the older model), and at the long end, it indeed doesn't focus as well as a constant f2.8 zoom in very low light (under Ev 4 or so, which is pretty dim), but I really didn't expect it to. . . there's about 1/3 the light available for the AF sensor to work with compared with f2.8. With the newer model, it's f4 at the long end, so there's about 1/2 the light doing the same comparison.

Pentax has never been known as having a leading edge very low light AF sensor. SAFOX VII was sometimes very frustrating at very low light levels, but there's been constant improvement from model to model. The newest models (w/SAFOX IX) are really getting there, and are definitely competent, but if you want the best performance in this aspect, you're best off looking for the fastest glass in the class to give the system an edge.

Scott
03-23-2011, 09:15 PM   #7
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I have the Tamron as well and find it focusses faster than the kit lens. I have the Sigma 50-150 HSM and find it extremely fast as well.
03-24-2011, 12:57 AM   #8
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QuoteOriginally posted by snostorm Quote
Hi drakkar,

I have the Sigma 17-70 f2.8-4.5 (the older model), and at the long end, it indeed doesn't focus as well as a constant f2.8 zoom in very low light ..
Scott
Thanks. Could fit well, I remember shooting landscape at 70 mm and rather low contrast it could start hunting even if the light wasn't too low (winter time in Norway so it probably wasn't too good either). Anyway I noticed a significant improvement with the Tamron. I don't mind a bit noise, as long at it is focusing.

QuoteOriginally posted by gebco Quote
I have the Tamron as well and find it focusses faster than the kit lens.
That's my impression as well.

03-24-2011, 04:30 AM   #9
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QuoteOriginally posted by drakkar Quote
QuoteOriginally posted by gebco Quote
I have the Tamron as well and find it focusses faster than the kit lens. I have the Sigma 50-150 HSM and find it extremely fast as well.
That's my impression as well.
How would this compare to Sigma HSM? I mean Sigma HSM vs. Tamron.
03-24-2011, 05:01 AM   #10
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I got the Sigma 17-70 mm f/2.8-4.0 HSM, so it was this lens I (sometimes) had an AF issue with. How fast it is I cannot remember, my attention was drawn towards it tended to blast right trough focus. I don't feel the Tamron as slow (feel is the key word here), it just makes a bit noise focusing.

I mean - I own a K-x and this rather cheap but still OK zoom lens, and that is placing me in a segment where "pretty fast" is fast enough.
04-05-2011, 10:43 PM   #11
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QuoteOriginally posted by hramat Quote
How would this compare to Sigma HSM? I mean Sigma HSM vs. Tamron.
The Tamron is screw drive; the Sigma is HSM. I used both recently tracking childrens' speed skating races. I felt that the Sigma focus was keeping up whereas the Tamron struggled a bit. I was positioned at a spot where the skaters came around the corner and toward me.
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